Biosolarization and Chemical Disinfection as Strategies to Enhance Asparagus Yield and Quality in a Decline-Affected Plantation

Abstract

Asparagus decline syndrome (ADS) is a major challenge affecting asparagus production, leading to reduced yield and spear quality. This study evaluated the effectiveness of different control strategies, including biosolarization with Brassica carinata seed pellets, biosolarization with chicken manure pellets, and chemical disinfection with Dazomet. Field trials were conducted over three consecutive years to assess their impact on commercial yield, spear quality, and plant performance. Biosolarization with B. carinata seed pellets increased commercial yield by 17% and the number of spears per plot by 21%, compared to the control. B. carinata seed pellets and Dazomet improved spear weight by 196% and 170%, respectively, and increased diameter by 115% and 95%, respectively, in 2019. In 2021, chicken manure pellets and Dazomet treatments reduced hardness by 11% and °Brix by 5% and 4%, respectively. These findings suggest that biosolarization could be an effective strategy to mitigate ADS effects and enhance asparagus yield and quality. Furthermore, the results highlight the importance of considering biological control methods to manage ADS while preserving beneficial soil microorganisms. This study provides valuable insights for sustainable asparagus production, emphasizing the role of biosolarization as an alternative to chemical disinfection in ADS-affected fields.

1. Introduction

Asparagus (Asparagus officinalis L.) is a widely cultivated vegetable known for its nutritional benefits, distinctive flavor, and strong consumer demand. Grown in over 60 countries, it plays a crucial role in agricultural economies, generating significant revenue and employment opportunities, particularly in rural areas [1]. However, the long-term viability of asparagus production is increasingly threatened by asparagus decline syndrome (ADS), a condition first identified in Spain in the 1980s [2]. Since then, ADS has emerged as a major concern for asparagus growers worldwide due to its detrimental effects on yield, spear quality, and plant longevity [3,4].

The global significance of asparagus is reflected in its extensive cultivation, with over 1.6 million hectares dedicated to its production and an annual output surpassing 8 million metric tons. Spain ranks among the leading asparagus-producing countries, with Andalucía accounting for the majority of the national production area [5]. Spain has over 14,200 hectares dedicated to asparagus cultivation, with a production of 65,000 tons, making it the fifth largest producer globally and the second largest in Europe after Germany [5]. The value of this crop was approximately EUR 65 million in the Andalusia region alone in 2023 [6]. The economic success of this crop is largely driven by its high market value, particularly for premium-quality spears that meet consumer preferences for size and uniformity [6].

Asparagus cultivation, while agriculturally and economically significant, is severely challenged by ADS, which substantially reduces both yield and spear quality [3]. In affected fields, plants exhibit stunted growth, decreased vigor, and premature death, significantly shortening the productive lifespan of plantations. This often forces farmers to abandon infected plots and seek new cultivation areas, intensifying land-use pressures [7,8]. ADS is considered a complex issue influenced by both biotic and abiotic stressors, with root and crown rot caused by Fusarium spp. identified as a primary contributing factor. Pathogens such as F. solani, F. oxysporum f. sp. asparagi, and F. proliferatum invade the plant’s vascular system, disrupting water and nutrient transport and ultimately leading to characteristic symptoms like reduced spear size, wilting, and plant decline [9,10].

Given the severity of ADS, various strategies have been explored to mitigate its impact. While utilizing resistant cultivars is a potential solution, the considerable genetic and pathogenic diversity of Fusarium spp. complicates efforts to breed for resistance [1]. Some cultivars, such as Dariana, Plasenesp, and Morado de Huétor, have demonstrated lower susceptibility to F. solani in pathogenicity trials, compared to the highly susceptible cultivar Grande, which is widely used. The agronomic performance of Dariana and Plasenesp, along with the organoleptic quality of Morado de Huétor, suggests their potential use in areas previously affected by ADS [2]. However, breeding efforts to develop resistance against F. oxysporum remain a priority [11].

Chemical control strategies are generally ineffective in eradicating Fusarium from infected seeds or crowns, although short-term protection can be achieved through chemical treatments applied to propagation material [7]. Surface sterilization of planting material using commercial bleach solutions has been proposed as a means of reducing pathogen presence before planting. However, current regulatory restrictions on agrochemicals in the European Union limit chemical control options, necessitating the evaluation of alternative soil disinfection methods to reduce Fusarium inoculum levels and enhance productivity.

