Health Parameters of Potato Tubers under the Influence of Soil Applied Bio-Preparations and Bio-Stimulants

Abstract

Increasing consumption of processed potatoes and consumer preference for buying potatoes washed and packed in transparent packages are a reason for increasing quality standards for potatoes. Processing and trade require potato tubers with smooth skin and without signs of disease, such as common scab, black scurf, and silver scurf. It is necessary to introduce protective measures to reduce the growth of pathogens causing these diseases and, at the same time, are safe for the environment and the consumer. To meet these requirements, the effects of application to soil and treatment of seed potatoes in the following solutions were examined: biological control agents (BCAs): Pythium oligandrum (BCAPo), Bacillus subtillis str. QST 713 (BCABs); microbial soil additives (MSADs): Efficient microorganisms (EM), UGMax soil conditioner, Biogen Rewital (BR); plant growth promoter (PGP): Ecklonia maxima (PGPEm) for the infection of the tubers by Streptomyces scabies (S.s), Rhizoctonia solani (R.s) Helminthosporium solani (H.s) and potato yield. Average Disease Severity Index (DSI) for common scab (S.s) 62.0%, black scurf (R.s) 57.88%, and silver scurf (H.s) 54.24%, obtained from three growing seasons, indicate their significant economic importance. The bio-preparations used significantly reduced their intensity. The effectiveness of protection for individual pathogens varied and was highly dependent on hydrothermal conditions. The analyzed preparations showed E between 8.0% and 50.8% against S.s. However, a stronger effectiveness was found in relation to H.s (12.9–56.6%) and R.s (19.5–69.2%). In years with water deficit, PGPEm and MSADs are more effective than BCAs in protecting potato tubers from skin diseases and contribute to higher yield increases. There was a significant negative correlation between the total DSI and the potato tuber yield.

1. Introduction

Potato (Solanum tuberosum L.) is one of the most important crops grown in the world. Only Saccharum officinarum L. (sugarcane), Zea mays L. (maize), Oryza sativa L. (rice), and Triticum aestivum L. (wheat) outweigh its production [1]. Potato tubers have a high nutritional value and are rich in protein of high biological value, vitamins C, B₁, B₃, and B₆, dietary fiber, and minerals [2,3,4,5]. Antioxidants such as phenolic compounds, including chlorogenic acid and catechin acid, as well as lutein, zeaxanthin, and anthocyanins, are extremely valuable for human health [6,7,8]. Sulfur-containing amino acids, which are relatively rare in other food products, are a very important nutrient of potato [5,9].

The high nutritional value of potato and the fact that the unit of surface produces significantly more protein than in the cultivation of other plants makes it a plant that should be included in the global strategy to combat hunger [10]. According to the FAO [11], with the growing world population, there must be an almost 50% increase in agricultural production by 2050. Unfortunately, this may have negative effects, such as arable land degradation or greenhouse gas emissions. Therefore, more emphasis should be placed on sustainable development in food production. According to Gustavsena [5], this is ensured by, inter alia, the production and consumption of food with a lower carbon footprint and minimal use of land for food production. These conditions are met by the potato since it has a low CO₂ footprint and low soil requirements compared to other alimentary crops.

Potato is used for direct consumption, processing into French fries and chips, as well as for the production of dry products and the extraction of starch [12]. Direct consumption dominates in European countries, but consumption of French fries and chips is clearly increasing [5,13]. The trend toward the consumption of processed potatoes, as well as consumer purchases of potatoes washed and packed in small, transparent consumer packs, makes the processing and marketing of potatoes subject to increased health standards for tubers [13,14,15,16]. In such a situation, potato skin diseases such as black scurf (Rhizoctonia solani), silver scurf (Helminthosporium solani Durieu and Mont), and common scab (Streptomyces scabies Thaxter) are also of economic importance. The priority for many potato producers is to obtain tubers with smooth skin and without any blemishing lesions.

S. scabies is a dominant pathogenic species present in soils around the world [17,18]. The saprotrophic nature ensures that even in the absence of a host plant, it does not lose its viability for a long time. It can therefore be a fixed source of inoculum during subsequent growing periods [19]. After infection, a shallow or deep suberization appears on the surface of the potato tubers, which blemishes the skin of the potato and requires deep peeling. The shape of the infected tubers is often deformed. The severity of symptoms of common scab depends on the amount of inoculum in the soil, the virulence of the Streptomyces species, the susceptibility of the variety, and the environmental conditions [20]. Loria et al. [21] reported that the symptoms of scab might also appear on many other root crops, including Daucus carota L. (carrot), Beta vulgaris L.(beetroot), Pastinacea sativa L. (parsnip), Brassica rapa L. subsp. rapa (turnip), and Raphanus sativus L. var. sativus (radish).

The fungus R. solani Kühn (Thanatephorus cucumeris (Frank) Donk), which is spread all over the world, has an even wider economic range. It attacks important species of arable crops belonging to families Solanaceae, Poaceae, Braccicaceae, Asteraceae, and many others [22,23,24,25]. R. solani is the cause of rhizoctoniosis; its symptoms are varied and usually related to parts of the plant. It causes gangrene of the shoots, roots, stem bases, and storage organs, wounds, and, less often, leaf spot disease [26]. Regardless of the geographical distribution, R. solani is an important pathogen of potatoes and other root vegetables [27,28,29,30,31,32,33]. The level of losses caused during potato cultivation depends on the severity and time of the plant infection. What is dangerous is the rotting of shoots, which occurs during the emergence. On the other hand, later infections affect potato tubers (black scurf). Black hardened fungal hyphae (sclerotia) contaminate the periderm of potato tubers, contributing to the deterioration of quality. Severe infection is often accompanied by the diminution of tubers, which may disqualify their suitability for processing. Furthermore, sclerotia are the source of infection in the next growing season, and spores that form in the soil do not lose their ability to infect for several years [34,35].

According to Errampalli et al. [36], potato is the only host for the fungus Helminthosporium solani. Silver scurf, a disease caused by this fungus, is relatively common, but symptoms on unwashed tubers may remain unnoticed. Hyphae of the fungus H. solani spread between cells and in cells, forming an air space under the periderm of the tubers [37]. The result of the infection includes extensive stains with silver gloss. Water is lost through the damaged skin during storage, the tubers shrink, and the loss of mass may reach 17% [16,38,39]. An important factor in reducing the disease severity is to ensure adequate sanitary conditions and to reduce inoculum near the tubers by removing crop and soil residues [13]. However, planting infested potato tubers is the main source of H. solani in the soil.

The presented disease units are caused by unrelated phytopathogens with varying life cycles, range of hosts, and different symptoms that can be present on the same potato tubers at the same time. Knowledge of the causes of diseases is crucial for the successful management of potato production. In practice, the fight against pathogens living in soil and carried with seed potatoes is not easy [20,21,30,32,40,41,42,43,44]. It should be highlighted that the use of chemical protective agents may be ineffective if environmental conditions promote the dispersion of pathogens or if fungicide-resistant genotypes appear [45,46,47]. The use of synthetic pesticides has numerous times been shown to lead to a loss of soil biodiversity and an accumulation of pathogenic bacteria, fungus-like organisms, and fungi in soil [42,43,44,48]. The introduction of organic matter to soil is one of the more effective methods of restoring soil biodiversity and balance between populations of beneficial and pathogenic microorganisms [42,49,50]. Different types of microbiological preparations, effective microorganisms, and soil conditioners can play a similar role [51,52]. Many authors point out that their use in the cultivation of different plant species has had a positive effect on the yield, the content of nutrients, and bioactive substances [53,54,55,56,57]. In addition, they have increased plant resistance processes against biotic factors resulting in better plant health [32,58].

