Pathogenicity, Aggressiveness, and Fig Cultivar Susceptibility to Alternaria alternata and Bipolaris sorokiniana as Twig Dieback Pathogens on Fig Trees

Abstract

Branch dieback symptoms are becoming increasingly common in fig (Ficus carica L.) orchards throughout southern Iran, the country's primary fig-producing region. During surveys of commercial orchards, fungal isolates from Alternaria spp. and Bipolaris spp. were obtained from symptomatic branches of the economically significant ‘Sabz’ cultivar. Through morphological characterization and phylogenetic analyses based on the translation elongation factor 1-α (tef1), glyceraldehyde-3-phosphate dehydrogenase (gapdh), and major allergen alt a 1 (Alta1) genes, for Alternaria alternata isolates, and the tef1 gene for B. sorokiniana isolates, these fungi were identified to species level. Pathogenicity tests on detached shoots demonstrated that all isolates caused brown lesions and wood discoloration, with notable differences in aggressiveness. Principal component analysis (PCA) of aggressiveness data divided the A. alternata isolates into four distinct groups. Isolates from symptomatic trees in Ij, Fars Province, were significantly more aggressive than those from other areas. Pathogenicity assays revealed that both fungal species produced necrotic lesions on fig buds, leaves, and 1-year-old saplings. A. alternata consistently resulted in more severe symptoms than B. sorokiniana. Additionally, in greenhouse trials on saplings, both conidial suspensions and mycelial plugs confirmed pathogenicity, with the mycelial plug method generally leading to more extensive lesion development. Furthermore, fig cultivars showed varying susceptibility, as PCA and heatmap clustering based on six pathogenicity traits grouped the cultivars into three categories, identifying ‘Gilasi’ as the most susceptible and ‘Matti’ as the most resistant. Under the conditions tested, the findings of this study support the involvement of A. alternata and B. sorokiniana as pathogens contributing to branch dieback and discoloration of wood and inner bark tissues in fig trees from southern Iran. These findings establish A. alternata and B. sorokiniana as emerging threats to fig production in Iran and underscore the importance of incorporating resistant cultivars, such as ‘Matti’, into future disease management strategies.