In February 1999, localized areas in a commercial field of Vidalia sweet onions in Toombs County, GA, exhibited symptoms that included elongated pale to light tan lesions on older leaves as well as some totally collapsed leaves. A dark, sooty to purple-gray fungal growth also was observed on affected leaves. Both symptoms and signs were observed during a period of wet, cool weather. Microscopic observation of affected tissues revealed nonseptate mycelia and dichotomously branched sporangiophores, which tapered to short sterigmata, giving rise to pyriform to fusiform sporangia. Based on these observations, the disease was determined to be downy mildew of onion caused by Peronospora destructor (Berk.). The dimensions of 25 sporangia averaged 25.1 ± 3.8 × 64.2 ± 4.3 μm, falling within the range of those currently reported for P. destructor (1). The disease was isolated to the field in which it was initially reported and did not cause extensive damage. Its failure to progress and spread may have been due to the warm, dry conditions that prevailed subsequent to symptom detection. This is the first report of P. destructor in Georgia.
Reference: (1) H. F. Schwartz and S. K. Mohan. 1995. Diseases of aerial parts caused by fungi. Pages 20--24 in: Compendium of Onion and Garlic Diseases. The American Phytopathological Society, St. Paul, MN.