Organic amendments have been investigated as a means of improving soil conditions and suppressing soilborne pathogens [12]. The application of poultry manure pellets and olive mill compost, in combination with soil solarization, has been tested under controlled conditions. Laboratory results indicate that poultry manure pellets are particularly effective in reducing F. oxysporum and F. solani populations [10], but their efficacy in field conditions remains uncertain. Additionally, biofumigation with Brassicaceae species, which release isothiocyanates with fungicidal properties upon enzymatic hydrolysis, has gained attention as a potential ADS control measure. Brassica, Raphanus, Sinapis, and Eruca species are commonly used, not only for their fungicidal effects but also for their contributions to soil organic matter and structure improvement [13]. Recent studies suggest that the effectiveness of biofumigation depends on factors such as soil composition, environmental conditions, and the specific Fusarium species present, highlighting the need for region-specific validation of this approach [14].

Among the control strategies, two methods have been extensively tested: chemical soil fumigation, which effectively controls pathogens [15] but requires evaluation under local agro-environmental conditions, and the combination of organic amendments with soil solarization (biosolarization), which has demonstrated efficacy under controlled conditions [16]. Further research is necessary to assess these approaches in field conditions and to optimize ADS management strategies for sustainable asparagus production.

In light of the challenges posed by ADS, a study was conducted to evaluate the effects of three treatments—biofumigation with poultry manure pellets, biofumigation with Brassica pellets, and soil disinfection with Dazomet—on asparagus yield and quality over a three-year period in Spain. The hypothesis was that these treatments would differentially influence the incidence of ADS, thereby affecting both the productivity and quality of asparagus spears. The study aimed to provide insights into the effectiveness of these strategies in mitigating ADS and improving asparagus cultivation outcomes.

2. Materials and Methods

2.1. Plant Material and Growth Conditions

The research utilized Asparagus officinalis L., specifically the Grande variety, obtained from INTERSEMILLAS S.A. (Valencia, Spain). Seeds were germinated in forestry trays at a seed nursery starting on 7 March 2018. Seedlings were then moved to trial fields showing decline syndrome on 11 June 2018. The planting frame used was 1.3 m between the rows and 0.33 m between the plants. Local farming practices were followed. Irrigation in the plot was carried out using the traditional furrow flooding system, from the end of the harvest period (May) until just before the crop entered senescence (mid to late August), with an approximate frequency of one irrigation every 20 days, depending on weather conditions and the soil’s water retention capacity. Fertilization was performed using commercial macro- and micronutrient fertilizers, including ammonium sulphate nitrate (FERTIBERIA S.A., Madrid, Spain) and potassium nitrate (Multi-K^®, Haifa Iberia, Madrid, Spain), to meet plant nutrient demands. Multi-K^® was composed of 46% potassium oxide and 13% potassium nitrate. Fertilization was applied in granular form in early February, prior to the sprouting of the first spears, using ammonium sulphate nitrate and potassium nitrate as the main sources of nitrogen, sulfur, and potassium. After fertilizer application, a mechanical cultivator pass was carried out to incorporate the fertilizer into the soil profile. This operation was preferably conducted before a rainfall event to facilitate nutrient incorporation through mechanical means or after rainfall, in which case no cultivation was performed, relying on soil moisture for fertilizer dissolution and movement into the soil.

2.2. Experimental Design, Treatments, and Plant Sampling

The experimental plots had an area of 12 × 4.2 m², and the trial followed a randomized complete block design with four treatments and four replicates per treatment, resulting in a total of 16 plots. A total of 108 plants were transplanted per plot. To prevent cross-contamination between treatments due to irrigation, each plot was isolated by a soil irrigation dike measuring 50 cm in height and width. Additionally, a 2 m separation was maintained between plots. The experiment was conducted in an asparagus-growing area in the province of Granada, specifically in the locality of El Jau (37°11′58.4″ N, 3°44′23.4″ W).

Four treatments were evaluated: (T1) untreated control; (T2) biofumigation with Brassica carinata seed pellets (5000 kg/ha) using Biofence^® (Triumph Italia, Cerealtoscana Group, Livorno, Italy), containing 6% N, 7% P₂O₅, 2.6% K₂O, 4.4% SO₃, 0.9% MgO, and 84.2% organic matter; (T3) biofumigation with chicken manure pellets (5000 kg/ha) using Riger^® (Ferm O Feed, Helmond, The Netherlands), composed of 4% N, 3.4% P₂O₅, 3.2% K₂O, 0.9% MgO, 65% organic matter, and 8% Ca; and (T4) soil disinfection with Dazomet (600 kg/ha) using Basamid^® (Certis Europe, Elche, Spain), containing 98% Dazomet (Figure 1).

