By Clint Thompson
Downy mildew remains the biggest disease concern for Georgia’s Vidalia onion farmers. It won’t be long before producers must protect their year’s investment, says University of Georgia Extension Vegetable Plant Pathologist Bhabesh Dutta.
“This disease is so aggressive and can damage the crop so fast, all of these programs need to be initiated right (in late February). That’s when we start seeing it. We need one aggressive spray before the onset of the disease,” Dutta said. “Based on my fungicide trials, none of the materials look great but some materials displayed more levels of efficacy against downy mildew. Those materials include Ridomil Gold Copper, Omega 500, and then we have Zampro and even Bravo.”
Dutta said the disease has been sporadic, but it has been observed the last two years. It was first discovered on March 23 in 2020; Feb. 18 in 2019; March 19 in 2018; April 13 in 2016; and April 20 in 2015.
The first symptoms of downy mildew occur on older leaves as light green to pale yellow, 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. This could cause the total collapse of leaf tissues. Infected bulbs are reduced in size and typically don’t store well. In severe cases, 100% yield losses have been reported.
“If the conditions are perfect, if you miss (the time) of not spraying, you are risking your entire crop,” Dutta said. “Either you do it (spray) and at least feel safe, but if you don’t do it and it happens, you’re on your own.”