Field Responses of Almond Accessions to Diaporthe-Associated Biotic Stress in Southern Italy

Phenotypic response to twig canker and shoot blight varies substantially across almond germplasm. We hypothesized that genetically diverse almond accessions grown in a common orchard environment would exhibit genotype-dependent differences in symptom severity under natural disease pressure. Thirty-three almond accessions of different geographic origins were evaluated under field conditions in southern Italy. Disease incidence and severity were assessed, and symptomatic tissues were subjected to mycological and molecular analyses. Significant differences among accessions were detected for both disease incidence (I) and McKinney’s Disease Index (McK). ‘Merced’, ‘Nonpareil’, and ‘Genco’ exhibited high levels of disease incidence and severity, characterized by twig canker, shoot blight, and shoot clustering, with incidence values of 50% or higher, whereas local or less widely cultivated genotypes such as ‘Vesta’ and ‘Rana Gentile’ showed low disease incidence and severity. The Diaporthe complex represented the predominant fungal group isolated from symptomatic tissues, with D. foeniculina accounting for more than 80% of the identified Diaporthe isolates, whereas D. rudis was detected at lower frequencies. Other fungal taxa, particularly Botryosphaeriaceae, were recovered at lower frequencies. The observed variability supports the use of almond germplasm collections to identify genotypes with reduced symptom expression that may support breeding programs aimed at improving disease resilience.