Isolation, Identification, and Pathogenicity of Alternaria alternata Associated with Apple Branch Canker in Al-Jabal Al-Akhdar Region, Libya

Abstract

Apple (Malus domestica Borkh.) represents a major fruit crop of global economic importance. Despite its widespread cultivation, fungal diseases infecting branches and vegetative tissues continue to be a major concern, adversely affecting orchard productivity and compromising fruit quality. The present study investigated the fungal pathogen associated with branch canker symptoms observed in apple orchards located in the Al-Jabal Al-Akhdar region, Libya. Symptomatic branch samples exhibiting bark cracking, canker formation, and progressive dieback were collected and subjected to fungal isolation under laboratory conditions. Isolates were purified on potato dextrose agar (PDA) medium and identified according to morphological and microscopic characteristics using standard taxonomic keys. Pathogenicity assays were conducted on healthy apple seedlings to verify the causal relationship between the isolated fungus and disease development. Morphological observations demonstrated that the recovered isolate belonged to Alternaria alternata (Fr.) Keissl., characterized by dark olivaceous to brown colonies and multicellular conidia arranged singly or in short chains. Conidial dimensions ranged from 14.05–14.38 µm in length and 8.07–9.81 µm in width, with 2–8 transverse septa. Artificial inoculation induced typical symptoms, including bark necrosis, branch canker formation, chlorotic lesions, and progressive tissue collapse. Re-isolation from infected tissues successfully fulfilled Koch’s postulates. The findings confirm the pathogenic role of A. alternata in apple branch canker development under local environmental conditions. This study represents one of the first detailed reports concerning the occurrence and pathogenicity of A. alternata associated with apple branch canker in eastern Libya and provides baseline information for future epidemiological and disease management studies