Cucurbit powdery mildew of melon incited by Podosphaera xanthii: Global and western US perspectives

Cucurbit powdery mildew (CPM) is a major problem of melon (Cucumis melo L.) production worldwide, that is mostly caused by two fungi: Podosphaera xanthii (Px; formerly Sphaerotheca fuliginea) and Golovinomyces cichoracearum (DC) V.P. Heluta (Gc; formerly Erysiphe cichoracearum). The two species may co-infect in some areas of northern Europe, but Px dominates in warmer climates around the world. Forty-six races of Px have been reported on melon based on sets of race differentials that range in number from as few as two to as many as 28. The CPM research community and seed industry are undertaking steps to define uniform sets of CPM race differentials and objective criteria for race nomenclature and designations. Breeders and pathologists must also consider another aspect of CPM that stems from its obligate parasitic nature: race stability as defined by a given set of CPM race differentials. This review summarizes the reported races of Px largely from the published literature. It also presents annual profiles from 2002 through 2011 of Px populations in the Central and Imperial valleys of California, and Yuma, Arizona. It is suggested that a large fraction of the races are not relevant to most Px resistance breeding, which will be done on a regional basis for subsets of races.