Cucurbit powdery mildew is a chronic and major disease problem of cantaloupe and honeydew production worldwide. Early infection can kill plants or drastically reduce fruit yield; late infection reduces fruit quality. The cucurbit powdery mildew responsible for powdery mildew disease in the USA is Podosphaera xanthii. Two races of P. xanthii were known in California until 2003 when a new race, S, was observed in Imperial Valley by Jim McCreight, melon breeder with USDA–ARS, Salinas and Mike Coffey, Plant Pathologist at UC Riverside. Since then, Race 1 has been found most frequently (from 2014 through 2016) in Imperial Valley, as well as Race 2 (years from 2014 through 2016) and Race S (two years from 2014 through 2016). More recently, Race 3.5 was observed in the Woodland area in 2014, 2015 and 2016 following appearances by Races 1, 2, and S in the preceding four years. Race 5 was observed in a field planting in 2016 near Five Points, where Races 1 and S were observed in 2003 and 2007, respectively. Race SD, first isolated from Imperial Valley cucurbit powdery mildew samples, is currently predominant in a Salinas greenhouse. Cucurbit powdery mildew races on melon in the Central Valley have been geographically and seasonally variable in recent years. Race variation compromises and confounds genetic host plant resistance to cucurbit powdery mildew in melon. This project has characterized P. xanthii race variation on melon in California based initially on field samples in 2015 and 2016, and supplemented with greenhouse samples. The aim has been to identify and characterize unique sources of resistance to the most prevalent races present since 2003 to the present 2018 season.