Due to the concern of protecting biodiversity, the environment, and the consumer, biological preparations applied to soil and onto plants will supplant chemical plant protection products. In line with these requirements and increasing health standards for potato tubers, six preparations that are authorized for use in organic potato production systems were tested. The aim of this study was to (I) determine the extent to which the soaking of healthy seed potatoes in the following solutions: biological control agents (BCAs) (Polyversum WP, Serenade ASO), plant growth promoters (PGPs) (Kelpak SL), and microbiological soil additives (MSADs) (Em Farma^TM, UGMax soil conditioner, Biogen Rewital), as well as their introduction into the soil just before potato planting, shape the health of the surface of the tubers and the total yield of the potato. In this study: (II) the magnitude of the problem of the occurrence of blemishing lesions caused by S. scabies, R. solani, H. solani was determined; (III) it was verified which preparations reduced the growth of the pathogens assessed most strongly, taking into account seasonal variations in hydrothermal conditions; (IV) the relationship between the severity of individual units of disease and potato tuber yield was assessed.

2. Materials and Methods

2.1. Plant Material

Potato tubers were harvested each year (2015–2017) from a field experiment carried out in Łukomierz (51°13′20″ N; 18°55′00″ E), which is located in the southwestern part of the Łódzkie Province.

The experiments were carried out on the potato cultivar Tajfun. This variety is recommended for cultivation in this part of Poland. Information on culinary suitability and resistance to pathogens and nematodes is provided in

Table 1. Characteristics of the potato cultivar Tajfun.

No.	Highlighted Characteristics	Description
1.	Maturity time	Medium early
2.	Flesh color	Pale yellow
3.	Skin color	Yellow
4.	Tuber shape	Oval
	Shape regularity 1–9	Regular 7.0
5.	The size of the tubers 1–9	Very big 9.0
6.	Shallowness of eyes 1–9	Shallow 7.1
7.	Fertility	Very big
8.	Cooking type	B-BC (B—general purpose, C—mealy)
9.	Resistance to late blight on a 9°	Susceptible (5°)
10.	Resistance to PVY on a 9°	Field resistant (7°)
11.	Resistance to PLRV on a 9°	Resistant (8°)
12.	Resistance to nematode	Quite resistant to the potato cyst nematode

in scale 1–9; 1—estimate the worst, 9—estimate the best [56].

. The parameters of the potato cultivar under examination indicate its suitability for cultivation in the organic system.

2.2. Field Experiment Design

The study concerned the assessment of the protective effect of biological preparations (two biological control agents (BCAs), one plant growth promoter (PGP), and three microbial soil additives (MSADs)) on potato tubers. Each preparation was applied to the soil just before the potatoes were planted. Additionally, sprouted seed potatoes were dipped in it. The applied doses of the preparations were in accordance with the manufacturer’s guidelines. The experiment included 6 variants differentiated in terms of the preparations used and a control treatment. Due to the high risk of Colorado potato beetle (Leptinotarsa decemlineata) as well as diseases such as potato late blight fungus (Phytophthora infestans) and alternariosis (Alternaria spp.), during the growing season, the same protection was applied to all study treatments. When the youngest development stages of L. decemlineata larvae appeared, SpinTor 240 SC preparation was used at a dose of 0.15 L ha⁻¹ (up to 3 treatments were performed during the growing season). In accordance with the recommendations for organic crops, a copper-containing preparation, Miedzian 50 WP, was used for protection against P. infestans and Alternaria spp. It was applied three times on uninfected plants without exceeding a total dose of 6 kg of pure copper per hectare. In contrast, weed control was carried out mechanically throughout the experiment. During the growing season, the potatoes were protected according to the scheme presented in

Table 2. Scheme of plant protection treatments.

Designation of Protection Treatments		Preparate	Preparations and Their Doses
			Before Planting		During Vegetation
			Application to Soil	Dressing Seed Potatoes	The Foliar Application
BCAs	BCAPo *	Polyversum WP	200 g·ha−1 in 300 L of water	10 g·kg−1 of seed potatoes (soaking for 30 min)	3 × SpinTor 240 SC 0.1 l ha−1 + 3 × Miedzian 50 WP 2 kg ha−1
	BCABs	Serenade ASO	10 L ha−1 in 500 L of water	400 mL·kg−1 of seed potatoes (soaking for 30 min)
PGP	PGPEm	Kelpak SL	4 L ha−1 in 800 L of water	0.4% (soaking for 5 min)
MSADs	EM	Em Farma™	20 L ha−1 in 300 L of water	1% (soaking for 5 min)
	UG	UG Max soil conditioner	1.2 L∙ha−1 in 300 L of water	0.5% (soaking for 30 min)
	BR	Biogen Rewital	1 L∙ha−1 in 500 L of water	0.2% (soaking for 4 h)
Control	C	no protection	300 L of water	In distilled water for 30 min

* The article will use the abbreviations of individual variants of the protection treatments used.

.

The one-factor field experiment was conducted using a random block method in four replicates. The size of the observation and collection plot was 20 m², and the total working area was 560 m². Each year, spring barley was the potato forecrop. In autumn, 30 t·ha⁻¹ cattle manure was applied and incorporated into the soil by deep plowing. Before winter, phosphate–potassium fertilizers (80 kg P₂O₅ ha⁻¹ and 120 kg K₂O∙ha⁻¹) were also introduced to the soil. Nitrogen fertilizers in the amount of 80 kg N·ha⁻¹ were applied in spring. Immediately prior to planting, the preparations listed in Table 2 were introduced into the soil. The seed potatoes were certified (class A), with 40–50 mm in size. They were subjected to germination, and biopreparations were applied to their surface prior to planting at doses indicated in Table 2. Potato tubers were planted in the second fortnight or week of April with a spacing of 30 × 67.5 cm. Potatoes were harvested during technical maturity.

2.3. Characteristics of the Preparations Used

The experiment used preparations authorized for use in organic crops and approved by the National Institute of Hygiene.

• Biological control agents (BCAs):

  -

  Polyversum WP—106 oospor Pythium oligandrum in 1 g of the agent (Biopreparaty Ltd., Uherce, Czech Republic);

  -

  Serenade ASO—Bacillus subtillis str. QST 713—13.96 g L⁻¹ (minimal concentration 1.042·1012 CFU·L-1) (Bayer AG, Leverkusen, Germany).

• Plant growth promoter (PGP):

  -

  Kelpak SL—auxins—11.0 mg·L⁻¹ and cytokinins—0.031 mg∙ L⁻¹ obtained from Ecklonia maxima algae (Kelp Products International (Pty) Ltd., Simon’s Town, South Africa).

• Microbial soil additives (MSADs):

  -

  Em Farma™—microorganism strains or consortia thereof produced by natural fermentation processes based on species commonly occurring in nature, involving microorganisms, not genetically modified, with the highest standards of hygiene and quality (a useful blend of bacteria that supports the natural growth of useful microorganisms found in agricultural environments and is safe for humans and the environment), organic cane molasses, revitalized, unchlorinated water, salt, mineral complex (Probiotics, Poland);

  -

  UG Max soil conditioner—a liquid concentrate containing microorganisms (acid bacteria, photosynthetic bacteria, Azotobacter, Pseudomonas, actinomycetes, and yeasts) as well as macronutrients (g·L⁻¹: K—3.5; N—1.2; S—1.0; P—0.5; Na—0.2; Mg—0.1) and micronutrients (g·L⁻¹: Zn—20.0; Mn—0.3), (Bogdan Sp. zoo, Bolesławowo, Poland);

  -

  Biogen Rewital—contains 108 CFU·g⁻¹, a microbiological composite of non-pathogenic bacteria (cellulolytic, nitrifying, sanitary, lipolytic) and a starter medium (Bio-Gen Ltd., Łódź, Poland).

