Epidemiological Analysis of Purple Blotch Disease on Onion under the Climatic Conditions of El-Minya Governorate, Egypt

Purple blotch, caused by the fungus Alternaria porri, is a serious disease affecting onion crop in
Egypt. This study aimed to analyze the epidemiology of purple blotch under the climatic conditions of ElMinya governorate, Egypt. Infected onion samples were collected from six areas in middle Egypt. The
systemic survey was carried out to record the severity of purple blotch disease in three onion major
growing districts of El-Minya governorate, Egypt, during 2020-2021 and 2021-2022 crop seasons. The
survey for disease, severity, distribution, and spread was carried out at physiological maturity. Significant
increase in onion purple blotch DI, DS%, the Area Under Disease Progress Curve (AUDPC) and the
disease pressure was higher in the first than in second seasons. Also, there is a significant increase in both
disease parameters in Giza-6 than Giza red cvs. When recorded every two weeks, from the 11th the
standard week to the 22nd standard week. Based on the mean data, a multiple regression equation was
developed to predict the disease. The results show that in 2021-2022, two weather parameters; the
maximum temperature <20oC and relative humidity (80%), spell contributed to an increase in the
infection rate. The maximum AUDPC was recorded in February-March of each season. The severity of
purple blotch disease in onions varies significantly across different locations in Egypt, influenced by both
environmental conditions and pathogen virulence. The variation in disease severity can be attributed to
factors such as local climatic conditions, agricultural practices, and the presence of different pathogenic
isolates.