Control of Apple Scab in Commercial Orchards Through Primary Inoculum Management

Abstract

Apple scab, caused by Venturia inaequalis, is one of the most important diseases in apples in all production regions and its sustainable control is still a challenge. The aim of this work was to optimize the control of apple scab through different environmentally friendly inoculum management strategies, specifically the removal of fallen leaves in winter and the treatment of ground leaves with the biological agent Trichoderma asperellum (T34 BIOCONTROL^®) to inhibit or prevent inoculum development in commercial orchards. The results obtained from 4 years of trials in commercial orchards demonstrated that the combination of fungicide treatments and leaf litter management, particularly through aspiration, significantly reduced the development of apple scab in comparison with strategies commonly used by growers that are based solely on fungicide application. Both the incidence and severity of the disease in leaves and fruit decreased by over 90% when inoculum management and fungicide treatments were combined. These results highlight that reducing the source of inoculum by removing fallen leaves is an effective strategy that complements fungicide or biological control agent applications. In conclusion, combining eco-friendly strategies with standard fungicides and monitoring environmental conditions can help to reduce the frequency of phytosanitary applications, ultimately contributing to the goal of minimizing their use in the control of apple scab.

1. Introduction

Apple scab, caused by the ascomycete fungus Venturia inaequalis (Cke.) Wint., is one of the most important apple (Malus domestica) diseases in all production regions, being more severe in regions such as the Mediterranean, where frequent spring rainfall triggers ascospore release and infection [1]. This disease has a significant economic impact and is a critical factor affecting the viability of apple production. Disease management is based on fungicides, requiring 10 to 20 applications per season on susceptible varieties and accounting for over 70% of pesticide treatments in conventional orchard farming [2]. Most current commercial varieties are highly susceptible, with direct losses potentially reaching 100% of the harvest if no protective measures are taken [3].

The disease is polycyclic, beginning in early spring with the release of ascospores from pseudothecia contained in the ground litter (source of primary inoculum). These ascospores are ejected during rainfall and carried by the wind, causing primary infections in the green parts of apple trees, including leaves, buds, flowers, and fruits. After some time, lesions appear as necrotic spots, where conidia are produced, leading to secondary infections throughout the apple tree’s annual cycle. Rain plays a key role in triggering the release of the primary inoculum and creating favorable conditions for the spores to infect green tissues [3]. Primary infections are therefore of high importance and must be effectively suppressed to avoid additional costs due to fungicide applications during the summer and even in subsequent years, as well as to reduce the risks of infection on the fruits, which are highly vulnerable. Many cycles of secondary infections may then occur during summer depending on the frequency of precipitation and the presence of weather conditions conducive to infections if successful primary infections developed during spring.

The models identify weather factors that favor infection and thus identify when a fungicide is necessary. The most well-known model is the Mills model, developed in the mid-20th century in the USA [4]. This model is currently integrated into the warning systems of the Plant Health Service of the Generalitat de Catalunya. In recent years, modifications and refinements have been introduced to the Mills model in countries where the disease is prevalent [5,6]. However, in the Girona region, the RIMpro model is more commonly used. Its main advantage over the Mills model is its ability to predict the development of the primary inoculum, allowing for the more accurate timing of spore release and emission periods. Despite this, depending on the year, 10 to 15 fungicide applications are still required annually to control the disease.

In the current context, the reduction of plant protection products is becoming an imminent requirement, as outlined in the European Commission’s ‘Green Deal’ and the ‘Farm to Fork’ strategy [7]. This strategy calls for a substantial reduction in the use of chemical pesticides, fertilizers, and antibiotics. Specifically, it aims for a 50% reduction in the use and associated risks of chemical pesticides by 2030. To meet this goal, the European Commission mandates that the strategic plans developed by Member States for implementing the future Common Agricultural Policy (CAP) must be more ambitious, focusing on significantly reducing the use of chemical pesticides and the risks they pose. Consequently, it is crucial to develop and implement new sustainable control strategies that may effectively reduce the need for phytosanitary treatments.

