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 Mike Coffey, Plant Pathologist at UC Riverside. More recently, Race 3.5 was observed there in 2012 by Jim McCreight. 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 genetic host plant resistance to cucurbit powdery mildew in melon. This project has characterized P. xanthii race variation on melon in California for he last 2 years, 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.