Screening for resistance to cucurbit powdery mildews (Golovinomyces cichoracearum, Podosphaera xanthii)

In this chapter is summarized the basic knowledge about cucurbit powdery mildew (CPM) which is the major cause of losses in cucurbit production worldwide. The disease is caused by two obligate biotrophic ectoparasites, Golovinomyces cichoracearum s.l. (Gc) (syn. Erysiphe cichoracearum s.l.) and Podosphaera xanthii (Px) (syn. Sphaerotheca fuliginea). They can be distinguished easily under light microscopy according to the morphology of conidia and their germ tubes. Powdery mildew species differ in host range, ecological requirements, and geographic distribution. Broad pathogenic variation is represented by the existence of different pathotypes and races. Fungicide resistance of CPMs is a serious problem worldwide, therefore suitable sources of resistance and resistance breeding are considered as very important for all cucurbit crops. There are briefly described the basic methods of pathogen determination, isolation, cultivation and maintenance, including preparation of plant material for screening. The methods of adult leaf disks inoculation, disease assessment and calculation of degree of infection (DI) are described, as well as determination of pathogenic variability (pathotypes and races) by both species of cucurbit powdery mildew. (author)