Assessing alternative strategies to control almond red leaf blotch through the reduction of Polystigma amygdalinum inoculum in leaf litter

Abstract

Background Red leaf blotch (RLB), caused by Polystigma amygdalinum, is a major foliar disease of almond trees in Mediterranean and Middle Eastern regions. While preventive fungicide applications are the main control strategy, cultural practices aimed at reducing pathogen inoculum in leaf litter are gaining relevance. This study evaluated the efficacy of four chemical treatments on fungal biomass and ascospore production in leaf litter and assessed the impact of two cultural practices—urea application and leaf litter removal—on airborne inoculum levels and disease incidence under field conditions. Results Urea and lime sulfur significantly reduced ascospore production by 99% and 94%, respectively, compared to the untreated control. Urea was the only treatment that also significantly reduced fungal biomass. In field trials, leaf litter removal reduced airborne ascospores by 60% but did not significantly affect RLB incidence or severity. Urea application reduced airborne ascospores by 45% and led to a modest yet statistically significant 4% reduction in disease incidence. Conclusion This study highlights the potential of cultural practices to reduce the primary inoculum of P. amygdalinum in almond orchards. Urea application offers dual benefits by reducing both fungal biomass and ascospore release, contributing to modest disease suppression. For more effective and sustainable RLB control, these practices should be integrated into a broader management strategy that includes the selection of tolerant almond cultivars, preventive fungicide applications guided by forecasting models.