Fungicide resistance spells trouble for Georgia, Virginia vegetable farmers

By for CAES News

Popular vegetables like broccoli and kale are among the crops that could be in danger from Alternaria leaf blight — a disease that can cause spots on some brassica crops and render them unmarketable — which has developed resistance to a once-dependable fungicide that Georgia farmers rely on, according to Bhabesh Dutta, University of Georgia Cooperative Extension plant pathologist.

Dutta recommends producers stop using Quadris on brassica crops, which include cabbage, collards, kale, mustard greens and broccoli. The fungicide is the main one farmers currently use when treating for the disease. Although further research is required to confirm his hypothesis, Dutta believes a new species of Alternaria may be responsible for the outbreak of disease. The species normally associated with Alternaria leaf blight differs from the disease that has recently been observed in Georgia’s brassica fields.

Tift County, Georgia, vegetable farmer Bill Brim is among the brassica farmers concerned about the development.

“Alternaria has become resistant to Quadris, so it’s not as good as it once was,” said Brim, co-owner of Lewis Taylor Farms in Tifton, Georgia, which includes about 1,500 acres of brassica crops. “We’ve got a little bit left in our arsenal to use for Alternaria. We just need to get something back in there we can use.”

Dutta is conducting a research trial evaluating different varieties of Alternaria leaf blight, along with different fungicide programs against this disease, at the Blackshank Farm on the UGA Tifton campus.

Dutta, an assistant professor of plant pathology in UGA’s College of Agricultural and Environmental Sciences, emphasizes the need to develop an integrated pest management (IPM) program to fight Alternaria leaf blight, especially in broccoli and leafy brassicas.

“We do have some other groups of fungicides that we’ll need to rotate, but considering how effective Quadris has been for our vegetable farmers, this resistance is a huge hit on our growers,” Dutta said.

Alternaria leaf blight first became a problem in Georgia in 2016, but has gotten significantly worse over the past two years.

“Alternaria is a foliar pathogen. Symptoms first appear on older leaves as small, dark spots that gradually enlarge with concentric rings. As the disease progresses, younger leaves can also become infected. In severe cases, infection can occur that results in rot on heads. Infection is exacerbated by humidity and extended periods of leaf wetness from overhead irrigation or frequent rainfall,” Dutta said.

Farmers can employ alternative methods to help prevent the disease from becoming more widespread in their fields. Since the pathogen can survive in crop debris, Dutta recommends farmers bury their crop debris when their spring and fall crops are harvested.

Because the disease propagates and spreads through overhead irrigation, growers should use drip irrigation or a form of subsurface irrigation to help reduce the splashing effect of the pathogen, Dutta said.

Excessive rainfall Georgia in January and February led to outbreaks of the disease this year.

“We have to try to manage this issue with good resistance-management techniques, such as rotating different modes of action in order to preserve the chemistries that we have,” said Jeremy Kichler, Colquitt County Extension coordinator. “Hopefully, if we implement good resistance-management strategies, then we can effectively manage this disease.”

According to the UGA Center for Agribusiness and Economic Development, cabbage production in Colquitt County accounted for more than $42 million in farm gate value in 2017. Colquitt County, which produces approximately 6,500 acres of cabbage in the fall and spring, has experienced severe disease outbreaks.

The production of brassica crops is a profitable industry for Georgia farmers. According to the UGA Center for Agribusiness and Economic Development, the state farm gate value for cabbage was $53.6 million in 2017.

Georgia is not the only state experiencing problems with Alternaria leaf blight. As early as 2015, broccoli growers in Virginia’s Northern Neck region reported severe Alternaria head rot in fields where Quadris was the primary fungicide used. During 2015 and 2016, some growers experienced complete crop failures from this pathogen. 

Virginia Tech researchers led by Steve Rideout, director and vegetable plant pathologist at Virginia Tech’s Eastern Shore Agricultural Research and Extension Center in Painter, Virginia, have determined isolates of Alternaria that possess resistance to Quadris. 

To learn more about vegetable production in Georgia, see http://extension.uga.edu/topic-areas/lawn-garden-landscapes/fruits-vegetables.html.

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