For this reason, an environmentally friendly strategy should focus on reducing primary inoculum production to increase control efficiency and to replace synthetic pesticides, at least in part. In this way, sanitation efforts aimed at removing the overwintering inoculum of Venturia inaequalis have been proven to reduce both initial disease pressure and the amount of chemicals required for control during the growing season [8,9]. A reduction in the spring inoculum can significantly reduce the incidence of apple scab in orchards throughout the growing season [10]. This reduction could delay the onset of an apple scab epidemic, potentially resulting in fewer pesticide applications for effective disease management [11]. In this aspect, since the pathogen overwinters in leaves on the orchard floor, effective sanitation practices such as leaf shredding, chemical treatments, or urea applications are essential [8,12]. Shredding leaf litter in controlled environments has shown to reduce scab incidence by as much as 90% [8]. However, in practice, inoculum reduction through shredding or leaf litter removal can range from 50–65% due to several factors [12]. In Belgium, the use of a prototype leaf-shredding machine reduced scab incidence on shoot leaves by 75% [13].

Alternatively, spray applications to reduce leaf litter can serve as an effective substitute for shredding and removal [12]. The use of urea alone or combined with adjuvants has been shown to significantly enhance leaf litter degradation, while also reducing both the number and fertility of pseudothecia [2]. Biological control, particularly using fungi antagonists such as Trichoderma to reduce ascospore production, has shown potential [14].

Thus, strategies that reduce the primary inoculum may be an option for effectively managing apple scab in an environmentally friendly manner. This involves minimizing pathogen availability, thereby enhancing control measures, and potentially reducing reliance on synthetic fungicides and pesticides. The objective of our study was to optimize apple scab control through inoculum management strategies, including the removal of fallen leaves in winter to decrease inoculum availability and the treatment of leaves with the biological agent Trichoderma spp.

Over four consecutive campaigns from 2020–2023, trials were conducted in commercial orchards to evaluate whether the integration of these sanitation measures could increase the efficacy of standard fungicide sprays in controlling apple scab. To improve overall quality while minimizing the environmental impact, this approach represents a significant advancement towards more sustainable apple production methods. By focusing on inoculum management and exploring biological control options, our research contributes to the promotion of environmentally friendly apple growing practices.

2. Materials and Methods

2.1. Locations and Conditions of the Assays

The trials were conducted during four consecutive campaigns in different commercial apple orchards, all located in Baix Empordà of Girona province (Spain). During 2020 and 2021, medium-sized plot assays were carried out in two apple orchards (Figure 1): one in Ullà with trees of the Royal Gala variety grafted onto an M9 and planted at a spacing of 3.75 m × 1.2 m, a central axis training system, and under integrated production; and the other located in Fonolleres with trees of the Golden variety grafted onto a G11 and spacing of 3.5 m × 1.0 m, a wall training system, and under organic production. During 2022 and 2023, large-sized plot assays were carried out in six orchards, all of the Royal Gala variety and under integrated production (Figure 2): two were located in La Tallada d’Empordà and four orchards were located in Ullà. All the orchards had trees grafted onto M9 and planted at a spacing of 3.75 m × 1.2 m and a central axis training system. During the assays, fertilization, pruning, and herbicide treatments were conducted following standards normally used in commercial apple orchards in the region. All orchards had irrigation through a drip system and had an anti-hail net.

2.2. Experimental Design and Treatments

In assays performed in 2020 and 2021, two management inoculum strategies were tested. Collecting fallen leaves by aspiration (ASP; leaf vacuuming during winter) and Trichoderma treatment (TRI; application of Trichoderma on ground leaves) were compared with a reference treatment (REF; standard application of common phytosanitary products in the area) and a non-treated control (NTC). Four plots of at least 1000 m² and with a minimum of 4 rows were defined in each orchard. The characteristics of the trials and the areas covered by each strategy and the control are summarized in

Table 1. Specifications of the trials carried out during 2020 and 2021 for controlling the primary inoculum of apple scab. In all treatments, fungicides were applied for Apple scab control except in the NTC.

