As the joke goes, there are only two things certain in life: death and taxes. Another item can surely be added to that list: the presence of some amount of leaf spot in a peanut field.
Bob Kemerait, University of Georgia Extension plant pathologist, says, based on his observations of leaf spot in 2009, this was the first year that he had seen diligent producers fail in some part of their leaf spot control program. Why was that?
“Late leaf spot was very aggressive last year, and wet weather was abundant.” he says. “Typically, what would happen is the grower would not compensate for wet weather by tightening the spray interval.”
A More Conservative Approach
Jay Chapin, Clemson University Extension peanut specialist, says some South Carolina producers need to be more conservative in their leaf spot management programs.
“This an area where we need to improve our game plan,” he says. “We have improved on white mold, but on our most susceptible Virginia-type varieties, we sometimes limp along to the finish line. If the weather turns against us, we may not even make it to the finish line without defoliation and pod loss.
“In South Carolina, we have a challenging combination of some very leaf spot susceptible varieties and consistent late leaf spot pressure. Phillips, Gregory, Brantley, Perry and Champs are highly susceptible to late leaf spot in that order, with Phillips and Gregory being the worst.
“On these varieties, we need to be more conservative, particularly at the 60 days after planting treatment. Options include increasing the Bravo rate to 1.5 pints in tebuconazole tankmixes, adding five ounces of Topsin to Bravo and tebuconazole tankmixes or adding six ounces of Headline to Convoy or tebuconazole treatments.
It’s also important for the fungicide to stay on the leaf long enough.
“How long does the material need to be on the leaf? If you are irrigating, certainly wait a minimum of 24 to 48 hours,” he says.
In a study, Chapin showed that even when the product was only on the leaf for two hours, most products reduced leaf spot defoliation somewhat. However, as a protectant, the longer the fungicide is on the leaf, the more protection is offered.
“Headline was the exception, after only two hours, leaf spot protection was basically as good as when there was no washoff for 24 hours. Headline alone is unreliable for white mold control, but it is exceptional for leaf spot protection in rainy weather.
“If less weather-fast fungicides get washed off, we can be greatful that we probably get improved white mold control along with some leaf spot control, and remember to shorten the interval until the next spray, especially on the varieties mentioned previously.”
Leaf Spot Fungicides
Kemerait says producers using a tebuconazole for leaf spot need to tankmix it with another fungicide.
“It was probably the single most commonly used fungicide in peanuts last year, but it’s also used in soybeans and corn,” he says. “It is less expensive than other fungicides, and used effectively in low-to-moderate risk situations, you can probably come out ahead using it.”
• Compensate for wet weather by shortening spray interval
• Allow fungicide product to stay on leaf for 48 hours if possible
• Tankmix tebuconazole with another fungicide product
• Weigh the cost of the fungicide with the efficacy provided
• Initiate spray program based on planting date and efficacy of fungicide
But premium leaf spot products, such as Provost and Headline, really stand out.
“When you are looking to maximize yields, you have to take the cost of a cheaper fungicide against the efficacy of a more premium fungicide.”
Planting Date And Fungicide Efficacy
Producers should initiate leaf spot fungicide programs based on both planting date and the efficacy of the fungicide.
“You can now wait until 45 days after planting to begin spraying if you use a product such as Headline with greater systemic activity,” Kemerait says.
Additionally, some of the new varieties in the Southeast have improved leaf spot resistance. PG