Cucurbit Powdery Mildew Race Variation on Melon in California's Sacramento Valley

More than 40 physiological races of cucurbit powdery mildew (CPM) incited by Podosphaera xanthii (syn. Sphaerotheca fuliginea) have been reported on melon (Cucumis melo L.) worldwide. Races 1 and 2, which were first described in the 1930s, are common throughout the United States, with the exception of California and Arizona. Race S was first observed in 2003 in the lower desert areas of Imperial Valley, CA, and Yuma, AZ, when it caused widespread disease in commercial fields and was observed to infect all of the common CPM race differentials of melon in use at that time. Race variability has been detected annually in those areas since that time. Race S was observed in 2007 at Five Points in the San Joaquin Valley of California, which is ca. 460 km from the lower desert and is a major area for summer melon production. Field studies were done in the Davis–Woodland area of the Sacramento Valley in order to further characterize the variation of races of P. xanthii in California. The tests were established using transplants in randomized complete-block designs on 26 May 2011 and 28 June 2012, and evaluated on 26 August and 7 September, respectively. The tests included 11 of the 13 commonly used P. xanthii race differentials, plus 22 other melon cultivars and accessions and two watermelon (Citrullus lanatus L.) cultivars to further characterize the CPM populations present in these tests. The 2011 test, which was near Davis, indicated presence of race SW, a variant of race S that infects watermelon. A test in 2012 at that same location indicated a unique race to which PI 414723 and PI 124111 were completely resistant; the other common differentials and ‘Crimson Sweet’ and ‘New Hampshire Midget’ watermelons were susceptible. A different race profile was observed in a field near Woodland in 2012: PI 414723, MR-1, PI 124112, PI 313970 were resistant, PI 124111 and ‘Crimson Sweet’ watermelon varied in their reactions, and all other common differentials were susceptible. These data indicate seasonal and spatial variation in the population of P. xanthii races in the Davis–Woodland area. There have been recent anecdotal reports of races 3.5 and 5, in the Sacramento Valley of California, but neither race was present in these tests.