2018 ASHS Annual Conference

Cucurbit powdery mildew (CPM) is caused most frequently by two fungal species, Golovinomyces orontii and Podosphaera xanthii (Px), that are highly variable in virulence. The former organism has not been reported on melon in the U.S. since the 1960s, while the latter organism is consistently reported on melon across the country. Variability in CPM virulence, recorded as different physiological races, was first observed in the U.S. in 1938, when race 2 appeared shortly after the release of race 1-resistant ‘PMR 45’ for Imperial Valley, Calif. production. Px race 3 appeared in 1977 in Weslaco, Texas. Px race 1 was present on spring and fall season melons in Imperial Valley in 2001–02, but in 2003 a new race, designated S, was observed in a spring melon production field and race differential research plots. Race S infected all the CPM melon race differentials in common use at that time. Px races 1, 2, and S occurred on spring melons in Imperial Valley, 2004–17. In Yuma, Ariz., ~80 km distant from the Imperial Valley research plots, race S first appeared in Spring 2004, and recurred there during the spring seasons of 2005, 2007–08. Px race S has not been observed on fall melons in Yuma or Imperial Valley; only race 1 was observed in those areas in Fall 2002–07. Px races 2, 5, 3.5, and S were present at 3 locations in the Central Valley, Calif. in August and September 2003–17. Px race SD, which infects all known Px melon race differentials was first isolated via single spore transfers from race S in 2003, and has been present in a greenhouse at Salinas since 2012. Twelve California field isolates and their single-spore sub-strains collected 2014–15 revealed additional virulence variation, including one similar to race pxCh1, in controlled-inoculation greenhouse tests of 11 CPM race differentials. Such virulence variation has not been reported in other parts of the U.S., with the exception of one unconfirmed report of race S in Georgia. Px races 1 and 2 occur in Texas and New York. Px race 1 occurs along the Atlantic coast, though there was evidence of race 2 at a low level on fall melons in Charleston, S.C. Px race 2 consistently occurs in the Gulf of Mexico coastal areas of Florida.