Orchard	Years	Location	Production	Treatments 1	Action Date	Variety	Plot Area
1	2020 2021	Ullà, Girona, SPAIN	Integrated	ASP	17 January 2020–26 January 2021	Golden	0.13 ha 0.13 ha
				TRI	6 March 2020–11 March 2021
				REF	-		1.54 ha 0.80 ha
				NTC	-
2	2020 2021	Fonolleres, Girona, SPAIN	Organic	ASP	17 January 2020–26 January 2021	Gala	0.07 ha 0.08 ha
				TRI	6 March 2020–11 March 2021
				REF	-		0.09 ha 0.04 ha
				NTC	-

¹ ASP: leaf aspiration; TRI: Trichoderma application; REF: without leaf litter treatment; NTC: non-treated control.

and Figure 1. The external rows of each block were considered buffer zones while the central rows were the evaluation zone. Four replicates of ten trees were randomly selected and labeled in the central rows for disease assessment.

In assays performed in 2022 and 2023 in each orchard, the aspiration strategy (ASP) was compared with a reference treatment (REF), in which the same standard fungicides were applied in all orchards. The characteristics of the trials and the areas covered by each treatment are summarized in

Table 2. Specifications of the trials carried out during 2022 and 2023 for the control of the primary inoculum of apple scab. In both treatments, fungicides were applied for Apple scab control.

Orchard	Years	Location	Treatments 1	Action Date	Variety	Plot Area
1	2022 2023	La Tallada d’Empordà, Girona, SPAIN	ASP	6 February 2022–22 February 2023	Gala	0.27 ha
			REF	-		0.24 ha
2	2022 2023	La Tallada d’Empordà, Girona, SPAIN	ASP	6 February 2022–22 February 2023	Gala	1.21 ha
			REF	-		0.65 ha
3	2022 2023	Ullà, Girona, SPAIN	ASP	6 February 2022–22 February 2023	Gala	0.52 ha
			REF	-		0.23 ha
4	2022 2023	Ullà, Girona, SPAIN	ASP	7 February 2022–23 February 2023	Gala	0.79 ha
			REF	-		0.20 ha
5	2022 2023	Ullà, Girona, SPAIN	ASP	7 February 2022–23 February 2023	Gala	0.63 ha
			REF	-		0.34 ha
6	2022 2023	Ullà, Girona, SPAIN	ASP	7 February 2022–23 February 2023	Gala	0.65 ha
			REF	-		0.24 ha

¹ ASP: leaf aspiration; REF: without leaf litter treatment.

and Figure 2. In both plots of each orchard, the external rows of each block were considered buffer zones while the central rows were the evaluation zone. Four replicates of ten trees were randomly selected and labeled in the central rows for disease assessment.

2.3. Treatment Performance

For aspiration treatment, the overwinter inoculum was removed in January or February depending on the year. The inoculum was removed by raking and vacuuming the fallen leaves with a tractor-driven vacuum machine (TC125 Turf Collection System, John Deere Ibérica, S. A., Parla, SPAIN). The collected leaves were then burned to deactivate the inoculum.

For Trichoderma treatment, a single application was applied onto the leaves on the ground with the T34 strain of T. asperellum provided by Dr. María Isabel Trillas Gay from the Autonomous University of Barcelona and commercialized as T34 BIOCONTROL^® (IQV Agro España, Mollet del Vallés, SPAIN). The application was performed in March at a dose of 0.5 g/L (concentration 10¹² ufc/kg) and an application volume of 600 L/ha, when the median temperature was above 10 °C and after a rainy period. The application was performed with a Stihl SR 420 (Stihl Inc., Virginia Beach, VA, USA) backpack sprayer.

