Impact of Alternaria Leaf Spot and Meteorological Conditions on Growth and Yield of Ocimum basilicum L. cv. Genovese in South Banat, Serbia

Alternaria leaf spot caused by Alternaria alternata affects Ocimum basilicum L. worldwide, yet its occurrence and impact in Serbia have remained unknown. This study identified the causal agent of leaf spot on O. basilicum cv. Genovese using molecular, morphological, and pathogenic analyses, and assessed the effects of disease and meteorological conditions on herb yield and morphometric traits. From 109 sampled plants in the preliminary survey (2021 and 2022) and field experiment (2023–2025), 59 isolates were obtained from symptomatic leaves, stems, branches, and seeds. Isolates were preliminarily identified based on the ITS rDNA region. Pathogenicity tests were done on detached leaves, stems, and seedlings. For selected A. alternata isolates, identification was confirmed based on additional regions (Alt a1, ATP, and CAL) and morphology (PDA, PCA, and V8 media), while pathogenicity was confirmed on whole plants. Disease incidence varied from 0% (2023) to 34.87% (2024) under warm and favourable moisture conditions. Significant reductions in plant height, plant weight, number of branches, and leaf size indicated the combined effects of disease and unfavourable meteorological conditions, while dry herb yield was reduced by up to 59.8%. Temperature and precipitation between harvests played a key role in disease development and yield loss. This study provides the first comprehensive molecular, morphological, and pathogenic characterization of A. alternata causing basil leaf spot in Serbia and demonstrates the importance of climate–disease interactions for basil production. These findings provide a basis for targeted disease monitoring, early detection, and climate-informed disease management strategies. Future studies should validate these findings in additional basil cultivars and production environments.