The following preparations approved for organic crops were applied on leaves:

-

A biotechnical agent for controlling Leptinotarsa decemlineata—SpinTor 240 SC—spinosad: Spinosyn A, Spinosyn D (a substance from the group of macrocyclonic lactones)—240 g·L⁻¹, (22.72%) (Dow AgroSciences Polska Sp. z o.o., Poland);

-

Miedzian 50 WP—copper in the form of copper oxychloride—50% (500 g Cu·kg⁻¹) (Synthos Agro Sp. z o.o., Oświęcim, Poland).

2.4. Soil Conditions

Soil samples were taken using Egner’s stick each year prior to setting up the experiment. They were transported to an approved laboratory of the District Chemical and Agricultural Station in Kraków, where, according to the methods in place at chemical–agricultural stations in Poland, chemical and physicochemical properties of the soil were determined.

The following were determined in the soil samples: pH using the potentiometric method, nitrate nitrogen content by colorimetry using a spectrophotometer, humus using the Tiurin method, and available macronutrients (P, K, Ca, and Mg) by methods commonly accepted at the chemical–agricultural station [59,60,61,62,63]. The content of microelements was also indicated in the soil [64,65,66,67,68]. The results of the soil arable layer analysis are shown in

Table 3. Chemical properties of the arable layer of soil.

Specification	2015	2016	2017	Mean
pH H2O	5.69	6.08	5.83	5.87
Salinity (g NaCl·L−1)	0.21	0.14	0.24	0.20
Humus content [%]	1.73	1.89	1.45	1.69
The content available in the soil layer of 0–30 cm (mg·L−1)
N-NO3	16.00	<14.00	<14.00	14.67
P	29.00	36.00	28.00	31.00
K	108.00	118.00	199.00	141.67
Ca	182.00	270.00	258.00	236.67
Mg	10.00	44.00	38.00	30.67
Cl	<27.00	<27.00	39.00	31.00
Cu	0.89	2.02	1.02	1.31
Zn	2.59	12.54	5.02	6.72
Mn	60.12	76.00	78.00	71.37
Fe	234.60	268.64	173.8	225.68
B	0.10	0.20	1.10	0.47
S-SO4 (mg∙100 g−1 soil)	0.35	0.32	1.72	0.80
Pb (mg·kg−1)	12.26	16.81	12.63	13.90
Cd (mg·kg−1)	<0.33	0.33	0.37	0.34
Ni (mg·kg−1)	1.50	4.04	3.02	2.85

.

The potatoes were grown on podzolic soil made from light, strongly sandy clay with a granulometric composition of fine sand, which is classified as a good rye complex [69,70].

The soil had an acid reaction (pH = 5.86) and low humus content (1.69%). The arable layer was rich in potassium, but there were shortages of other macroelements and microelements, such as Cu, Zn, and B, and excessive iron content. The heavy metal content did not exceed the permitted standards.

Weather conditions in the years under investigation were varied (Figure 1). They were characterized by the Sielianinov hydrothermal index (K):

$$K=\frac{p}{0.1·\Sigma t}$$

where: p—sum of precipitation for the examined period (mm); t—average daily air temperature (°C).

The analyzed years of the study had different precipitation patterns and amounts (Figure 1). The growing season in 2015 was the driest and the warmest. Only 246.4 mm of rain was recorded between April and September. During the first growing season (April–June), the total precipitation was 127.6 mm, and in the second (July–September), it was 118.8 mm. Sielianinov (K) ratios indicated optimal humidity (K = 1.5) only in July, and in the remaining months, they indicated drought (K = 0.5–0.9). In contrast, 2016 can be considered the most optimal year in terms of water supply (358.6 mm) and coverage of potato demand. However, after a very wet July (K = 2.3), a significant precipitation deficit was recorded in August and September (K = 0.5). The study period from April to September 2017 was wet (543.6 mm of precipitation) and warm. During April–June, the sum of precipitation was 243.6 mm, and between July and September, it was 300.0 mm. Potato plants were sufficiently preserved in moisture.

2.5. Assessment of the Occurrence of Infectious Diseases of the Skins of Potato Tubers and Yields

Potato tubers were harvested in the second fortnight or week of September. The surface of the adjacent soil was cleaned from each tuber plot and washed under running water. The total yield is expressed in t·ha⁻¹. Afterward, the yield increase in individual treatments was calculated and expressed as a percentage in relation to the control.

The health assessment of the tubers was carried out immediately after harvesting on 100 tubers which were randomly taken from a batch of tubers obtained from individual plots (28 samples).

The evaluation of the tuber infection by R. solani was carried out according to the EPPO PP 1/32 (3) [72] standard, which adopted a four-stage scale of infection of the bulb surfaces.

According to the scale:

0—no disease symptoms; 1—1% surface of tubers covered with sclerotia of R. solani; 2—5%; 3—10%; 4—above 15%.

Since there is no EPPO-developed scale for potato tubers infection by Streptomyces scabies and Helminthosporium solani, the 5° scale developed by Driscoll et al. [73] for common scab was used.

According to the scale:

0—no disease symptoms, 1—1–10% of tuber surface with common scab/with symptoms of silver scurf (stains with changed tuber skin color); 2—11–20%; 3—21–50%; 4—51–80%; 5—81–100%.

Intensification of the potato tuber diseases was expressed by the disease severity index (DSI), the average level of infection of the sample, and the percentage of tubers affected by the particular disease, respectively.

DSI was computed according to the formula described by Townsend and Heuberger [74]:

$$DSI=\frac{{\displaystyle \sum _{i}n\cdot v}}{i\cdot N}\cdot 100\%$$

where DSI—disease severity index; n—number of plants with a given degree of infection; v—degree of infection; i—highest degree of infection; N—total number of plants analyzed.

Average degree of infection of infected tubers (ADIIT) was determined in a batch of potatoes with clear symptoms of skin diseases.

The effectiveness of the protection procedures studied was calculated according to the Abbott formula [75].

$$E=\frac{A-B}{A}\cdot 100\%$$

E—effectiveness; A—infection in the control combination; B—infection in the protected combination.

2.6. Statistical Calculations

The obtained results were subjected to a bivariate analysis (ANOVA) and repeated Tukey’s honestly significant difference tests, with a significance level of 0.05. Tukey’s comparative tests allowed for detailed analyses of the mean values by isolating statistically homogeneous groups. The presence of the same letter next to the means indicates that there are no statistically significant differences between them. The coefficients of variation (CV) or standard errors (SE) were determined for the assessment of the variation.

Correlation analyses and validation of significance were also conducted in the study. In addition, linear regression models were established for selected dependencies.

The calculations were performed with the statistical software package Statistica 9.1 (StatSoft Inc., Tulsa, OK, USA).

3. Results

3.1. The Effect of Hydrothermal Conditions and Protection Measures on Potato Tubers Health

The intensity of infectious diseases in potato tubers is the result of a number of co-influencing factors: environmental; agro-technical, including the biopreparations used; and biotic interactions between organisms that colonize the tubers.

Three disease units, i.e., common scab (S. scabies), black scurf (R. solani), and silver scurf (H. solani), were observed on potato tubers in the studied growing seasons immediately after harvest. The analysis of the variance showed that the biological preparations used, the growing years, and the interactions between them had a significant impact on all the assessed parameters of infection by the culprits of individual diseases (

Table 4. ANOVA F-test probabilities for the effects of parameters.