Disease management throughout the growing season relied on fungicide applications, which were applied across all plots regardless of the inoculum management strategy. Control of infections was carried out with the conventional strategy of the region, preventively covering the infections indicated by the RIMpro model. In the case of integrated production orchards, this was achieved preferentially with contact products (Captan and Dithianone), with the possibility of incorporating curative treatments if conditions require it, preferably dodine, cyprodinil, or pyrimethanil from the green tip to F2, and IBEs, kresoxim-methyl, and fluxopyroxad from F2 to BBCH 69. For organic orchards, lime sulfur was applied as a preventive strategy. The application rate was 1000 L/ha using a commercial 3000-liter multi-fan sprayer (AMP Sprayers, Vilobí, Spain). Depending on the amount of rainfall during the risk period (March to May), application frequency varied between orchards and years.

2.4. Disease Assessment

The evaluations were performed according to the presence and evolution of symptoms and were performed on the central rows of each plot. For this, 4 randomized blocks (repetitions) consisting of 10 trees were established. The presence of lesions was evaluated in June around BBCH 74. In each replicate, 10 leaves of 20 shoots and 100 fruits were chosen at random and distributed on both sides of the trees. The incidence was assessed as the number of infected leaves or fruits over the total. To evaluate the severity, each leaf or fruit was assessed according to the following semi-quantitative categorical severity index (SI): 0, no symptoms observed; 1, up to 10% of the affected leaf surface or 1 lesion per fruit; 2, 10–50% of the affected leaf surface or 2–3 lesions per fruit; 3, more than 50% of the affected leaf surface or more than 3 lesions per fruit. Then, the following formula was used:

S = ((SI₁ + SI₂ + …SI_i)/n × 3) × 100

(1)

where S is the severity (0–100); SI is the disease severity index for each leaf or fruit; i is the number of fruits or leaves, n is the number of leaves or fruits; and 3 is the maximum level of severity.

2.5. Weather Conditions

Weather data, including daily minimum and maximum temperatures, relative humidity, wetness, and rainfall, were obtained from an automatic weather station placed in the experimental orchards. Temperature and relative humidity were measured every 10 min and wetness and rainfall every 20 s. Mean temperature, relative humidity, duration of wetness, and total rainfall were recorded every hour.

2.6. Data Analysis

All data are presented as mean ± standard deviation (SD). To statistically analyze the results, student’s t-tests and one-way analysis of variance (ANOVA) were applied and significant differences (p < 0.05) among the treatments were determined using a post hoc Tukey HSD test when necessary. Analyses were performed using the statistical package JMP (v16, SAS Institute Inc., Cary, NC, USA).

3. Results

The effect of different inoculum management strategies on the development of apple scab was tested during different years in different plot size assays. The level of natural infections in the trials was different depending on the year. In 2020 and 2021, the incidence of apple scab in the non-treated control was too high, with values of around 100% and 75% in leaves and 75% and 60% in fruits, respectively. In the other years, i.e., 2022 and 2023, the incidence in the non-treated control was low, with values of around 3% and 1.5% in leaves, respectively. The incidence in fruits in the non-treated control in these years was also very low, with an incidence of around 1.5% in 2022 and with scarce presence in 2023. This can be explained by the climatic conditions, particularly cumulative rainfall, which is known to play a significant role in the development of apple scab. Thus, the year 2020 showed the highest spring rainfall, with 140.2 mm recorded in Ullà and 200.3 mm in La Tallada d’Empordà. Similarly, 2021 was a relatively rainy year, with a total rainfall during spring of 91.4 mm in Ullà and 102.1 mm in La Tallada d’Empordà. In contrast, the springs of 2022 and 2023 experienced much lower rainfall, with values below 70 mm in both Ullà and La Tallada d’Empordà in both years.

