Alternaria and Curvularia leaf spot pathogens show high aggressivity on watermelon, and are emerging pathogens in cucurbit production

Fungal leaf spot pathogens of cucurbits cause significant yield losses. They cause extensive leaf necroses and defoliation, reducing host photosynthesis. They increase risks of fruit sunscald, and can cause substantial crop damage. Alternaria cucumerina has been recognized as the causal agent of leaf spot disease of cucurbits, and recent studies have identified other Alternaria species, and other emerging pathogens such as Curvularia. This study characterized 25 isolates obtained from infected watermelon and cucumber leaves from Hungary, Spain, and Kosovo. Morphological characterization and molecular analyses using TEF1-α, HIS3, and ITS gene regions identified Alternaria alternata and A. arborescens, and for the first time on this host, the genus Curvularia. Detached leaf assays of ten isolates on 73 watermelon accessions showed variation in isolate pathogenicity. The tested Curvularia isolate was the most aggressive, followed by the A. arborescens and A. alternata isolates, although A. alternata was the most frequently identified species. These results highlight the potential for emerging fungal pathogens causing cucurbit leaf spot, such as Curvularia sp., and show that these fungi can cause damage on economically important plants. This study also showed differing resistance within the watermelon collection, indicating potential for the plant introduction (PI) accessions as sources of resistance breeding.