By Clint Thompson
Vidalia onion producers must be aware the calendar is nearing the timeframe for when downy mildew disease usually occurs in Southeast Georgia.
University of Georgia Extension Vegetable Plant Pathologist Bhabesh Dutta spoke about onion diseases at the Southeast Regional Fruit and Vegetable Conference in Savannah, Georgia, in early January. He highlighted when disease outbreaks have occurred in recent years and what fungicides growers should be implementing.
“The last four outbreaks occurred around Feb. 21 to Feb. 28. Late February is the window,” Dutta said. “January to February, what we are recommending is be light on Bravo and phosphites, depending on weather conditions. But once you hit late February, go with at least one application of Omega, Ridomil Gold Copper or Orondis Ultra, in rotation with Bravo.”
The first symptoms of downy mildew occur on older leaves as light green to pale yellow in color, which turn to tan and brown as the lesion ages. In Georgia, early symptoms can be diamond-shaped lesions that are mottled with pale and green areas interspersed. As colonization progresses, lesions may girdle the entire leaf which can cause the total collapse of leaf tissues.
Dutta said Omega 500, Ridomil Gold Copper, Orondis Ultra and Zampro have moderate efficacy on downy mildew, while Bravo and Reason have shown moderate-to-low efficacy. Phosphite and Bravo are to be used as a rotational partner. Revus and Previcur Flex have shown no efficacy.
The Reason fungicide was re-evaluated based on a grower’s recommendation. It performed ‘okay’ in consecutive two-year field trials.
Growers are also reminded that they are not to make more than six applications of Omega 500 per acre per year.