Powdery mildew caused by the obligate biotrophic fungus Podosphaera xanthii poses a serious threat to melon (Cucumis melo L.) production worldwide. Frequent occurrences of the disease in different regions of South Korea hints at the potential existence of several races which need to be identified. The races of five isolates collected from different powdery mildew affected regions were identified based on the pathogenicity tests of these isolates on eight known differential melon cultigens namely, SCNU1154, PMR 45, WMR 29, PMR 5, MR-1, PI124112, Edisto 47 and PI414723. None of the isolates have shown same disease responses to those of the known races tested in this study and in previous reports on these identical differential melon cultigens. This indicates that the tested uncharacterized isolates are new races. Among the isolates, the isolates from Hadong, Buyeo, Yeongam and Gokseong have shown same pathogenicity indicating the possibility of these isolates being one new race, for which we propose the name 'race KN1'. The isolate of Janghueng have also shown unique disease response in the tested differential melon cultigens and hence, we identified it as another new race with a proposed name 'race KN2'. Report of these new races will be helpful in taking effective control measures in prevalent regions and for future breeding programs aimed at developing varieties that are resistant to these race(s).
Keywords: Podosphaera xanthii; melon; pathogenicity; powdery mildew; race.