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The first reported occurrence of onion downy mildew (ODM), caused by Peronospora destructor (Berk.) on Vidalia sweet onions (Allium cepa L.) in Georgia was in 1999 (1). The term Vidalia refers to a group of sweet onion shortday, Granex-type varieties that are grown in 20 southeastern counties in Georgia near the city of Vidalia. However, the majority of Vidalia onions are grown in 13 counties surrounding Toombs and Tattnall counties. Although the disease is not common every year, two epidemics have occurred since 1999. We report here on the impact of a recent outbreak of ODM on the $100-million Vidalia onion crop in 2012. Symptoms of ODM were first observed in early February 2012. Symptoms included elongated leaf lesions with purple-gray sporulation, tip dieback, and collapsed leaves (Fig. 1). Microscopic observation of symptomatic leaves showed numerous, dichotomously branched sporangiophores with pyriform to fusiform sporangia (Fig. 2) and non-septate mycelia. Based on the morphology and molecular identification (rDNA ITS sequencing), the pathogen was confirmed as P. destructor. Continuous periods of wet, cool weather with average low nighttime temperatures of 10.2 to 11.3°C, and a hard freeze followed by high humidity with foggy mornings in late February and early March in southeastern Georgia (Georgia Automated Environmental Monitoring Network, www.GeorgiaWeather.net) favored pathogen sporulation and disease development. Favorable weather persisting from late February through early March triggered repeated cycles of sporangial formation and resulted in severe and continued infection in the region. It is uncertain where the initial sources of inoculum originated. ODM forms oospores in infested debris and bulbs in cull piles and these may survive for up to 5-6 years (2,3). A genetic diversity study on ODM populations from diverse locations in the Vidalia onion growing region in Georgia indicated that a single source of inoculum may have been involved (V. Parkunan and P. Ji, unpublished). Thus, it is possible that oospores from a previous epidemic in 2007 survived to initiate the epidemic.

An Epidemic of Downy Mildew caused by Peronospora destructor on Vidalia Sweet Onions in Georgia in 2012