Prior to the experiment, all plots displayed intense symptoms of decline syndrome, with a high rate of crop mortality. The plots used for the trial had a history of repeated asparagus cropping cycles and had exhibited severe symptoms of ADS in previous years. On 4 May 2018, Brassica and chicken manure pellets were manually and uniformly spread onto the soil. Dazomet was distributed using a calibrated manual fertilizer spreader. These treatments were applied once as soil disinfectants prior to transplanting the asparagus seedlings. After the applications, all treatments were incorporated into the soil with a rotavator. A light irrigation was then conducted to increase soil moisture, and the plots were subsequently covered with a 120-gauge transparent totally impermeable film (TIF) for 25 days. At the end of this period, the plastic cover was removed, and the asparagus seedlings were transplanted.

2.3. Climatic Conditions

Meteorological data were collected from the closest weather station to the experimental site, located at IFAPA Camino de Purchil in Granada, Spain (Figure 2). The dataset included monthly averages of relative humidity (%), precipitation (mm), and temperature (°C) registered from April 2018 to May 2021. This information was obtained from the publicly available database managed by the Spanish Ministry of Agriculture, Fisheries, and Food (https://servicio.mapa.gob.es/websiar/SeleccionParametrosMap.aspx?dst=1, accessed on 10 March 2025).

2.4. Harvest

In the first year (2019), harvesting took place between February and March to prevent excessive depletion of the young plants. In the following years (2020 and 2021), the harvest period extended from mid-to-late February to early May, with its duration depending on the climatic conditions of each year and unforeseen events, such as the COVID-19 lockdown in 2020. Throughout each harvest season, spears from all plants within each plot were collected three times per week—on Mondays, Wednesdays, and Fridays. Harvested spears were cut to a standard length of 24 cm, corresponding to the commercial size for green asparagus. After harvest, spears were classified into two categories: marketable and non-marketable. Marketable spears met the commercial standards in terms of size, shape, tip closure, and absence of defects. Non-marketable spears, which contributed to the non-commercial yield, included those that exhibited deformities, open or loose tips, insect damage, mechanical injuries, or other imperfections, as well as spears that did not reach the minimum size required for commercialization. The same harvested spears were also used for measuring spear length and diameter and for assessing quality parameters.

2.5. Quality of Spears

To evaluate quality parameters, spears were sampled at three key moments during the harvest season (early, mid, and late). The average of all collected spears at these time points was used for the quality assessment.

Unit Weight: The weight of each harvested spear was individually measured using a Cobos AJ-1200CE Complet balance (Cobos Precisión, S.L., Barcelona, Spain).

Diameter: Spear diameter was recorded at three positions—basal, middle, and apical—using a Mitutoyo digital caliper. The final value was obtained by averaging these measurements.

Hardness: A penetrometer from T.R. Turoni (Forlì, Italy), distributed by Decco Ibérica (100–1000 g) was used to measure hardness at the basal, middle, and apical sections. The average of these values was calculated.

Volume: Spear volume was estimated by modeling it as a truncated cone using the following formula:

$$\mathrm{V}={\displaystyle \frac{1}{3}}·\mathsf{\pi }·\mathrm{h}·({\mathrm{R}}^2+{\mathrm{r}}^2+\mathrm{R}·\mathrm{r})$$

“R” represents the basal radius, “h” the spear length, and “r” the apical radius.

°Brix: The concentration of total soluble sugars (°Brix) within the spear juice was determined using a Hanna Instruments HI 96,801 Digital/Optical Refractometer (Hanna Instruments, Woonsocket, RI, USA). This involved placing a small amount of extracted juice onto the refractometer’s sensor for immediate measurement.

Juiciness: Juiciness was determined as the percentage of juice extracted relative to the total sample mass. To obtain this value, small spear sections from all harvested spears within each plot were homogenized, weighed, and pressed using a Craftsman hydraulic press to extract sap.

