1. Introduction
Maize (
Zea mays L.) is widely used in agriculture and the food industry all over the world, including Poland. Grain and maize silage are used mainly as feed in livestock production. In 2018, over 1.2 million hectares of maize were cultivated in Poland, of which 645,000 were allocated for grain production and 601,000 for silage [
1]. Corn is also processed into a wide variety of food and industrial products.
The corn market is one of the most dynamically developing agricultural markets. This is due to the versatility of the use of corn. It is a spring crop, very tolerant to precropping, which in Polish conditions is gradually replacing winter crops after frost. In the south, southwest and west of Poland, varieties are grown for grain much later than in other regions of the country. To the contrary, in northern Poland, much earlier varieties with a lower yield potential are grown. When maize is used for silage, slightly later varieties are chosen, which are characterised by higher productivity in both grain yield and green matter per hectare [
2].
Most of the plantations are exposed to a variety of infections, of which fungal ones are very frequent. The most common fungal foliar diseases in maize observed in Poland are: northern corn leaf blight caused by
Exserohilum turcicum (syn.
Helminthosporium turcicum (Pass.) K.J. Leonard & Suggs), eyespot caused by the fungus
Kabatiella zeae Narita & Y. Hirats (syn.
Aureobasidium zeae (Narita & Y. Hirats) Dingley) and common corn rust caused by an infection by
Puccinia sorghi Schwein [
3]. The lesions caused by these fungi may become so numerous that the maize leaves are destroyed, leading to loss of yield due to lack of available carbohydrates in the grains [
3].
Additionally, the occurrence of common smut caused by the fungus
Ustilago maydis (DC.) Corda (syn.
Ustilago zeae (Link) Unger) and head smut caused by
Sphacelotheca reiliana Kühn was observed on maize fields in Poland [
3]. All these infections, however, rarely result in financial loss.
Moreover, there are several fungal pathogens that cause ear and kernel rots in corn that may reduce yield and grain quality. A quite common disease in Poland is red ear rot, caused by
Fusarium graminearum Schwabe. Another disease for cobs is Fusarium ear rot, also known as pink ear rot, caused by several
Fusarium species, the most common of which include
Fusarium verticillioides (Sacc.) Nirenberg (syn.
F. moniliforme Sheldon) and
Fusarium proliferatum (Matsushima) Nirenberg [
4]. Among other fungal diseases for maize, Fusarium ear rot is crucial for the quantity and quality of grain products used in human and animal nutrition [
5,
6]. The
Fusarium species produce metabolites called mycotoxins, which contaminate human food and animal feed as well as cause disorders of the digestive system and kidney and liver functions [
7,
8,
9,
10]. The most common mycotoxins produced by several toxigenic
Fusarium species are deoxynivalenol, trichothecenes, zearalenone, fumonisins and moniliformin [
11,
12].
The severity of fungal infections in maize production is largely dependent on environmental conditions (weather, temperature, wind, rain) as well as the choice of cultural practice (no till/minimum till, corn-on-corn rotation, elimination of weed infestation) and farming system (organic or integrated). The susceptibility of varieties used for breeding also plays a significant role [
3,
12,
13,
14].
Maize can be grown in both integrated and organic systems [
15,
16]. The organic cultivation of plants reduces the costs of cultivation and is becoming increasingly popular among Polish farmers [
17,
18].
Organic farming is a farming system consisting of balanced crop and livestock production within the farm. It relies on the use of correct crop rotation and other natural methods to maintain or enhance biological activity and soil fertility, as well as on the selection of plant species, varieties and animal breeds, taking into account their natural resistance to disease. Organic farming not only plays an important role in producing pesticide-free food, but also enhances and maintains the biodiversity and natural values of agricultural production spaces. It also increases employment in rural areas through the amount of work needed to produce organic products [
19]. In an integrated cultivation system, the use of plant protection products is allowed if the level of presence of the pest exceeds the harmfulness threshold. In contrast, the organic system excludes chemical plant protection products against pests and pathogens, and weeds are controlled mechanically.
Meanwhile, modern hybrid varieties of maize, adapted to more intensive growing conditions, may show less resistance to fungal diseases under less than optimal cultivation conditions. For this reason, it is necessary to assess maize varieties in terms of their suitability for organic cultivation.
The aim of the study was to assess fungal infection in maize grown in organic and integrated farming systems between the years 2017 and 2019. In addition, the species composition of fungi within the genus Fusarium and the incidence of Alternaria alternata (Fr.) Keissler in both systems of cultivation were analysed. Moreover, the study also includes a comparison of three maize varieties as regards their response to fungal pathogens in order to indicate which varieties can be recommended for cultivation in the organic system.
