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 inEgypt. 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. Thesystemic survey was carried out to record the severity of purple blotch disease in three onion majorgrowing districts of El-Minya governorate, Egypt, during 2020-2021 and 2021-2022 crop seasons. Thesurvey for disease, severity, distribution, and spread was carried out at physiological maturity. Significantincrease in onion purple blotch DI, DS%, the Area Under Disease Progress Curve (AUDPC) and thedisease pressure was higher in the first than in second seasons. Also, there is a significant increase in bothdisease parameters in Giza-6 than Giza red cvs. When recorded every two weeks, from the 11th thestandard week to the 22nd standard week. Based on the mean data, a multiple regression equation wasdeveloped to predict the disease. The results show that in 2021-2022, two weather parameters; themaximum temperature <20oC and relative humidity (80%), spell contributed to an increase in theinfection rate. The maximum AUDPC was recorded in February-March of each season. The severity ofpurple blotch disease in onions varies significantly across different locations in Egypt, influenced by bothenvironmental conditions and pathogen virulence. The variation in disease severity can be attributed tofactors such as local climatic conditions, agricultural practices, and the presence of different pathogenicisolates.