Infection of Potato Tubers by:	Parameters of Potato Tuber Infection			Efficiency of Protection [E]
	Disease Severity Index [DSI]	Average Degree of Infection [ADI]	Percentage of Infested Tubers [PIT]
Rhizoctonia solani
Preparates	***	***	***	***
Years	***	***	***	***
Preparates × years	***	***	***	***
Streptomyces scabies
Preparates	***	***	***	***
Years	***	***	***	***
Preparates × years	***	***	***	***
Helminthosporium solani
Preparates	***	***	***	***
Years	***	***	***	***
Preparates × years	***	***	***	***

*** significant at p < 0.001.

and

Table 5. Potato tuber peel diseases depending on the applied biopreparations and the growing season.

Specification		C **	BCAPo	BCABs	PGPEm	EM	UG	BR
Rhizoctonia solani
Disease Severity Index	2015	55.4 b * ± 1.2	47.4 d ± 3.1	33.3 fgh ± 4.9	26.9 ijk ± 7.0	29.6 hi j ± 1.9	38.6 e ± 1.2	28.5 hij ± 3.0
	2016	50.3 cd ± 3.8	15.5 m ± 7.2	20.0 lm ± 7.9	31.5 ghi ± 5.7	25.8 jk ± 5.7	35.5 efg ± 5.1	22.8 kl ± 12.2
	2017	68.0 a ± 4.8	37.8 ef ± 5.1	38.8 e ± 5.9	54.8 bc ± 1.5	54.8 bc ± 2.0	50.0 cd ± 2.8	45.5 d ± 2.5
Average degree of infection	2015	2.2 b ± 3.8	1.9 c ± 3.4	1.3 efg ± 5.2	1.1 hij ± 7.8	1.2 fghi ± 5.4	1.5 de ± 2.2	1.1 ghij ± 5.1
	2016	2.0 bc ± 3.8	0.6 l ± 7.2	0.8 kl ± 7.9	1.3 fgh ± 5.7	1.0 ij ± 5.7	1.4 def ± 5.1	0.9 jk ± 12.2
	2017	2.7 a ± 4.8	1.5 de ± 6.2	1.6 d ± 5.9	2.2 b ± 1.5	2.2 b ± 2.0	2.0 b ± 2.8	1.8 c ± 2.5
Percentage of infested tubers	2015	74 abc ± 2.7	72 abcd ± 5.6	63 cdef ± 5.3	32 jk ± 15.3	45 ghij ± 3.8	56 dfgh ± 16.8	40 hijk ± 10.0
	2016	83 ab ± 9.3	27 k ± 12.3	36 ijk ± 7.9	53 efghi ± 6.3	47 fghij ± 15.2	47 fghij ± 7.1	38 ijk ± 17.5
	2017	88 a ± 10.2	63 cdfe ± 5.3	60 cdfg ± 4.7	86 ab ± 11.6	82 ab ± 12.7	69 bcdf ± 2.5	62 cdfg ± 7.2
Effectiveness of protection	2015		14.3 l ± 18.7	40.0 fgh ± 7.4	51.5 cd ± 6.6	46.6 cdef ± 2.2	30.4 ij ± 2.6	48.5 cde ±3.2
	2016		69.2 a ± 3.2	60.2 b ± 5.2	37.3 ghi ± 9.6	48.8 cde ± 6.0	29.3 ij ± 12.2	54.7 bc ± 10.1
	2017		44.5 defg ± 6.4	43.0 efg ± 7.8	19.5 kl ± 6.3	19.5 kl ± 8.2	26.5 jk ± 7.9	33.1 hij ± 5.0
Streptomyces scabies
Disease Severity Index	2015	73.2 a ± 2.0	66.7 b ± 3.0	62.8 bc ± 4.1	45.4 hij ± 3.8	52.4 efg ± 4.5	57.2 cde ± 4.9	67.3 ab ± 2.8
	2016	59.0 cd ± 3.9	37.8 klm ± 6.6	32.4 mno ± 3.7	41.4 ijkl ± 2.9	47.0 ghi ± 4.1	50.5 fgh ± 4.3	50.6 fgh ± 6.3
	2017	53.8 def ± 3.5	30.0 no ± 4.4	26.6 o ± 5.4	39.8 jkl ± 2.2	35.6 lmn ± 5.1	41.0 ijkl ± 6.1	43.2 ijk ± 2.3
Average degree of infection	2015	3.7 a ± 2.0	3.3 ab ± 3.2	3.1 bc ± 4.1	2.3 gh ± 3.8	2.6 def ± 4.5	2.9 cde ± 4.9	3.4 ab ± 3.0
	2016	3.0 cd ± 3.9	1.9 ij ± 6.6	1.6 jkl ± 3.7	2.1 hi ± 2.9	2.4 fgh ± 4.1	2.5 efg ± 4.2	2.5 efg ± 6.3
	2017	2.7 de ± 3.5	1.5 kl ± 5.2	1.3 l ± 5.4	1.9 ij ± 11.3	1.8 ijk ± 5.1	2.1 hi ± 6.1	2.1 hi ± 3.3
Percentage of infested tubers	2015	97 a ± 3.4	91 abc ± 5.7	86 bcd ± 2.3	77 de ± 4.3	80 cde ± 3.5	94 ab ± 2.1	93 ab ± 3.6
	2016	92 ab ± 3.1	73 efg ± 6.0	56 hi ± 5.1	75 ef ± 4.4	73 efg ± 8.1	80 cde ± 3.5	88 abc ± 3.2
	2017	86 bcd ± 2.3	63 gh ± 5.3	52 i ± 5.4	71 efg ± 6.1	65 fgh ± 2.7	71 efg ± 6.1	80 cde ± 6.1
Efficiency of protection	2015		8.9 h ± 30.9	14.1 fgh ± 24.5	38.0 bc ± 6.3	28.4 cdef ± 11.4	21.9 efg ± 17.5	8.0 h ± 31.5
	2016		35.9 bc ± 11.7	45.1 ab ± 4.5	29.7 cde ± 6.8	20.3 efg ± 15.9	14.4 fgh ± 25.4	14.2 fgh ± 37.7
	2017		44.2 ab ± 5.6	50.8 a ± 5.7	26.0 def ± 6.2	33.8 cd ± 10.1	23.8 efg ± 19.5	19.7 fg ± 9.2
Helminthosporium solani
Disease Severity Index	2015	63.6 a ± 2.9	54.3 b ± 2.8	53.6 b ± 1.1	39.2 ef ± 4.6	50.4 bc ± 2.2	43.2 de ± 2.3	46.0 cd ± 2.9
	2016	52.6 b ± 5.0	28.8 ij ± 3.9	27.2 jk ± 7.5	34.3 gh ± 3.9	45.8 cd ± 2.3	42.0 de ± 2.1	39.0 ef ± 1.7
	2017	46.5 cd ± 3.3	23.6 kl ± 6.1	20.2 l ± 5.9	31.0 hij ± 7.4	33.0 hi ± 4.7	38.6 efg ± 5.9	34.8 fgh ± 4.1
Average degree of infection	2015	3.2 a ± 2.9	2.7 b ± 2.8	2.5 bc ± 11.1	2.0 ef ± 4.6	2.5 bc ± 2.2	2.2 de ± 2.3	2.3 cd ± 2.9
	2016	2.6 b ± 5.0	1.4 ijk ± 3.9	1.4 jk ± 7.5	1.7 fghi ± 3.3	2.3 cd ± 2.3	2.1 de ± 2.1	2.0 ef ± 1.7
	2017	2.3 cd ± 3.1	1.2 kl ± 6.1	1.0 l ± 5.9	1.5 hij ± 7.0	1.7 fghij ± 4.7	1.9 efg ± 5.9	1.7 fgh ± 4.1
Percentage of infested tubers	2015	78 a ± 2.6	70 abcd ± 2.9	66 abcde ± 3.0	56 efg ± 5.1	66 abcde ± 5.2	59 cdefg ± 2.9	62 bcdef ± 3.2
	2016	73 ab ± 7.1	54 efg ± 3.7	55 efg ± 6.0	66 abcde ± 3.0	69 abcd ± 8.6	76 a ± 3.7	70 abcd ± 2.9
	2017	71 abc ± 6.1	50 fg ± 8.9	48 g ± 19.5	55 efg ± 10.8	58 defg ± 7.7	61 bcdef ± 7.1	62 bcdef ± 7.2
Efficiency of protection	2015		14.4 j ± 14.4	15.7 j ± 5.7	38.4 cd ± 7.3	20.8 ghij ± 8.6	32.1 def ± 4.8	27.7 efgh ± 7.5
	2016		45.2 bc ± 4.8	48.3 ab ± 8.0	34.8 de ± 7.2	12.9 j ± 15.3	20.2 hij ± 8.4	25.9 fgh ± 4.9
	2017		49.3 ab ± 6.3	56.6 a ± 4.5	33.4 def ± 14.8	29.1 efg ± 11.5	16.9 ij ± 28.5	25.2 fghi ± 12.4