2.6. Statistical Analysis

To evaluate the impact of the treatments, a one-way analysis of variance (ANOVA) was performed, setting a 95% confidence interval. Means and standard errors for each treatment were calculated from nine individual measurements per parameter. Differences between means were then determined using Fisher’s least significant difference (LSD_0.05). Furthermore, a two-way ANOVA was utilized to assess the influence of asparagus decline syndrome (ADS) and year on the outcomes. To investigate correlations among the physiological, agronomic, and quality attributes of the asparagus spears, a Spearman’s rank-order correlation analysis was executed using all collected data. In addition, a pathway analysis was performed to explore the potential direct and indirect relationships among variables. Statgraphics Centurion 16.1.03 was the statistical software employed for all analyses.

3. Results

3.1. Yield

In 2019, all treatments exhibited similar commercial yield values and total spear numbers per plot, with no significant differences detected among them. However, in 2020, increases in commercial yield and spear number were observed in T2 and T3, although the differences were not statistically significant (

Table 1. Total yield (kg/ha) for both commercial and non-commercial asparagus spears at the close of the harvest seasons in 2019, 2020, and 2021.

		Commercial Yield (Kg/ha)	Non-Commercial Yield (%)	Total Spear Number Per Plot
2019	T1	164.99	12	50.25
	T2	160.73	14	55.50
	T3	117.43	14	52.50
	T4	145.46	11	46.25
	p-value	NS	NS	NS
2020	T1	872.90	2	303.20
	T2	990.85	1	344.17
	T3	914.06	2	317.50
	T4	866.79	1	301.08
	p-value	NS	NS	NS
2021	T1	2987.80 b	5	1041.05 b
	T2	3609.81 a	5	1257.77 a
	T3	3362.38 ab	5	1171.56 ab
	T4	3097.45 b	5	1079.25 b
	p-value	*	NS	*
	Multivariant analysis
	Treatment (T)	*	NS	*
	Year (Y)	***	***	***
	T × Y	*	NS	*

The differences between means were compared using Fisher’s least significant difference (LSD) test. Significance levels are expressed as * p < 0.05; *** p < 0.001; and NS (not significant), with p > 0.05. Values with different letters indicate significant differences.

). In 2021, significant increases in these parameters were recorded for T2, reaching 3609.81 kg/ha (17% higher than T1) and 1257.77 spears per plot (21% higher than T1). These values were significantly higher than those of the other two treatments, with T4 and T1 yielding only 3097.45 kg/ha and 2987.80 kg/ha, respectively, and presenting a lower number of spears per plot (Table 1).

Regarding non-commercial yield, no significant differences were observed among treatments over the three years (Table 1). In 2019, non-commercial yield accounted for approximately 11.95%, 13.79%, 13.76%, and 10.94% of the total commercial yield for T1, T2, T3, and T4, respectively. In 2020, a substantial decline in the ratio of non-commercial spears was recorded in 2020, with percentages approximating 1.53%, 1.29%, 2.23%, and 1.25% of the total commercial yield for T1, T2, T3, and T4, respectively (Table 1). In 2021, these values increased to 5.12%, 5.62%, 4.67%, and 3.79% (Table 1).

3.2. Quality

In 2019, significant differences were observed in spear weight, intermediate diameter, unit volume, and °Brix values. All three disinfection treatments significantly outperformed the spear weight of the untreated control. Thus, B. carinata pellets increased this parameter by 196%, chicken manure pellets by 130%, and Dazomet by 170%. Significant differences were found in the spear diameter, with biofumigation with B. carinata seed pellets producing the thickest spears (115% increment), followed by chemical disinfection (95% higher) and chicken manure (81% higher) treatments, all of which were significantly larger than the control. In terms of unit volume, Dazomet-treated plants showed the highest values, with an 18% increment with respect to T1, whereas biofumigation and chicken manure treatments performed similarly, all significantly higher than the control (by 14% and 4%, respectively). The highest °Brix values were recorded in the chemically disinfected treatment, whereas biofumigation and chicken manure resulted in significantly lower sugar contents than the control (by 3%) (

Table 2. Spear quality metrics across treatments and years (2019, 2020, and 2021).

