Molecular and Morphological Identification and Pathogenicity of Fusarium Species Causing Melon Wilt in Russia

Fusarium wilt of melon, caused by <i>Fusarium</i> fungi, results in sizeable economic losses worldwide. In Russia, data on the species composition of the causative pathogens of this disease on melon are lacking. From 2022 to 2025, 19 <i>Fusarium</i> isolates from the Volgograd and Rostov regions were included in a study that included species identification using molecular phylogenetic analysis of the <i>tef1</i> and <i>rpb2</i> loci, morphological description, and pathogenicity assessment against the host plant and other members of the Cucurbitaceae family. Four <i>Fusarium</i> species were found to be involved in the pathogenesis of Fusarium wilt of melon in Russia: <i>F. clavus</i> (37% of the total number of isolates), <i>F. annulatum</i> (21%), <i>F.</i> cf. <i>inflexum</i> (21%), and <i>F. brachygibbosum</i> (21%). All identified species were isolated in the Volgograd Region, while only <i>F.</i> cf. <i>inflexum</i> and <i>F. brachygibbosum</i> were isolated in the Rostov Region. This study reports for the first time the high pathogenicity of <i>F.</i> cf. <i>inflexum</i> and <i>F. brachygibbosum</i> species associated with melon wilt. Morphological variability and different aggressiveness of isolates of the species <i>F. brachygibbosum</i> and <i>F. clavus</i>, isolated in the Volgograd (-V) and Rostov (-R) regions in different years, were observed. The isolate <i>F. brachygibbosum</i>-V showed high aggressiveness both at the sprout and seedling stages, while the isolate <i>F. brachygibbosum</i>-R was characterized by moderate aggressiveness only at the sprout stage. High pathogenicity of the species isolated from melons was established for other cucurbit crops. <i>F.</i> cf. <i>inflexum</i> was also pathogenic for watermelon and pumpkin, and <i>F. brachygibbosum</i> was pathogenic for pumpkin. The obtained data will have practical value for the development of biological control measures against <i>Fusarium</i> fungi and will be used in a melon breeding program for resistance to Fusarium wilt.