Disease problems of sunflowers

Sunflower, the second most important oilseed crop in the world, was developed as a crop in eastern Europe, but is a native of North America. The pathogens causing many of its major diseases are also native to North America. Although a profitable crop, sunflowers have fairly low value per hectare. Disease control must, therefore, be inexpensive, by resistant varieties or seed treatment or cultural practices, rather than repeated field application of chemicals. Rust, a limiting factor in many countries, has been successfully controlled everywhere by resistance from wild sunflowers discovered in Canada about 1950. New races are posing problems in Argentina and Australia, but new resistance may be available. Verticillium wilt has been destructive in some areas but resistance is available from wild sunflowers and Russian high‐oil varieties. Downy mildew, highly destructive in many countries, has been effectively controlled by two genes from the original rust‐resistant material. A new race attacking this resistance was discovered in 1980: resistance to it appears to be available. Sclerotinia stalk rot and head rot, caused by a pathogen with wide host range, is much harder to control by breeding. Leaf spot diseases have long been a limiting factor in some European countries and elsewhere, but were considered minor in North America until recently. Broom rape, a root parasite which almost destroyed the crop in the USSR and elsewhere in eastern Europe, does not attack it in North America. It is controlled by resistant varieties.