In trials performed in 2020 and 2021, the assays were designed in medium-sized plots of around 1000 m² each. In the UTC plots, the incidence of scab on leaves was >99% in 2020 and about 75% in 2021. In the same plots, the severity of disease on the leaves was >90% in 2020 and about 60% in 2021. In these assays, aspiration of fallen leaves (ASP) and the application of T. asperellum (TRI) were tested compared to a reference treatment (REF) and a non-treated control (Figure 3 and Figure 4). All treatments reduced disease compared to the non-treated control (Figure 3); the ASP strategy was most effective, achieving an efficacy of more than 95% in reducing apple scab in both fruits and leaves.

In orchard 1, during 2020, the ASP treatment significantly reduced the incidence and severity to 1.13% and 0.38% in leaves and 1.0% and 0.42% in fruits, respectively, compared to the REF, where the incidence and severity percentages reached 4.38 and 1.54% in leaves and 6.5% and 3.17% in fruits, respectively. On the other hand, the TRI treatment resulted in leaf incidence and severity values of 5.88% and 2.08%, respectively, and 6.22% and 2.58% in fruits, which were not significantly different from the values observed in the REF treatment. During 2021, the incidence and severity in leaves were significantly reduced from 76.13% and 61.63% in the NTC to 6.88% and 2.46% in ASP treatment, as well as from 79.22% and 63.4% in control fruits to 0.75% and 0.33%, respectively. Although these percentages were lower than those seen for the REF treatment, significant differences were only found in leaves but not in fruits. Again, values observed in the TRI treatment, including incidence and severity of 4% and 1.35% in leaves and 1.22% and 0.58% in fruits, were not significantly different with respect to the REF treatment, except in the case of severity in leaves. It is important to note that the ASP treatment reduced symptoms to a greater extent compared to the REF treatment in most cases, though not all differences were statistically significant.

Similar results were found in orchard 2, where the incidence and severity registered during 2020 for the ASP treatment were significantly decreased from 54.25% and 27.08% in the REF treatment to 28.75% and 11.83% in leaves and from 5.25% and 2.08% to 0.75% and 0.33% in fruits (Figure 4). In contrast, for TRI treatments, the values of incidence and severity were not different from those in the REF treatments. The efficacy of the treatments was lower in orchard 2 because it was managed under organic production. During 2021, the incidence and severity in leaves were significantly reduced from 43% and 25.21% in the REF treatment to 23.38% and 9.83% in the ASP treatment, as well as from 20.72% and 9.75% in control fruits to 8.75% and 3.92%, respectively. The TRI treatment slightly reduced the incidence and severity in leaves compared to the REF, though no differences were reported in fruits. The results suggest that the inoculum management strategy based on aspiration improves control efficacy in comparison with the reference treatment, which only applied phytosanitary products on the trees, while the strategy based on the application of Trichoderma did not improve the reference strategy and was discarded in the following assays performed in large-sized plots.

In the large-scale trials conducted in 2022 and 2023 across six commercial orchards, only two strategies were compared, the REF, based on fungicide treatments, and the ASP, based on the basal fungicide treatments plus primary inoculum management through vacuum aspiration. As has been mentioned above, an important reduction in disease incidence and severity rates was observed due to unfavorable meteorological conditions, especially in 2023, where no fruit incidence was detected in any plot. In both years, the ASP treatment significantly reduced the incidence and severity of apple scab compared to the REF strategy in most of the plots (

Table 3. Incidence and severity of apple scab registered in leaves and fruit during 2022 in 6 commercial orchards, where fallen leaves were collected by aspiration (ASP) and compared with a reference treatment (REF). Data were obtained in mid-June 2022. Data are means ± SD of n = 4. Different letters (lowercase letters for leaves and capital letters for fruits) show significant differences between treatments according to a Student’s t-test (p < 0.05).

