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 (six years 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.
Powdery mildew is a limiting factor in melon production in most years in all growing regions of California. Genetic resistances to races one and two of Podosphaera xanthii have been available for about 80 years in commercial cultivars. Fourteen years ago, in 2003, such commercially available resistance broke down in Imperial County with the emergence of Race S. Its recurrence and widespread distribution in Imperial County is testament to its potential. Fungicides used to combat the disease added to production costs, and insensitivity to several fungicides with different modes of action is now widespread in California. Incorporation of genes in commercial melon varieties for resistance against these new races is a critical need at this time. For effective breeding of resistance, it is essential to know the pathogen and its potential to overcome resistance.
The project involved collaboration between a plant pathologist and a plant breeder both with many years of experience working with powdery mildews including Podosphaera xanthii. A set of melon powdery mildew race differentials were utilized to characterize powdery mildew strains, both field collections (FC) and in some cases single spore derivatives (SS). Melon accessions containing potentially new resistance genes have been identified and are readily utilizable as sources of powdery mildew resistance. Techniques for building-up the inoculum and preserving it using state-of-the-art cryogenic methods were developed. Each sample was typed for pathogenic race identity using a standard set of race differentials. The melon powdery mildew database contains information on field collections (FC) and single spore isolates (SS) derived from them.
This website was supported by the Specialty Crop Block Grant Program at the U.S. Department of Agriculture (USDA) through Grant 14-SCBGP-CA-0006. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA.