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DFP-RonSmith-FloodedFields.jpg Ron Smith
A wet planting season that extended into summer created ideal conditions for disease pathogens to develop in West Tennessee cotton. UTIA Extension specialist Heather Kelly recommends consistent scouting to develop a management plan.

Wet season puts crops at risk of foliar diseases

Scout for cotton disease before spraying

Don’t be too quick to pull the trigger on target spot sprays in West Tennessee cotton. But be prepared to apply fungicides if symptoms, field history and weather patterns indicate.

Other crops are at risk, too, the result of an unusually wet spring and early summer.

“We had our first reports of target spot after (tropical storm) Barry,” says University of Tennessee Institute of Agriculture associate professor and Extension specialist Dr. Heather Kelly.

“We’ve had the heat and the weather that target spot likes,” Kelly says. “We had a good shower yesterday (July 29) and canopies will not dry out much today. Scout, but don’t spray too soon. Producers who sprayed before Barry probably had to spray again.”

Favorable weather

She recommends that if a grower sees lesions and has a field history of target spot infection, in addition to favorable weather for diseases to develop, fungicide application might be justified. Growth stage spraying will not be as significant for disease control as scouting for the disease. She says the typical window for target spot development and fungicide application in Tennessee is the third week after bloom, in addition to the other cautions.

“If they are already picking up target spot the third week and have the field history, they might go ahead and spray,” she says, but cautions that she has picked up target spot later in the past two years, five weeks after bloom. “We see it a little later in West Tennessee. It’s a field-by-field decision and scouting is the key. If they see lesions, they should assess growth stage and the weather to make fungicide application decisions.”

Weather has been conducive to target spot, “soggy and hot. Still, with cotton, we do not want to go too soon. If a tropical storm is forecasted, we may want to get ahead of that. If growers are not likely to get back in the field for two weeks, they might want to get ahead of it.”

She recommends patience. “I want to be mindful of pricing and the bottom line for farmers. Our guidance is to scout, save money and hold off on fungicides until needed.”

Kelly also notes that target spot often looks much worse than it is. “With target spot, it often pays to hold off. Our surveys show that fungicides are needed less than 20 percent of the time to protect yield. It looks bad, a lot of defoliation, but yield is still there.”

Other pathogens

Weather has been a challenge throughout spring and into summer, Kelly says, setting the stage for disease infection. “We’ve seen more Verticillium wilt than we would have expected, too. It’s not widespread, and even in fields where we found it, we’re not finding a lot.”

She adds that waterlogged fields could have resulted in potassium deficiencies. Standing water prevented plants from getting to potassium and allowed leaf spot to come in. “We expect that to clear up as fields dry.”

Kelly also cautions cotton growers to be aware of a new disease, cotton leaf roll dwarf virus (CLRDV), vectored by aphids. “We’ve identified it for the first time in Tennessee. We are not sure of the effect on yield. We identified it in a research field in Jackson. But a week later, we could not find symptomatic plants. Cotton looked fine.”

The virus has been identified in other parts of the Cotton Belt, including Mississippi, Alabama, Florida and Georgia.

“We hope to begin processing samples in-house here in Jackson, instead of sending them off to a lab. We can do more research in-house. We want to find out what it means when symptoms disappear. Is it still there or has plant defense mechanisms kicked in? It’s not clear now what is going on in the field.”

Monitor fields

Kelly says this is not a doomsday issue. “It’s just one more thing we have to monitor. Some areas have identified fields with significant yield loss, she says. “But other things may have been going on in those fields, too. And other fields that tested positive for the virus didn’t show an effect on yield.”

Typically, early infection means the highest potential for yield loss. “That might not be the case with this virus,” she says. “We’re not sure. Our symptoms in positive fields faded away. They did see some yield loss in Alabama, but it is not clear how much can be attributed to the virus. It came on late in Mississippi and no yield loss occurred.

“We do know that commercial varieties are susceptible. And it is present in Brazil. Testing is ongoing in Alabama and other places.”

Kelly says it is concerning that in Brazil the virus overcame resistance that was deployed in varieties there.

Other crops

She notes that growers have identified some disease in corn. “But nothing can be done about it. If the corn was at high risk and producers sprayed at tassel, it’s probably okay. If fields were at low risk, it’s probably okay.”

She says soybeans are at high risk for disease infection this year. “Growers should know their varieties and know what to scout for.

“We are seeing some frogeye showing up in research plots. That usually occurs at the mid- to late-R-3 stage. If producers are not seeing disease symptoms, they should hold off until R-4 if they can.

“Be mindful of the weather. If they don’t scout for two weeks, they may see more problem than expected.”

Kelly says with current soybean markets some producers are spraying with less expensive fungicides to save money. “I recommend combo fungicide applications. Do not use just a straight strobie/QoI; it will fail because of resistance in the frogeye pathogen, and Septoria brown spot also has resistance to that fungicide group. We have a lot of great options that all do a good job, some better than others.”

She recommends growers consult a fungicide efficiency table that rates each fungicide for each disease, from poor to excellent. “Then scout to pick the product, based on the disease and the variety that will have the best efficacy.”

It’s been a busy spring and early summer, Kelly says, and she recommends farmers continue to scout consistently, know their varieties’ susceptibilities, and watch weather patterns to develop a sensible and economical fungicide application plan.

It’s not a good year to waste money.

TAGS: Crop Disease
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