Treatment		Incidence (%)		Severity (%)
		Leaves	Fruits	Leaves	Fruits
Orchard 1	REF	2.88 ± 0.63 a	3.75 ± 1.26 A	0.96 ± 0.21 a	1.58 ± 0.63 A
	ASP	0.75 ± 0.65 b	0.00 ± 0.00 B	0.25 ± 0.22 b	0.00 ± 0.00 B
Orchard 2	REF	2.88 ± 0.48 a	3.05 ± 0.84 A	1.04 ± 0.32 a	0.92 ± 0.32 A
	ASP	1.50 ± 0.71 b	1.25 ± 0.50 B	0.50 ± 0.24 b	0.42 ± 0.17 B
Orchard 3	REF	0.38 ± 0.25	0.00 ± 0.00	0.13 ± 0.08	0.00 ± 0.00
	ASP	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00
Orchard 4	REF	3.00 ± 0.91 a	2.25 ± 0.50 A	1.00 ± 0.30 a	1.00 ± 0.27 A
	ASP	1.25 ± 0.50 b	0.50 ± 0.58 B	0.42 ± 0.17 b	0.25 ± 0.32 B
Orchard 5	REF	1.25 ± 0.65 a	1.25 ± 0.50 A	0.46 ± 0.28 a	0.42 ± 0.17 A
	ASP	0.13 ± 0.25 b	0.25 ± 0.50 B	0.04 ± 0.08 b	0.08 ± 0.17 B
Orchard 6	REF	3.25 ± 0.65 a	3.00 ± 0.82 A	1.08 ± 0.22 a	1.25 ± 0.50 A
	ASP	0.88 ± 0.63 b	1.50 ± 0.58 B	0.29 ± 0.21 b	0.50 ± 0.19 B

and

Table 4. Incidence and severity of apple scab registered in leaves and fruit during 2023 in 6 commercial orchards, where fallen leaves were collected by aspiration (ASP) and compared with a reference treatment (REF). Data were obtained in mid-June 2023. Data are means ± SD of n = 4. Different letters (lowercase letters for leaves and capital letters for fruits) show significant differences between treatments according to a Student’s t-test (p < 0.05).

Treatment		Incidence (%)		Severity (%)
		Leaves	Fruits	Leaves	Fruits
Orchard 1	REF	1.00 ± 0.41 a	0.00 ± 0.00	0.38 ± 0.21 a	0.00 ± 0.00
	ASP	0.00 ± 0.00 b	0.00 ± 0.00	0.00 ± 0.00 b	0.00 ± 0.00
Orchard 2	REF	0.13 ± 0.25	0.00 ± 0.00	0.04 ± 0.08	0.00 ± 0.00
	ASP	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00
Orchard 3	REF	0.50 ± 0.41 a	0.00 ± 0.00	0.17 ± 0.14 a	0.00 ± 0.00
	ASP	0.00 ± 0.00 b	0.00 ± 0.00	0.00 ± 0.00 b	0.00 ± 0.00
Orchard 4	REF	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00
	ASP	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00	0.00 ± 0.00
Orchard 5	REF	1.13 ± 0.48 a	0.00 ± 0.00	0.38 ± 0.16 a	0.00 ± 0.00
	ASP	0.13 ± 0.25 b	0.00 ± 0.00	0.04 ± 0.08 b	0.00 ± 0.00
Orchard 6	REF	0.38 ± 0.25 a	0.00 ± 0.00	0.13 ± 0.08 a	0.00 ± 0.00
	ASP	0.00 ± 0.00 b	0.00 ± 0.00	0.00 ± 0.00 b	0.00 ± 0.00

).