* homogeneous groups. Tukey’s test HSD, p = 0.05; ** C—Control; BCAPo—Polyversum WP; BCABs—Serenade ASO; PGPEm—Kelpak SL; EM—Em Farma™; UG—UG Max soil conditioner; BR—Biogen Rewital

). These factors also had a significant impact on the effectiveness of the protection of the studied preparations.

Based on the obtained results, it is impossible to establish unambiguously which of the diseases dominates in Poland. The severity of the diseases under consideration (DSI) was variable in individual growing seasons. The overall effect of the tuber surface infection by R. solani, H. solani, and S. scabies was the highest in 2015 (Figure 2). It was a growing season in which the highest level of severity of the infection caused by S. scabies and H. solani was found. Application of biological preparations to soil and soaking of seed potatoes in them improved the phytosanitary status of the surfaces of potato tubers. Regardless of the years, tubers from the treatment where BCABs had been used had the best health. Slightly higher levels of tuber infection, and at the same time at similar levels, were reported in BCAPo and PGPEm treatments. In general, the preparations of the MSADs group, and in particular the UG soil conditioner, protected the potato tubers from pathogens to a lesser extent.

Parameters, such as the average degree of infection determined according to a scale and the percentage of tubers with disease symptoms, are used to determine the severity of an infectious disease (DSI). However, each of them is of great utilitarian importance, particularly with regard to diseases whose symptoms blemish the surface of the tubers and thus determine their commercial value. When analyzing the share in the total yield of tubers with disease symptoms, it can be stated that silver scurf had the most stable occurrence (Figure 3). This is evidenced by the fact that between 2015 and 2017, the number of tubers with spots caused by H. solani remained at a similar level, i.e., between 57.8% and 66.1%. Moreover, the average level of infection in the affected batch of tubers (ADIIT) in 2016 and 2017 was almost the same, namely 2.9 and 2.8, respectively, indicating that 20.6% and 19.6% of the tuber surface was affected by lesions. In the first year of the study, that level was 3.8, and changes on the skin of potato tubers caused by H. solani covered almost 50% of the surface.

With respect to other diseases, their incidence was more strongly determined by the weather conditions prevailing in the growing seasons. In the first two study seasons, the highest (88.2 and 76.7%) frequency of tubers with common scab symptoms was found, covering, on an average, 24–20% of the surface of the tubers (ADIIT 3.4 and 2.9) (Figure 3). In the last growing season, however, the Rhizoctonia solani fungus found the best conditions for growth since its scletoria were observed, on average, on 72.8% of the potato tubers. On average, they covered around 9.1% of their surface; ADIIT was 2.7 and practically remained unchanged in the remaining growing seasons, even though the share of infected tubers decreased significantly. Regardless of the years, biological preparations introduced into the soil and onto the surface of seed potatoes contributed to the reduction of the share of infected tubers in the total yield and decreased the ADIIT. The most spectacular 35% reduction in the share of tubers with black scourt symptoms was noted in the BR treatment.

The tested preparations, compared to the control, decreased the frequency of tubers infected by S. scabies and by H. solani within the range from 4.7 to 27% and from 8.7 to 17.7%, respectively. At the same time, these changes were accompanied by a reduction in the severity of the lesions. In the case of common scab, ADIIT amounted to 2.8–3.0 and indicated 19.9–21.5% of the surface of the tubers with scabs. In the control, on average, 23.6% of the potato tuber skin was covered with common scab (ADIIT = 3.4). The applied biological preparations reduced the development of H. solani to a clearly higher degree. Compared to the control, where, on average, 49% of the surface was covered with silver scurf, in other treatments, the surface was limited within the range from 20.3% to 29.7% (Figure 3).

On the one hand, weather conditions significantly modify the severity of diseases, but they also influence the efficacy of preparations used for protection. The severity of infectious diseases is reflected in the efficacy of the preparations used (Figure 4, Table 5).

The highest efficacy of the preparations was observed against black scurf, which reached significantly the highest value (49.9%) in 2016 compared to dry 2015 and wet 2017. On the other hand, the potato was best protected against silver scurf and common scab in 2017, with an average efficiency of 35.1% and 33.1%, respectively. In each growing season, the least effective protection was achieved against S. scabies, ranging from 19.9% in 2015 (the year of the most severe average intensity of the disease, DSI = 60.7%) up to 33.1% in 2017, ensuring the lowest infection of the potato (DSI = 38.6).

Compared to BCAs and PGPEm, BR and UG preparations (ranked into MSADs) proved to be the least effective form of potato protection against S. scabies. Among these, only EM gave the same protective effect as BCABs and PGPEm. Moreover, MSADs showed the least significant efficacy against H. solani, namely between 20.9 and 26.2%.

Etiological factors of plant diseases in interaction with the environment influence the pathogenesis process. The introduction of any natural substance or biological agent may significantly interfere with the plant–pathogen system. The direction of these changes is, to some extent, reflected by disease symptoms and morphologically altered tissue due to the interference of physiological and chemical processes in the potato tuber skin cells infected by pathogens. The previously generalized results are, therefore, insufficient to clarify pathosystems.

That is why in the following part, each pathogenic organism was considered individually in the context of seasonal hydrothermal changes, with the simultaneous pressure of the applied preparations.

The statistical analysis showed a significant interaction between years and the used preparations in determining the severity of symptoms of individual potato skin diseases, which at the same time indicates the efficacy of the applied preparations (Table 5).

In the first year of the study (2015), the optimal humidity for potato (K = 1.5) was observed only in June, April, and September, classified as dry and warm (K-0.9). In contrast, May was very dry without rain and cool (13.2 °C). On the other hand, a very large water deficit and very high temperatures (20.1–22.8 °C) were recorded in July and August. It is difficult to determine whether such hydrothermal conditions favored the potato tuber being infected by R. solani. The DSI value of 55.4% in the control treatment was considerably higher than in 2016 and lower than in 2017. It was observed that, under drought conditions, PGPEm, based on auxins and cytokines from Ecklonia maxima algae, contributed to significantly the strongest reduction in black scurf intensity. This is indicated by half of the value of all health parameters assessed and 51.5% efficacy protection. It became apparent that EM and BR preparations containing various microorganisms which were applied to soil and seed potatoes had a slightly lower efficacy but were not significantly different from PGPEm. In contrast, the effect of BCAPo protection was assessed only at 14.3% (Table 5). The highest level of protection (69.2% and 60.2%) was reported in 2016 in BCAPo and BCABs treatments. It is worth noting that hydrothermal conditions in the second year of the study satisfied the potato requirements the most. At the same time, the least severity of black scurf symptoms was noted. In addition to BCAs, tubers from the treatment where BR had been used were significantly less affected. It should be noted that in other growing seasons, the efficacy of this preparation was one of the largest (33.1–54.7%). In 2017, the beginning of the growing season was very wet and cool; extremely high precipitation (185.4 mm) and lower temperature (13.0 °C) also occurred in September, which favored the infection of the tubers by the R. solani fungus. It was high in the DSI control treatment, reaching 68.0%; on average, 9.0% of the surface of the tubers was covered with black scletoria of the fungus. Applied to the soil and onto the surface of the seed potatoes, BCAs based on Bacillus subtillis and Pythium oligandrum significantly reduced the growth of R. solani, DSI was 38.8% and 37.8%, respectively, and sclerotia covered about 3.7% of the surface of the tubers. The efficacy of these preparations was 43.0–44.5%, while for EM and PGPEm it was significantly the lowest (only 19.5%); the share of infected tubers in those treatments exceeded, as in the control, 80% (Table 5).

