Susceptibility of Almond (Prunus dulcis) cultivars to red leaf blotch caused by Polystigma amygdalinus

Polystigma amygdalinum is a biotrophic ascomycete responsible for the red leaf blotch (RLB) affecting almond worldwide. Small leaf blotches, developing into necrosis throughout the season, are reported to be associated with the disease. The work was aimed at studying the RLB susceptibility of varieties representing most of Apulian almond biodiversity and setting molecular protocol for pathogen detection. Field-observations on 237 varieties grown in an ex-situ biodiversity almond collection field located at the Centro di Ricerca, Sperimentazione e Formazione in Agricoltura “Basile Caramia” in Locorotondo, and on six varieties grown in an orchard located in Ruvo countryside, were conducted for at least two years. RLB infections occurred at the end of spring to summer and were particularly severe in the year 2022, reaching up to 46.7% of incidence on ‘Guara’. ‘Guara’ and ‘Lauranne Avijor’ were highly susceptible to the disease (Tukey test: p<0.01), while ‘Belona’, ‘Centopezze’, ‘Ficarazza’, ‘Genco’ and ‘Rachele piccola’ were the most tolerant (RLB never exceeding 6%). In 2023, in both the experimental fields, symptomatic leaves showing the typical RLB symptoms, or small red spots bordered by yellow halo (bacterial-like) or small necrotic spots, were singularly sampled from 33 varieties and used in molecular assays. A qPCR-based method was improved and used for P. amygdalinum detection in leaves. Our results confirmed the presence of the plant pathogen in the area, suggested different levels of susceptibility among P. dulcis varieties, and the mandatory use of suitable diagnostic tests to ascertain the presence of the pathogen when symptoms are not clearly attributable to RLB.