4. Discussion
In the present study, the highest incidence of fungal diseases in maize in both cropping systems was observed in 2018, when heavy rainfall and high relative humidity were recorded, especially in the second ten-day period of July and in August. It is probable that the high air humidity and the difference in the amount of precipitation between July and August had an impact on the frequency and occurrence of fungal diseases in maize plantations. This could explain the presence of
E. turcicum infections in both maize systems in 2018 because it thrives during periods of moderate (temperature between 20 and 28 °C), wet and humid weather [
35]. In 2017, rainfalls were also plentiful but short in duration and limited to the second ten-day period of the month. Meanwhile, in July 2018, rainfalls were more regular, which resulted in high relative humidity persisting for a long time. Such weather conditions favoured the spread of fungal diseases. It is possible that the influence of humidity was so great that it nullified the importance of other factors. Simultaneously, in 2018, a high yield of seeds was recorded by Księżak [
20], which means that fungal infestation did not reduce the yield. In our study, maize grown in 2019 in the organic system had a higher rate of fungal disease infestation. This can be related to the high amount of rainfall in August, when the samples were collected, and is also confirmed by the Księżak study [
20], where it is recorded that the 2019 maize yield was lower in organic farming, perhaps as a result of the fungal infestation. According to Księżak [
20], the maize varieties studied (Ambrosini, Sylvestre, Smolitop and Ricardinio), which were grown in the integrated system in 2017–2019, yielded on average 12.1% more seeds than those grown in the organic system.
The greatest differences in northern corn leaf blight and eyespot infestation emerged in 2019, when the relatively dry July and August weather was not very conducive to the development of fungal infections. In this case, the average degree of plant infection was distinctly higher in the organic cultivation system. The year 2019 was also marked by a clear difference in yields, which were lower in the case of plantations managed according to organic cultivation rules [
20].
In 2017–2019,
F. oxysporum,
F. verticillioides, F. graminearum and
F. poae were the most frequent
Fusarium species isolated from maize cultivated in both organic and integrated systems. There are many reports on climate change determining
Fusarium species population variability and
F. verticillioides distribution in Central European countries [
8,
36,
37]. Numerous studies confirm that moderate temperatures and a high level of moisture increase the infection rates of fungal diseases, especially the
Fusarium species [
8,
14,
38,
39,
40]. Red ear rot is caused by infections with
F. graminearum,
F. culmorum and
F. avenaceum during periods of moderate temperatures and high humidity [
40,
41], while pink ear rot, induced by pathogens such as
F. verticillioides,
F. proliferatum and
F. temperatum, has often been reported in southern regions of Europe where it was dry and warm [
42].
Fungi of the genus
Fusarium do not exist in isolation but compete with other microorganisms for niches in soil, plant debris and host plants. Their interactions and the balance between microbial communities are influenced by the prevailing environmental conditions. Temperature, water availability (a
w), aeration and light are key climatic factors influencing the production of
Fusarium inoculum. Marín et al. [
43,
44] showed that temperature and a
w significantly influence the growth and interaction between
F. moniliforme and
F. proliferatum and among
F. graminearum,
F. subglutinans,
F. proliferatum,
Aspergillus,
Penicillium and
Trichoderma species. According to Fadiji et al. [
45], organic farming has a direct impact on the diversity, functions and abundance of soil microbial communities and, thus, may be associated with improved yield, growth, enhanced plant resistance to abiotic and biotic stresses as well as improved soil health.
The predominant genus identified in organic maize was
Alternaria, but fungi from the
Fusarium genus were most commonly isolated from the integrated system. There are many reports in the literature comparing organic and conventional maize with wheat cultivation systems for the incidence of mycotoxin-producing fungi [
46,
47,
48]. Ariño et al. [
46] produced similar results to ours by obtaining more isolates of
Alternaria fungi in organic cultivation. They reported that organic maize showed a higher total fungal incidence but lower contamination of kernels compared to conventional maize. It is possible that the higher number of
Alternaria isolates in the organic system is related to the higher incidence of weeds, since herbicides as well as fungicides are prohibited in this type of growing. In our study, the sampling date in August coincided with the optimal period for
Alternaria spores, which is July–August. Furthermore,
Alternaria spores are more prevalent in manure-fertilised soils. Similarly, Lazzaro et al. [
48] observed that contamination by
Fusarium spp. was higher in conventional than in organic wheat. Bernhoft et al. [
47] also found a significantly higher incidence of
Fusarium spp. occurring in conventional Norwegian cereals, including wheat, compared to organic crops. The increased frequency of the presence of
Fusarium fungi in conventional cereal crops, including wheat, may be due to the use of ineffective pesticides against
Fusarium and mineral fertilisation. The intensive application of fungicides on cereals has selected for resistance in many cereal pathogens. Talas and McDonald [
49] conducted studies on the fungicide resistance of
F. graminearum isolates in a field population. Additionally, in the study by Yin et al., tebuconazole resistance was found in 1 isolate among 41 Chinese
F. graminearum isolates collected from 14 different locations [
50]. The main factors differentiating between conventional and organic farming are the use of pesticides to protect plants against pathogens and mineral fertilisation. Many studies are, thus, available on this subject [
51,
52,
53]. Research conducted by Henriksen and Elen [
52] found that large amounts of nitrogen fertiliser applied to wheat resulted in increased crop densities and a change in the canopy microclimate, which caused high moisture content and, therefore, more favourable conditions for the emergence of
Fusarium infection. In addition, Szulc et al. [
54] found that the highest infestation of corn plants by
Fusarium fungi occurred when broadcast seeding of NP fertiliser was applied. In contrast, the lowest plant infestation was observed when in-row sowing of NP fertiliser was applied at a depth of 15 cm.