In the growing seasons studied, the preparations used significantly reduced the growth of S. scabies on potato tubers (Table 5). Only in 2015, application to soil and dipping of seed potatoes in BR preparation did not have a significant impact on the assessed common scab intensity parameters on potato tubers. In this case, the efficacy of protection was the lowest, namely 8.0%. Similarly, low efficacy (8.9% and 14.1%) was found in the BCAs variants, but their DSI values were considerably lower than in the control. The driest growing season was the most favorable for the growth of S. scabies. In 2015, PGPEm was the most effective (38.0%) of the preparations used. The second in line was EMs, with 28.0% efficacy.

In 2016 and 2017, the common scab intensity in the control treatment was at an almost identical, statistically undifferentiated level; DSI was 59.0% and 53.8%, respectively. The efficacy of the BCAPo and BCABs potato protection against common scab was the highest and at the same level, which did not differ significantly. In 2016, only the use of PGPEm matched the effect of BCAPo. Under hydrothermal conditions closest to optimal and with higher precipitation levels, the use of BCAs can be three or five times more efficient than in dry years.

Potato silver scurf (H. solani) is another disease whose severity significantly determined the interaction of the preparation used and the growing season (Table 5). The strongest growth of H. solani was recorded in the driest season (2015). In all the years of cultivation, PGPEm showed highly stable efficiency, which was not significantly varied and ranged from 33.4 to 38.4%. In the same way, BR preparation had a permanent effect, with its protective effect estimated at 25.2–27.7%.

In 2015, the highest recorded severity of silver scurf, when DSI the control treatment was as high as 63.6%, with more than 40% of the surface of the tubers covered by spots, PGPEm was found to be highly effective (38.4%). DSI was 39.2%, and lesions on tuber skins were reduced to 11%. In 2016 and 2017, BCAs (both the BCABs variant with the use of B. subtilis bacteria and BCAPo P. oligandrum) for treating seed potatoes and for soil application had the highest efficacy against H. solani. The protective effects ranged from 45.2% to 56.6% and did not vary significantly, nor were there significant differences in the assessed parameters of silver scab severity. In 2016, the EM preparation showed the lowest (12.9%) efficacy in potato protection against H. solani. In 2017, this effect was already at a level of 29.1%. In general, MSADs reduced the DSI and the degree of tuber infection by H. solani to a lesser extent than BCAs and PGPEm.

3.2. Total Potato Yield Depending on Hydrothermal Conditions and the Preparations Used

Hydrothermal conditions prevailing during the growing season are the main determining factor in plant yield, in particular, root vegetables with a long growing season and high water demand. The analysis of variance showed a significant impact of the years, the preparation used, and a significant interaction between them in the shaping of the total yield of potato tubers and the obtained yield increases (

Table 6. ANOVA F-test probabilities for the effects of parameters.

Effects	Parameter
	Crop	Yield Increase
Preparates	***	***
Years	***	**
Preparates × years	***	***

** significant at p < 0.01; *** significant at p < 0.001.

and

Table 7. Effect of biological preparations and years on the total yield of potato tubers (mean ± CV).

Specification	C	BCAPo	BCABs	PGPEm	EM	UG	BR
Crop [t]
2015	36.2 j * ± 1.5	36.7 j ± 1.9	38.0 j ± 5.6	44.2 gh ± 1.7	42.1 hi ± 1.7	38.4 j ± 1.8	41.7 i ± 1.3
2016	44.6 fg ± 1.4	48.4 cd ± 0.8	49.0 bc ± 1.2	51.8 a ± 1.3	51.2 ab ± 0.6	51.2 ab ±1.7	52.0 a ± 0.9
2017	41.3 i ± 1.4	46.5 def ± 1.4	45.9 efg ± 0.7	47.1 cde ± 0.8	48.8 c ±1.3	44.9 efg ±1.1	45.4 efg ± 0.3
Yield increase [%]
2015		1.4 g ± 140.2	4.9 fg ± 97.1	22.1 a ± 13.5	16.3 abc ± 18.9	6.1 efg ± 47.4	15.2 abcd ± 16.9
2016		8.5 defg ± 15.9	10 cdef ± 27.8	16.1 abc ± 18.8	14.8 bcd ± 11.0	14.8 bcd ± 20.9	16.6 abc ± 10.1
2017		12.6 bcde ± 5.9	11.2 bcdef ± 19.0	14.1 bcd ± 5.7	18.2 ab ± 2.0	8.7 def ± 18.1	9.9 cdef ± 14.2

* homogeneous groups Tukey’s test HSD, p = 0.05.

).

The years under study were characterized by variability in the amount and distribution of precipitation and temperature (Figure 1). In 2017, the sum of precipitation (543.6 mm) per the 6-month potato growing season was more than twice as high as the precipitation during the same periods of 2015, which were either dry or very dry. The total yield of potato tubers in 2017 was high (45.7 t·ha⁻¹) but significantly lower than that obtained in 2016 (49.7 t·ha⁻¹) (Figure 5). The growing season of 2016 was warm and provided the potato with an optimum water supply. In contrast, the water deficit that lasted for five months in 2015 resulted in a much lower yield, i.e., 39.6 t·ha⁻¹. For all years, all preparations applied to soil and onto the surface of seed potatoes promoted significantly higher yields. Treatments with PGPEm and EM achieved significantly the highest potato tuber yields. Compared to the control, yield increases represented 17.4% and 16.4%, respectively. BR preparation also had a substantial yield-forming potential, and the obtained yield rise was as high as 13.9%. In general, BCAs had a smaller impact on potato yield, which is indicated by half the lower yield increases (7.5%, 8.7%).

In the assessment of potato tuber yielding, the interaction of years with the preparation used proved highly important (Table 7). In the 2016 season with optimum rainfall, all MSADs and PGPEm precipitations gave the same non-significantly different yield, and average yield increases were between 14.8% and 16.6%. This means that the average yield per hectare in these treatments was higher by 6.6–7.4 t. The highest 22.1% (8.0 t) yield increase was recorded in the first year of the study owing to the PGPEm application onto the soil and seed potatoes.

Analyzing the increases in yield, it was found that, among the MSADs, UG preparation in dry and extremely wet years has a significantly lower yield-forming effect compared to EM PGPEm. On the other hand, the yield-forming activity of the BCAPo and BCABs preparations increased significantly with the increasing amount of precipitation per growing season. In this case, the yield increases of potato tubers of 1.4% and 4.9% in 2015 did not ensure that total yields did not differ significantly from those obtained in the control.

3.3. Assessment of the Effect of Potato Tuber Skin Diseases on Total Yield

The statistical analysis showed that there is a significant negative correlation between the increase in three diseases of the potato tuber skin and the total yield (

Table 8. Correlations between the individual and summary severity of potato peel diseases and the total yield.

