Occurrence and characterization of Stemphylium and Alternaria species associated with lettuce leaf spot in Algeria

Lettuce leaves (Lactuca sativa) with leaf spot symptoms were collected from an organic farm and from farm markets in Oran Province, Oran, northwest Algeria, during the 2022 and 2023 growing seasons. A total of 119 isolates with morphological characteristics of Stemphylium and Alternaria were obtained from brown necrotic lesions on lettuce leaves. Based on morphological and multi-gene sequencing analyses, these isolates were identified as Stemphylium lycopersici, S. vesicarium, S. eturmiunum, S. amaranthi, and S. gracilariae, and Alternaria in sections Alternaria and Eureka. Pathogenicity tests with 24 representative isolates were conducted on young lettuce plants under greenhouse conditions, and lesions similar to those observed on the field and market collected plants developed on leaves of inoculated plants. Stemphylium gracilariae and S. lycopersici were the most aggressive fungi, causing extensive leaf necroses and defoliation, while other Stemphylium and Alternaria isolates develop small necrotic spots on mature leaves. A series of cross-inoculation experiments compared disease severity when plants were inoculated with one of four Stemphylium species alone or paired with one of five Alternaria isolates (20 combinations total). Differences in pathogenicity of mixed inoculations compared to single inoculation were detected, but synergistic increases in leaf spot severity were not detected for any mixed inoculations. These results demonstrate the potential of emerging fungal pathogens to cause lettuce leaf spot, and that Stemphylium and Alternaria species combinations can damage economically important lettuce crops.