Comparing the results of the above studies, however, it should be stated that, in general, organic maize cultivation is more exposed to attacks from fungal pathogens, primarily because the use of chemical plant protection products, in particular insecticides, is prohibited in this system. Mazzoni et al. [
55] demonstrated the effectiveness of insecticides in reducing the infection of maize cobs by larvae. In addition, according to numerous reports, foraging by the European corn borer (
Ostrinia nubilalis) and western corn borer (
Diabrotica virgifera) in corn-growing areas promotes the spread of fungal diseases and, especially, the development of cob fusariosis [
56,
57,
58,
59]. In Szulc’s study, it was shown that as damage to maize plants due to
O. nubilalis increased, infestation by fungi of the genus
Fusarium spp. also increased [
60]. Mechanical damage caused by insect feeding allows the fungal pathogens to penetrate more easily into plant tissues. In the present study, we also observed a relationship between the occurrence of European corn borer larvae and the greater intensity of cob infestation by fungi of the genus
Fusarium in organic farming. Further analyses, however, were not conducted to confirm this fact.
Out of the three studied varieties, Ambrosini seems to be the best suited to organic cultivation. Only in the case of eyespot was this variety infected to a significantly greater degree under organic conditions than in the integrated system. In terms of the rest of the diseases, Ambrosini performed comparably in both cultivation systems. The Ricardinio variety turned out to be relatively resistant to fusariosis and rust, in contrast to Smolitop. The latter variety was also characterised by low seed yield, as well as seed weight and number of grains per cob [
20]. These differences, however, are rather a result of the genetic diversity and yielding abilities among the varieties. Nevertheless, the low yield of Smolitop may also be the result of its greater susceptibility to fungal diseases.
The use of appropriate agricultural practices in the cultivation of maize in either an organic or integrated manner contributes to the occurrence or reduction of fungal infections. In an organic system, tilling the soil between crops is necessary as a weed control technique because the use of herbicides is prohibited. It is clear that the most important inoculum sources for
Fusarium spp. are plant debris and, especially, maize stalks. This is confirmed by many studies [
38,
61,
62]. Vasileiadis et al. [
62] showed that, after conventional ploughing, the incidence of
F. graminearum,
F. temperatum and
F. culmorum was reduced, while the incidence of
F. verticillioides and
F. equiseti was enhanced by tillage. Steinkellner et al. [
63], however, reported that the composition of
Fusarium species is different depending on the soil layer and survival structures of the species. According to Nyvall and Kommedahl [
64],
F. verticillioides has the ability to survive in maize stalks at a depth of 30 cm due to higher soil moisture and poor decomposition of plant tissue. In our study, the incidence of
F. vertocillioides was noted in 2018 when the abundance of rainfall and air humidity was relatively high. Therefore, conditions favourable for the survival of the pathogen in harvest residues were present.
Conversely, there are relatively few studies on the occurrence of fungal diseases in maize in both organic and integrated systems. In integrated pest management, mineral fertilisation and chemical plant protection agents are used if the threshold of financial losses by pathogens or pests is exceeded. Such a practice limits the use of chemical plant protection products to the necessary minimum, thus reducing pressure on the natural environment and protecting the biodiversity of the agricultural environment. In our study on integrated maize cultivation, fungicides and insecticides were not used because the incidence of pathogenic fungi and pests did not exceed the thresholds of harmfulness. Therefore, our study did not include one of the factors that differentiate organic and integrated systems.
Taking the above into account, it is difficult to relate the results of our research to the existing literature comparing organic systems of cultivation with conventional ones. Therefore, there is a need to continue research on the incidence of fungal diseases on maize plants grown under organic and integrated systems in conditions prevalent in Poland. Moreover, the study should be extended to include other observations, for example the presence of mycotoxins in maize seed from plantations managed according to both farming methods.