Specification	Disease Severity Index—Crop			Summary Disease Severity Index—Crop
	Black Scurf	Common Scab	Silver Scurf
C	−0.18	−0.78 **	−0.68 *	−0.96 ***
BCAPo *	−0.82 ***	−0.92 ***	−0.94 ***	−0.99 ***
BCABs	−0.46	−0.89 ***	−0.86 ***	−0.95 ***
PGPEm	0.03	−0.53	−0.45	−0.35
EM	0.14	−0.53	−0.48	−0.90 ***
UG	−0.18	−0.41	−0.23	−0.83 ***
BR	−0.39	−0.53	−0.47	−0.92 ***

* Significant at p < 0.05; ** significant at p < 0.01; *** significant at p < 0.001.

). This applies to black scurf (R. solani), common scab (S. scabies), and silver scurf (H. solani). The preparations used affected the level of these correlations, as indicated by the values of these factors.

Significant dependencies between DSI for all disease units and yield were observed only in the treatment with BCAPo. In contrast, the BCABs treatment and the control had important dependencies for common scab and silver scab. Insignificant relationships between tuber infection by individual pathogens and tuber yield were found in the treatments where EM, UG, BR, and PGPEm were applied. There was also a negligible relationship between the total DSI and the total yield in the PGPEm treatment. Of the analyzed diseases of potato tubers, black scurf correlates negatively with the yield to a lesser extent than other diseases. The correlation coefficients take very low values (Table 8). Only in combination with BCAPo preparation is this interaction at a very high level, i.e., 0.82.

Apart from one case (PGPEm), the total DSI effect of the infection of the potato tubers by pathogens was significantly correlated with the total yield. The regression analysis carried out allowed determining the effect of the cumulative infection of the tuber surface by pathogens on the total yield of the potato tubers (Figure 6). The total DSI proved to be a very useful predictor in estimating the total yield of potato tubers.

The total DSI correlated the most with yielding in the BCAPo treatment and control, and the determination factors indicated that, respectively, 97.9% and 92.3% of yield variability is explained by the estimated regression equation (Figure 6). In these treatments, an increase of 1% in the total DSI of the potato tuber surface resulted in a decrease in yield by 0.13 and 0.249 t·ha⁻¹. In the BCABs, EM, and BR treatments, regression models explain 89.7–85.0% of the variation of the total yield from the total DSI. Any 1% increase in the total severity of potato skin diseases corresponds with a decrease in yield between 0.14 and 0.56 t·ha⁻¹.

4. Discussion

Plant diseases are an integral part of the cultivation systems and, at the same time, a very important factor in reducing the quality and size of yields [76,77]. The pathogen species, the development phase of the plant in which the first infections appeared, the manner of parasitism, the rate of infesting the plant organs, and the spread under certain environmental conditions determine the level of losses incurred. Knowledge of etiological factors and the functioning of the pathosystem is necessary when deciding on the control method and the preparations. Currently, an extremely important criterion for plant protection and plant production, in general, is to minimize and even withdraw the use of chemicals.

In our study, symptoms of common scab, black scurf, and silver scurf were diagnosed on the surface of the tubers of Typhoon potatoes. The mean severity (DSI) of these disease units in the control treatments was high, i.e., 62.0%, 57.9%, and 54.2%, respectively. The extent of the problem is even better reflected by other morbidity parameters, such as the proportion of tubers infected in the crop and the average degree of tubers infected (ADIIT). Taking these into account, on average, 91.7% of the tubers were infected with symptoms of cork-like scabs induced by S. scabies, taking about 42.9% of the surface (Figure 3). On average, black, contaminating sclerotia of R. solani covered 10% to as much as 81.7% of the surface of potato tubers. In contrast, extensive silver stains caused by H. solani covered as much as 47.68% (ADIIT = 3.74) and were identified on 74% of potato tubers. Since disease resistance is a genetic feature of potato varieties and environmental conditions are a factor determining health, it is difficult to find adequate tests in the literature. Taking these issues into account in the comparative assessment, we conclude that the phytosanitary status of potato tubers in Poland has deteriorated significantly over the last decade. In 2003–2005, the average DSI value for common scab was 8.2%, for black scurf 12.8%, and 20.1% for silver scurf [30]. The study by Kurzawińska and Mazur [78] indicates that between 2005 and 2007, common scab symptoms were shown in about 24.4% of tubers, and in our study, this indicator was as high as 91.7%. The assessment carried out in 1998–2000 shows that DSI for silver scurf was 19.3–37.6% [79]. In our study, the DSI of this disease varied from 30.5% to 50.0%, regardless of the factors studied (Figure 4). From an epidemiological point of view, such intensive infection disqualifies potato tubers as a seeding material [13,36,80]. However, in this study, we highlight the significance of the tuber skin pathogens as an important factor in assessing the quality of potato tubers in terms of their suitability for processing and direct consumption. Black scurf alone, accompanied by deformations and cracks in tubers, can cause a 30% loss in commercial yield [81]. Surface, oblique and deep necroses of tubers caused by S. scabies make them inadequate on the market [82]. These diseases are associated with losses during peeling and overall efficiency losses in the production of French fries and chips. The intensive growth of H. solani causes the thickening of the skin, which is worn down unevenly during the mechanical peeling of French fries and chips [36]. As a result, in the areas of the infected skin, during frying, these products burn and are ugly and untasty.

Potato peel diseases are a problem for potato growers and producers worldwide. This is due to, among other things, the lack of genetic resistance of potato varieties and the low efficiency of chemicals [13,16,82,83]. R. solani shows a unique resistance to synthetic fungicides [84,85].

In our study, an ecological strategy for combating soil pathogens threatening potato tubers was adopted. Soil is the environment on which agricultural production depends, with many biological, chemical, and physical interactions involving microorganisms [86]. In the competition process, the microorganisms contained in the BCAs reduce the phytopathogenic inoculum or inhibit their biological activity [83,87]. Bacillus subtilis bacteria are most commonly used in plant protection because of their high spore-forming capacity and their ability to adapt to different soil conditions [82]. Moreover, these Gram-positive sticks produce numerous compounds and bioactive metabolites (e.g., ribosomal peptides, polypeptides, volatile compounds) with strong antimicrobial properties [88]. The preparation Serenade ASO that was used in our study and based on B. subtillis str. QST 713 (BCABs) ensured the best phytosanitary condition of the potato tuber surfaces, irrespective of the growing season. The effectiveness of BCABs in the diseases analyzed was between 36.7% for S. scabies and 47.7% for R. solani (Figure 4). B. subtilis QST 713 colonizing the roots not only activates the natural defense mechanisms of plants but modulates their growth. The resulting increases in the yield of potato tubers were, on average, 8.7%, corresponding to 3.6 t∙ha⁻¹ (Figure 5). Song et al. [89] state that plants treated with Bacillus subtilis Bv17 strain produce larger yields due to the increase in the weight of the tubers, which results from an increase in dry matter, starch, protein content, and reducing sugars. When introducing a mixture of B. subtilis and Trichoderma harzianum into the soil, Wang et al. [90] obtained 23–32.2% yield increases and a 30.6–46.1% reduction in common scab severity. Lin et al. [91] report that Bacillus amyloliquefaciens species also reduces the common scab severity by 50%. An even stronger (76%) suppression of S. scabies was found under the influence of Bacillus altitudinis [92]. There is little information available in the literature about black scurf and silver scurf suppression by B. subtilis. In German studies, B. subtilis reduced the share of tubers with black scurf symptoms by 50% and common scab by 67% [93]. In contrast, the Tunisian strain B. subtilis V26, under greenhouse conditions, reduced the occurrence of black scurf by 63% [94].

