Diseases of sunflower in California.
Sunflower seed production has shifted from the north central states of North Dakota and Minnesota to California, primarily because of the longer growing season, but also to escape disease and insect problems of the Midwest. Between 1983 and 1988, the most prevalent diseases observed by county agricultural inspectors on sunflower in California were Rhizopus head rot (caused by R. oryzae), powdery mildew (caused by Erysiphe cichoracearum) and charcoal rot (caused by Macrophomina phaseolina), none of which are diseases of quarantine significance. Rust (caused by Puccinia helianthi) and Sclerotinia sclerotiorum wilt were the only quarantine diseases observed and these only sporadically. An intensive survey during 1989 detected Rhizopus head rot in 71% of surveyed fields, rust in 30%, powdery mildew in 25%, S. sclerotiorum wilt in 20%, Sclerotinia minor wilt in 11%, charcoal rot in 8%, and Sclerotium [Corticium] rolfsii wilt in 6%. Downy mildew (caused by Plasmopara halstedii) was detected for the first time in 11 years in California in one field during the unusually cool spring of 1989. Diseases that were not observed on cultivated sunflower in California included Phomopsis stem canker (caused by P. helianthi), Phoma black stem (caused by P. macdonaldii), Verticillium wilt (caused by V. dahliae), bacterial leaf spot (caused by Pseudomonas syringae pv. helianthi and P. cichorii), apical chlorosis (caused by P. syringae pv. tagetis), Septoria leaf spot (caused by S. helianthi), Alternaria leaf spot (caused by A. helianthi and A. zinniae) and broomrape (caused by Orobanche cumana and O. ramosa).