		Weight (g)	Intermediate Diameter (mm)	Hardness (g/cm2)	Unit Volume (dm3)	°Brix	Juiciness (%)
2019	T1	8.64 b	5.86 c	-	20.30 c	5.83 b	-
	T2	25.58 a	12.61 a	-	23.09 ab	5.70 c	-
	T3	19.90 a	10.62 b	-	21.07 bc	5.70 c	-
	T4	23.30 a	11.44 ab	-	23.92 a	6.20 a	-
	p-value	***	***	-	*	***	-
2020	T1	19.70	10.48	445.72	66.38	4.69	12.71
	T2	23.25	12.61	422.99	75.31	4.71	13.58
	T3	19.90	10.62	426.49	61.49	4.76	12.31
	T4	23.30	11.44	433.54	70.29	4.85	11.39
	p-value	NS	NS	NS	NS	NS	NS
2021	T1	18.69	9.40	468.31 a	47.97 b	5.42 a	13.14 ab
	T2	19.60	10.58	450.77 a	53.36 ab	5.40 a	12.36 b
	T3	17.79	9.27	418.48 b	46.39 b	5.15 b	14.32 a
	T4	20.53	11.54	418.31 b	58.26 a	5.21 b	13.05 ab
	p-value	NS	NS	**	*	***	*
	Multivariant analysis
	Treatment (T)	***	***	**	**	*	NS
	Year (Y)	NS	*	NS	***	***	NS
	T × Y	***	**	NS	NS	**	*

The differences between means were compared using Fisher’s least significant difference (LSD) test. Significance levels are expressed as * p < 0.05; ** p < 0.01; *** p < 0.001; and NS (not significant), with p > 0.05. Values with different letters indicate significant differences.

).

In 2020, none of the measured parameters exhibited statistically significant differences among treatments. Spear weight, diameter, hardness, unit volume, °Brix, and juiciness remained comparable across treatments (Table 2).

In 2021, spear hardness was significantly higher in the untreated control, compared to the other treatments (Table 2). Spear volume also differed significantly among treatments, with Dazomet-treated plants exhibiting the highest values (21% higher than control). °Brix values were highest in the untreated control and the biofumigation treatment, with significantly lower values in the chicken manure (5% lower) and Dazomet treatments (4% lower). Additionally, spear juiciness was significantly (16%) higher in plants treated with chicken manure, compared to those treated with biofumigation (Table 2).

Multivariate analysis confirmed that treatment effects were highly significant for spear weight, diameter, hardness, volume, and °Brix, whereas no significant effects were found for juiciness. Moreover, the interaction between treatment and year was significant for weight, diameter, °Brix, and juiciness (Table 2).

3.3. Correlation Analysis

The results revealed several statistically significant correlations between the evaluated asparagus spear parameters. Spear weight showed a strong positive correlation with diameter and a moderate correlation with volume. On the other hand, spear volume exhibited a strong negative correlation with soluble solids content, and commercial yield was positively correlated with volume but negatively correlated with soluble solids. In addition, non-commercial yield showed a strong negative correlation with volume and a positive correlation with soluble solids (

Table 3. Linear correlation matrix of the measured parameters.

	Weight	Diameter	Hardness	Volume	°Brix	Juiciness	Commercial Yield	Non-Commercial Yield
Weight	1.00	0.80 ***	−0.10	0.37 *	−0.17	−0.23	−0.16	−0.12
Diameter	0.80 ***	1.00	−0.08	0.41 **	−0.18	−0.41 *	−0.22	−0.14
Hardness	−0.1	−0.07	1.00	−0.06	0.35 *	−0.06	−0.01	0.09
Volume	0.37 *	0.41 **	−0.06	1.00	−0.81 ***	−0.31	0.49 ***	−0.86 ***
°Brix	−0.17	−0.18	0.35 *	−0.81 ***	1.00	−0.03	−0.32 *	0.90 ***
Juiciness	−0.23	−0.41 *	−0.06	−0.31	−0.04	1.00	0.21	0.17
Commercial Yield	−0.16	−0.23	−0.01	0.49 ***	−0.32 *	0.21	1.00	−0.39 **
Non-Commercial Yield	−0.12	−0.14	0.09	−0.86 ***	0.90 ***	0.17	−0.39 **	1.00

Significance levels are expressed as * p < 0.05, ** p < 0.01, and *** p < 0.001. Statistically significant correlations are marked in bold.

).

