Rhizopus oryzae Associated with Melanagromyza splendida and Stem Disease of Sunflowers (Helianthus annuus) in California

In September 2012, a female parental line in a Yolo Co., California, sunflower seed-production field began displaying external stem symptoms that could not be attributed to any known disease. Symptoms appeared to be associated with tunneling caused by an unidentified insect. Stems were collected, and Rhizopus oryzae (causal agent of Rhizopus head rot) and a minute fly, Melanagromyza splendida, were identified as the causal agent and associated insect, respectively. Further, R. oryzae was isolated from intact fly puparia. All commercial hybrids evaluated in the greenhouse were susceptible to stem infection by R. oryzae isolates. Yield implications and geographic distribution of this novel stem disease are unknown. This is the first report of R. oryzae causing stem disease in sunflowers, and of its association with M. splendida.