Pathogenicity, Aggressiveness, and Temperature Response of Paramicrosphaeropsis eriobotryae Associated with Almond Twig Dieback and Wood Necrosis

In this study, Paramicrosphaeropsis eriobotryae is identified and characterized for the first time as a novel and aggressive pathogen responsible for twig dieback in almond orchards. Field surveys across major almond-growing regions of Iran, the world’s seventh-largest producer, revealed trees exhibiting vascular necrosis, wood discoloration, and progressive dieback symptoms. Fungal isolates from symptomatic tissues exhibited morphological characteristics consistent with those of a member of the Didymellaceae family, within Ascomycota . Phylogenetic analyses based on protein-coding loci β-tubulin ( tub2 ) and RNA polymerase II second-largest subunit ( rpb2 ) confirmed the identity of isolates as P. eriobotryae . Pathogenicity assays on detached shoots and one-year-old almond saplings fulfilled Koch’s postulates, inducing dark brown necrotic lesions and internal vascular discoloration that mirrored field symptoms. Notably, principal component analysis (PCA) confirmed significant geographic variation in aggressiveness, with southern isolates exhibiting markedly higher aggressiveness than western counterparts. Disease development was strongly temperature-dependent, peaking at 25–30°C and substantially suppressed at 15°C and 35°C. Importantly, previous research has demonstrated the capacity for latent infection in asymptomatic hosts such as loquat and olive, underscoring a considerable biosecurity risk to global almond production. This study establishes P. eriobotryae as an emerging threat characterized by temperature-sensitive pathogenicity and regional variations in aggressiveness, underscoring the need for integrated management strategies.