Pathway analysis revealed contrasting direct and indirect effects of the studied traits on yield. Volume exhibited the strongest positive direct effect (1.14), although this was partly counterbalanced by a substantial negative indirect effect (–0.96), resulting in a moderate positive total effect (0.18). °Brix showed a high positive direct effect (0.91); however, it was almost entirely offset by a strong negative indirect effect (–0.92), leading to a negligible total effect (–0.01). Similarly, juiciness had a positive direct effect (0.40) and a moderate negative indirect effect (–0.29), with a modest positive total contribution (0.11). In contrast, weight and diameter exhibited small negative direct effects (–0.18 and –0.10, respectively) and very limited positive indirect effects, resulting in negative total effects (–0.08 and –0.10, respectively). Hardness showed a small negative direct effect (–0.08) but a relatively large positive indirect effect (0.25), leading to an overall positive influence on yield (0.17) (

Table 4. Direct, indirect, and total effects of asparagus spear traits on commercial yield based on path analysis.

	Direct Effect	Indirect Effect	Total Effect
Weight	−0.18	0.10	−0.08
Diameter	−0.1	0.00	−0.10
Hardness	−0.08	0.25	0.17
Volume	1.14	−0.96	0.18
°Brix	0.91	−0.92	−0.01
Juiciness	0.40	−0.29	0.11

).

4. Discussion

The trend toward higher commercial yields and number of spears per plot observed in the biofumigation treatments with B. carinata pellets and chicken manure suggests a potential beneficial effect of these strategies on asparagus performance. Although statistical significance was not always reached, particularly in the early years, the increase in commercial yield observed by the third year in the B. carinata treatment supports the idea that biofumigation may contribute to mitigating the impact of soilborne stressors. Other studies have also reported positive effects on spear production following the application of organic amendments and biofumigation with Brassicaceae species [17,18]. The presence of bioactive compounds with known biocidal properties in B. carinata [19] could have partially reduced biotic pressure, favoring better crop establishment and productivity. In contrast, chemical disinfection with Dazomet showed lower effectiveness, reinforcing previous concerns about its long-term efficacy in managing complex soilborne syndromes such as ADS [20].

Despite the positive effect of biofumigation, total yields remained notably lower than those reported for healthy asparagus fields in the same region (4000 kg/ha in 2021) [3] and well below the European average (5000 kg/ha in 2022) [21]. This highlights that although biofumigation may alleviate some of the biotic stress, it does not fully restore crop productivity to levels observed in non-declining plantations.

Regarding non-commercial yield, the absence of significant differences between treatments suggests that soil disinfection strategies had limited influence on spear morphology or development defects. Instead, environmental factors and pest pressures appear to have had a greater impact on the production of non-commercial spears, particularly during years with high aphid and cutworm incidences [22].

The overall improvement in commercial yield and reduction in non-commercial spear percentage over the years was consistent with the typical developmental trajectory of perennial crops like asparagus, where initial years focus on root system establishment, followed by progressive increases in yield and quality as the plants reach physiological maturity [3,23]. The lack of treatment effects on non-commercial yield further supports the notion that factors such as crop age, environmental conditions, and agronomic practices [17] are more decisive than soil disinfection treatments in influencing spear quality in declining fields.

In other studies, the application of organic amendments has been shown to influence asparagus spear quality traits, potentially increasing soluble solids content or reducing tissue hardness [24,25]. In the present study, the improvement in spear quality parameters observed following biofumigation with B. carinata seed pellets and chemical disinfection with Dazomet suggests that these soil treatments may effectively mitigate biotic stress, leading to enhancements in asparagus spear morphology. In particular, increases in spear weight, diameter, and unit volume under these treatments point to improved plant vigor and resource allocation, possibly due to reduced pathogen pressure [19]. These results are consistent with previous studies demonstrating that soil disinfection strategies targeting pathogenic organisms can positively influence shoot development and yield traits in perennial crops like asparagus [26,27].

The reduction in spear hardness associated with the biofumigation and chemical treatments supports the idea that decreasing pathogen loads alleviates physiological stress, resulting in less lignified and more tender tissues. This observation aligns with findings by de la Lastra et al. [26], who reported similar effects when implementing soil disinfection treatments. In contrast, untreated controls exhibited greater spear hardness, which could be attributed to enhanced lignification processes triggered by pathogen-related stress [26].

The effects of soil treatments on °Brix values were less consistent, with lower sugar content detected in spears from chicken manure and Dazomet-treated plants, compared to the untreated control. These differences may reflect complex interactions between plant metabolism, stress alleviation, and carbon allocation patterns, as previously proposed in studies evaluating asparagus quality under different agronomic conditions [28,29,30].

Interestingly, spear juiciness was notably higher in plants treated with chicken manure pellets, suggesting a potential benefit of organic amendments on water retention within plant tissues. Organic fertilization has been associated with improved water status and tissue quality in other vegetable crops [19,31], and its positive effect on asparagus spear juiciness observed here warrants further investigation.

