First report of Alternaria leaf blight of Radish (Brassica napus): A Growing Concern in Pakistan's Agriculture

Alternaria species are necrotic plant pathogens that cause dark spot disease to nearly one hundred host plants, affecting them from vegetative to reproductive stages. Infection typically appeared on leaves and stems as dark brownish to black spots that form concentric ring patterns as they enlarge, typically showing blight symptoms. The objective of the study was to characterize Alternaria species associated with radish leaf blight in Pakistan. In this study, a total of six isolates causing typical symptoms of leaf blight were collected from radish leaves (cv. Sufaid Pari) during a survey in February 2023 at the Vegetable Research Station Sahiwal (VRSS) Punjab, Pakistan. Being the first identification, all recovered isolates from symptomatic radish leaves were characterized based on morphological traits, pathogenicity and molecular analysis of internal transcribed spacer regions (ITS) of the 5.8S rDNA gene. Sequence analysis showed that radish isolates grouped to three Alternaria species: mainly A. brassicae, A. brassicicola and A. alternata respectively. Pathogenicity tests demonstrated that all isolates could produce similar necrotic lesions on detached radish leaves as observed during the field survey. Morphological characteristics act as a primary method for identifying Alternaria spp. although it is not feasible due to some Alternaria genera are morphologically related, consequently, molecular method of identification was sufficient for accurate species identification. This study represents is the first comprehensive investigation of Alternaria spp. causing leaf blight of radish in Pakistan.