LAUSR

Simple Summary Cucumber is an important vegetable crop for mankind. Cucumber target leaf spot, caused by Corynespora cassiicola, is a worldwide disease. The disease is commonly treated with fungicides, but due to the emergence of resistance, the efficacy of fungicides has declined. At present, control of this disease has been problematic. Spores of C. cassiicola play a significant role in the epidemiology of the disease. Owing to this, fungus cultivation is necessary to properly understand its life cycle as well as pathogenesis for the possible future development of an efficient and environmentally friendly control method. Therefore, in this study, different temperatures and moistures were evaluated on four substrates to see their impact on the sporulation of C. cassiicola. Furthermore, the relationship between spore size and pathogenicity was determined. The results showed that temperature and moisture affected not only spore production, but also spore size. In addition, spore size was found to be an important virulence determinant in C. cassiicola. Abstract Cucumber target leaf spot, caused by Corynespora cassiicola, is an emerging disease with a high incidence that causes severe damage to cucumbers on a global scale. Therefore, efforts need to be undertaken to limit the spread and infection of this pathogen, preferably by using environmentally friendly methods. In this study, the effects of temperature and moisture on the sporulation of C. cassiicola were investigated in vitro and in vivo. The novelty of our study refers to the observation of spore production and size as well as the revelation of a correlation between spore size and virulence. On potato dextrose agar (PDA) and cucumber−leaf extract agar (CEA), temperature played a critical role in spore production, which was strongly influenced by both temperature and moisture on detached leaves and cucumber seedlings. Maximum spore production was found at 30 °C on PDA and 25 °C on CEA, cucumber detached leaves and living plants. Lower spore productions were observed with a stepwise change of 5 °C. In addition, the largest spore production was found at 100% relative humidity (RH) in comparison to the other tested moisture. Moreover, moisture was found to be the most important factor affecting spore size, accounting for 83.09–84.86% of the total variance in length and 44.72–73.10% of the total variance in width. The longest−narrowest spores were formed at 100% RH, and the shortest−widest spores were formed at 75% RH. Furthermore, the result showed that larger spores of C. cassiicola were more virulent and small spores were avirulent. Our findings will contribute to the development of new strategies for the effective alleviation and control of cucumber target leaf spot.