Despite the overall positive effects of biofumigation and chemical disinfection, the lack of significant differences among treatments in 2020 highlights the overriding influence of environmental variability. Factors such as seasonal climate fluctuations may mask or modify treatment responses, underscoring the complexity of plant–soil–environment interactions [28,32,33]. Multivariate analysis further confirmed that both treatment and year significantly influenced spear weight, diameter, volume, and °Brix and that treatment effects varied depending on the specific growing season.

Altogether, these findings underline the potential of biofumigation and chemical disinfection to enhance spear quality traits in asparagus affected by ADS. Moreover, these findings complement previous results in which biofumigation with Brassica seed pellets and soil disinfection with Dazomet were shown to enhance the nutritional properties of asparagus spears, including their antioxidant capacity [34]. Nevertheless, the results also emphasize the necessity of integrating long-term studies that include soil health monitoring and microbial community analyses to fully understand the mechanisms driving these improvements and to develop more resilient management strategies for asparagus cultivation under biotic stress conditions.

Several statistically significant correlations were observed among the evaluated asparagus spear parameters. Spear weight displayed a strong positive correlation with spear diameter and a moderate positive correlation with volume, indicating that heavier spears tended to be thicker and more voluminous. This pattern aligns with previous findings in asparagus, where spear diameter was identified as a major contributor to overall spear biomass [27]. Spear diameter also showed a positive correlation with volume but a negative correlation with juiciness, suggesting that thicker spears may retain less water in their tissues, potentially influencing texture. Similar trends have been reported in other vegetable crops, such as celery and leafy vegetables [35,36].

Spear volume was strongly negatively correlated with soluble solids content (°Brix), indicating that larger spears tended to have lower sugar concentrations. This trend could be attributed to a dilution effect of soluble compounds within larger spears, a phenomenon previously reported in studies evaluating asparagus quality [3,37]. Commercial yield was positively correlated with spear volume but negatively correlated with soluble solids content, reinforcing the idea that larger spears are generally favored in the market, although this may come at the expense of certain organoleptic qualities. Non-commercial yield exhibited a strong negative correlation with volume and a positive correlation with soluble solids content, suggesting that discarded spears were generally smaller and contained higher sugar concentrations, likely reflecting suboptimal growth conditions. These findings highlight the importance of agronomic management practices in balancing yield and quality in asparagus production. Strategies such as biofumigation with B. carinata seed pellets and chemical disinfection with Dazomet could contribute to optimizing this balance, improving commercial yields while maintaining desirable sensory attributes [29,37].

Pathway analysis further clarified the relationships between spear traits and yield. Volume exhibited the strongest positive direct effect on commercial yield, despite a substantial negative indirect effect, indicating that larger spears directly enhance yield, even if other associated traits may partially mitigate this contribution. In contrast, °Brix displayed a high positive direct effect that was almost entirely offset by strong negative indirect effects, resulting in a negligible overall impact on yield. This suggests that although higher sugar concentrations may intrinsically favor yield, their association with smaller spear size diminishes their practical contribution to commercial production. Juiciness also exerted a moderate positive total effect, driven by a balance between positive direct and negative indirect influences. Meanwhile, weight, diameter, and hardness showed comparatively minor contributions, with negative or modest positive total effects. Altogether, these results emphasize the central role of spear volume as a primary driver of yield in asparagus while highlighting the complex interplay between physical and quality traits in shaping overall productivity.

5. Conclusions

This study demonstrates that although biofumigation treatments with B. carinata seed pellets and chicken manure pellets mitigate the impact of ADS in spear yield, overall production remained below the expected values for healthy crops, highlighting the persistent challenge posed by ADS. In terms of quality, biofumigation and chemical disinfection positively influenced spear weight, diameter, and volume, with treated plants generally producing thicker and heavier spears compared to the untreated control. However, variations in °Brix values and juiciness across treatments highlight the complex interactions between soil amendments, environmental conditions, and plant physiology. Notably, the untreated control exhibited higher lignification, reinforcing the role of soil-borne pathogens in spear texture deterioration. These findings and the interactions found between the spear quality parameters underscore the importance of sustainable soil management strategies to optimize both yield and quality. Future studies should explore long-term soil health dynamics and microbial community interactions to further refine these agronomic practices.