The representative of the kingdom of Chromista (class Oomycetes), Pythium oligandrum, is one of the best-known organisms used in plant protection for the longest [43,44,95,96]. According to Brozowa [97], P. oligandrum contributes to an increase in the soil population of antagonists, which reduces the density of many important pathogens, including R. solani. In addition, it induces resistance to biotic and abiotic stress in plants [98]. In the tests carried out, independent of the growing season, Polyversum WP (BCAPo) preparation, which contains spores of this organism, applied to the soil and onto the surface of seed potatoes showed the greatest efficacy (E = 42.7%) against R. solani, which means that the mean DSI was 24.3% lower than in the control sample. Similar protective effects (E = 45.3%) were obtained after the treatment of potato tubers with this preparation and a single application onto the plants [99]. In light of the study conducted, the efficacy of BCAPo is lower against H. solani (E = 36.3%) and S. cabies (29.7%) (Figure 4). This is confirmed in the study by Kurzawińska and Mazur [100], in which P. oligandrum efficiency against silver scurf was estimated at 6.8–16.8%. Treatment of seed potatoes and the foliar application of P. oligandrum protect the plant also against other important pathogens such as Phytophthora infestans, Alternaria alternata, and A. solani [3].

Organic farming systems use preparations that are currently included in the category of microbial soil additives (MSADs), the first product of which is effective microorganisms (EM). BCAs and MSADs containing populations of useful microorganisms, when introduced into the soil, increase microbial populations, biodiversity, and biological activity, which have a positive effect on plant health and potato yield [56,57,88]. This study has shown that all preparations (EM, UG, BR) classified as MSADs protect potato tubers against silver scurf to a much lesser extent compared to BCAs, and the level of their effectiveness is comparable and non-diversified significantly—between 20.9% and 26.2% (Figure 4). It has been demonstrated that EM, as much as BCAPo, suppresses the development of S. scabies, whereas the efficacy of UG and BR is significantly lower, i.e., 20.0% and 14.0%, respectively. The treatment of seed potatoes and the introduction of BR preparation to the soil reduces black scurf severity by 45.5%, which is the same as BCAs. In contrast, the suppression of R. solani by EM is significantly weaker (E = 38.3%). The mechanism of the impact of microorganisms introduced into the soil and applied onto the surface of seed potatoes is complex and multifaceted. It should be kept in mind that even a slight decrease in soil microbial diversity or changes in soil structure and functions may affect the availability and uptake of nutrients as well as plant health [99,101]. On the one hand, we have their proven protection against pathogens, and on the other hand, their yield-forming effect. This study shows that the diverse consortia of microorganisms included in MSADs cause higher increases in potato yields than the individual species of BCAs microorganisms. EM contributed to increases in the tuber yield by 16.4%, the Biogen Rewital (BR) preparation by 13.0%, and UGmax (UG) soil conditioner by 9.9% (Figure 5). In a multi-year study, Pszczółkowski and Sawicka [102] showed that EM increases the number of and the coefficient of reproduction of seed potatoes. In addition, the authors indicate that EMFarmaTM and Ema5TM preparations increase dry matter and starch content, decrease the content of reducing sugars and sucrose, and also have a positive impact on other qualities impacting the quality of processed potato products [56,57]. The increase in yields, starch, and vitamin C content in the Tajfun cultivar was observed due to the use of the soil conditioner UGmax [103]. This preparation, used at different doses and times, limited the percentage of tubers with common scab symptoms and decreased its intensity [32]. Insufficient scientific reports on the impact of MSADs on potato yields and health show that there is a need to fill this gap in research. In changing economic and climate conditions, when the growing global area of cultivation is suffering from drought, and there is a need to give up or reduce the use of chemicals to a minimum, MSADs preparations alongside BCAs may prove to be a very important part of cultivation technology of potato and other plant species. PGP also plays an important role in modern plant production technologies. The conducted experiment is indisputable proof that the Kelpak SL (PGPEm) biostimulant, containing auxins and cytokinins derived from the Ecklonia maxima algae, is most favorable for potato yields. Yield increases were, on average, 17.4%, which means that on average, a 7.0 t larger yield can be obtained per 1 hectare. According to Mystkowska [104], Kelpak SL was applied onto leaves three times during the growing season, which increased the cultivar Tajfun’s potato yield by 0.9 t∙ha⁻¹. Other studies show that, on average, 21.6% potato tuber yield increases were obtained after two sprayings with this biostimulant [105]. Cytokinins have an impact on the enhancement and growth of the plant root system [106,107]. Therefore, the results obtained are not surprising; they only confirm the yield-enhancing role of PGPEm [108,109,110]. The conducted research study has shown that PGPEm protects potato tubers against silver scurf better than MSADs (Figure 4). However, its effectiveness against R. solani (E = 36.1%) and S. scabies (E = 31.3%) is the same as that of EM. Unfortunately, information on the phytosanitary significance of PGPEm in potato production is nowhere to be found in the literature. In our assessment, the total effect of DSI of potato tuber infection by pathogens in the PGPEm treatment was not significantly correlated with the yield. In other treatments, there was a significant negative correlation between the severity of infection of potato tubers by R. solani, S. scabies, and H. solani and the total yield (the correlation coefficients ranged from R² = 0.695 to R² = 0.979) (Figure 6).

Weather conditions, mainly the amount and distribution of precipitation, determine potato growth, yield, and health. The Results chapter of this paper contains a comprehensive analysis of the effects of these factors on individual disease units, the effectiveness of the preparations used, and yields. To conclude, considering the high scale of the problem of potato tuber skin diseases and in view of the increasing water deficit in the region under consideration, the application of PGPEm and MSADs (EM, UG, BR) to soil and onto the surface of seed potatoes is more recommended than the use of BCAs. BCAPo and BCABs preparations have satisfactory effectiveness in years with an optimal supply of water or precipitation to the potato.

5. Conclusions

It has been shown that tuber skin diseases are a serious problem in the cultivation of the Tajfun cultivar of potato. In the unprotected treatment, the average DSI of common scab is 62.0%, black scurf 57.88%, and silver scurf 54.24%. Their incidence is significantly varied in growing seasons. With a large water deficit, the phytosanitary status of tuber surfaces deteriorates, especially the infection caused by S. scabies, where the DSI reaches as much as 73.2% and, in the case of H. solani, 63.6%. On the other hand, excessive moisture and lower temperatures during the ripening period of potato tubers encourage the development of black scurf (DSI = 68.0%).

Among the diseases analyzed, silver scurf had the most stable occurrence over the years. On average, the tubers infected with H. solani had a share between 57.8% and 66.1%, and the lesions covered 19.9–21.5% of the surface of the tubers.

The application of biological preparations to soil and onto the surface of seed potatoes improves the health of the tubers and increases their total yield. The effectiveness of the reduction of disease symptoms is the result of a significant interaction between the type of preparation used and the hydrothermal conditions prevailing during the growing season. In growing seasons with a large water deficit, the effectiveness of BCAs treatments is significantly reduced. A particularly low (8.9–14.4%) efficacy against all pathogens was observed after the use of BCAPo. It appears that during drought periods, PGPEm and MSADs are much more effective than BCAs in protecting against pathogens that affect the periderm of potato tubers and contribute to yield increases ranging from 6.1% to 22.1%.

A significant negative correlation was found between the cumulative severity of the symptoms of potato skin diseases and the yield after the application of BCAPo, BCABs, EM, UG, and BR.

We are convinced that in sustainable farming management systems, the risk of infectious diseases of potato tubers can be minimized by introducing MSADs, PGPEm, and BCAs into the soil and onto the surface of seed potatoes. We suggest that further research should take into account the use of these biological preparations to protect against pathogens contributing to losses during the storage period.