In 2022, ASP significantly reduced incidence and severity in leaves, except in orchard 3 where disease was scarce. The results obtained in Orchard 1 for the ASP treatment are highlighted, where the incidence and severity of fruit were reduced from 2.88% and 0.96% to 0.75% and 0.25%, respectively, when compared to the REF treatment. The level of incidence in leaves was close to 3% in the REF treatment in most orchards, while in ASP, it significantly decreased to values between 1.5% to around 0%. In fruits, the disease was totally reduced in orchard 1, while in orchards 4 and 5, the incidence was reduced from 2.25% and 1.25% in the REF to 0.5% and 0.25% in ASP. No disease was found on fruit in orchard 3 (Table 3). In 2023, incidence and severity on leaves were totally reduced in orchards 1, 2, 3, and 6 by ASP treatment, while in orchard 5, disease incidence decreased from 1.13% in the REF to 0.13% ASP. On the other hand, in 2023, no symptoms of apple scab were found in fruits due to a lack of appropriate climatic conditions (Table 4).

4. Discussion

Effective management of apple scab is essential to prevent significant losses while minimizing both its impact and the use of phytosanitary products, making it a critical area of study. It is important to highlight that in organic production, where fewer phytosanitary products are available for disease management, the need for effective control strategies becomes even more critical. This is especially relevant in the current context of the European Green Deal [15]. This situation represents a significant challenge to our goal of increasing sustainability in production while ensuring the fruits remain suitable for commercial purposes. In this context, new control strategies to reduce the primary inoculum can be a complement to fungicide strategies. In addition, intensive fungicide spray programs can lead to the selection of V. inaequalis strains that are less susceptible to these treatments. Implementing sanitation practices can help to mitigate this risk and support resistance management [16].

The results obtained from 4 years of trials in commercial orchards demonstrated that leaf litter management, particularly through aspiration, significantly reduced the development of apple scab. Both the incidence and severity of the disease in leaves and fruit decreased by over 90% when combined with fungicide treatments. These results highlight that reducing the source of inoculum by removing fallen leaves is an effective strategy that complements fungicide or biological control agent applications as described by other authors [8]. Similar results have been reported in studies performed with apple scab and other fungal diseases. Leaf shredding and removal significantly reduced the incidence and severity of apple scab in a French orchard by 50–80%, and by about 90% in North Carolina [8,17]. In addition, another study performed in France showed that leaf litter removal is more effective than shredding for controlling apple scab inoculum in organic orchards, reducing disease development and fruit damage [18]. In agreement with our results, other studies based on inoculum management, for example, leaf shredding [19] or collection of fallen leaves alone or combined with straw mulch in tree rows [20], have demonstrated their efficacy in managing apple scab, reinforcing the idea that these strategies reduce disease levels and can serve as an effective complement to enhance the efficacy of chemical fungicides, while also being economically efficient options for integrated and organic orchards compared to non-sanitized systems. Although leaf litter removal has been demonstrated to possess very good efficacy in several studies, the lack of specific machinery has limited its implementation at a commercial level.

The application of Trichoderma has shown a low consistency and the efficacy in most of the cases did not improve disease control in comparison with a reference strategy based on fungicide treatments. This is in agreement with other authors, who have described great variability among biocontrol agents, mainly depending on their capacity to grow in field conditions and to inhibit V. inaequalis development [21]. In contrast, other studies reported that the use of T. viride completely stopped the scab disease in seedling leaves in North Kyrgyzstan [22].

According to our results, while Trichoderma species are widely used in agriculture as biocontrol agents, and concretely against apple scab, their potential for controlling the V. inaequalis overwintering inoculum in apples remains underexplored and requires further research in order to define the most effective strains, as well as to define the best time for application to obtain consistent results in disease control.

5. Conclusions

Our results demonstrate that leaf removal during winter consistently reduces the pressure of apple scab, suggesting that incorporating this practice in the integrated control of apple scab can contribute to orchard sanitation, reducing dependence on fungicides and consequently reducing their use. In addition, the consistent use of this strategy of inoculum management can be a good way to clean problematic orchards, gradually reducing the inoculum pressure. This can be an important aspect to consider in organic orchards, where the limited number of fungicides makes the efficient control of apple scab difficult. For full implementation at a commercial level, the development of specific machinery is mandatory. In the case of Trichoderma usage to complement the fungicide strategy, additional studies are still needed to adjust the application strategy and improve control consistency.