The adage “it’s all in the timing” is never truer than when talking about peanut disease and nematode control.
And while improved varieties have been wildly successful in controlling these pests, they still represent the single largest input cost for growers.
Georgia 06G, Georgia 07W, Florida 07 and Tifguard all have a nice level of resistance to tomato spotted wilt virus and other pests, but growers still depend on fungicides for controlling leafspot management, says Albert Culbreath, University of Georgia plant pathologist.
“We don’t have varieties with a high-enough level of leafspot resistance that we can get by without spraying a crop. We’ve found a considerable level of moderate resistance in some of the lines, and we’re finding more tolerance, as well,” says Culbreath.
Early and late leafspot are very adept at knocking the leaves off the plants, he says, and yield reduction is very closely correlated with the amount of plant defoliation.
“The variety Tifguard has a low to moderate level of resistance to leafspot. Even the ones that are susceptible appear to be hanging onto their yields even in the presence of greater levels of defoliation. That tolerance could be used in programs such as the Peanut Rx program, a risk index for peanut production.
“We’re looking at what might be that critical level of defoliation with some of these new varieties. With the old variety Florunner, basically as soon as you started seeing noticeable defoliation, you knew you were losing pods. That doesn’t appear to be the case with these new varieties,” says Culbreath.
Peanut Rx — the result of a collaboration between Southeastern land grant universities and several fungicide manufacturers — continues to be evaluated and updated by researchers, says Bob Kemerait, University of Georgia Extension plant pathologist.
Fungicide amount based on risk
The program, he says, is based on putting the right amount of fungicide into a field based on risk.
“If you have a high risk from short rotations and growing soybeans and less resistant varieties, you need to spray at least seven times, and in a wet year, maybe eight or nine times to maintain yields,” says Kemerait.
But if you’ve got good rotation, resistant varieties and good management, you may be able to get by with only four or five sprays, he adds. “The important thing is that the prescription program is a tangible way to benefit from using good management.”
Traditionally, and prior to 2006, most growers were spraying their peanut crop an average of seven times a season with fungicides.
Soil-borne diseases also continue to be costly for peanut producers to control, says Tim Brenneman, University of Georgia Extension plant pathologist.
“Peanuts are unique in that they flower above ground and set the crop underground, and that works very well except it exposes them to a lot of pathogens and a lot of diseases, because these pathogens reside in the soil,” says Brenneman. “We’re working with root-knot nematodes, CBR and white mold or stem rot.”
Georgia 06G, which is grown on 80 percent of Georgia’s acreage, is highly susceptible to root-knot nematode, he says. “To compound the issue, we’ve lost Temik, one of our best nematicides. Thankfully, we have the Tifguard peanut variety, which has a remarkable level of resistance to root-knot nematodes,” he says.
CBR or cylindrocladium black rot is scattered around the state, says Brenneman. “It’s not in every field, but if you have it, it’s probably your No. 1 concern because it’s so devastating. It’s a root-infecting pathogen, so if you have it, it’ll affect young secondary roots, eventually move into the taproot of the plant, and end up killing the whole plant.”
CBR is affected by cool, wet conditions, he adds. “We had unfavorable conditions this past year for CBR, especially at the beginning of the growing season. Even though Georgia 06G is very susceptible, we saw very little in growers’ fields in 2011. As the weather changes, the disease spectrum also changes,” he says.
There’s also stem rot or white mold, which is now the most damaging peanut pathogen in the state, says Brenneman. When conditions are favorable, the fungus can grow rapidly down the rows, killing stems above ground and rotting pods below ground.
Crop rotation also can reduce the disease, but when conditions are favorable, it can still cause significant losses. Hot weather favors disease development, and the hotter summers in recent years have led to severe epidemics.
Other line of defense
“The other line of defense against stem rot is the use of fungicides,” he says. “We have very good fungicides, but the problem is getting them to the target where they need to be. Infections are down on the bottom of the plant, the pods and lower stems. You can have the best fungicide in the world, but it’s of little use if you can’t get it down where it needs to be.”
One method recently developed by the University of Georgia is to spray fungicides at night when the peanut leaves are folded, and when high relative humidity reduces evaporation of spray particles, allowing them to be deposited deeper in the peanut canopy. This has resulted in greatly improved control of white mold with no additional fungicide inputs, says Brenneman.
“In the daytime, the leaves are opened, and you get that interlocking layer of foliage and canopy. It’s a very difficult crop to spray down through. At night, those leaves are folded down, and you can much more effectively spray and get your fungicide down to where it needs to be.”
Researchers also are looking at making fungicide sprays for white mold early in the season, says Brenneman.
“When you think of white mold, you normally think of the noticeable symptoms that are found below ground. What we’re realizing now is how early that fungus can begin infecting a plant. With hot conditions, the fungus infects earlier, and it’s below ground. You won’t see it at this time, but the fungicide will work better if you get it out there ahead of the disease. When disease gets started, and you’re trying to shut it down, it’s much more difficult to control.”
These fungicide applications are applied in a high volume of water and a narrow band over the young plant, about three weeks after planting, he explains. At this time, there is little foliage, and the fungicide can be concentrated on the crown of the plant and even washed down toward the roots.
These products are systemic, therefore moving into the plant and back up with the growing tissue.
Most of these sprays, says Brenneman, are also effective on leaf spot, thus allowing fewer sprays later for foliar diseases. “In limited trials, this application has dramatically improved white mold control, particularly with the hotter temperatures early in the season like we’ve been experiencing.”
Under current spray programs, growers usually don’t spray at all until 30 to 40 days after planting. “But we don’t think about white mold until about 60 days after planting. Last year, we discovered that we need to be thinking about white mold as early as 21 days after planting. We’re re-evaluating the entire schedule, and we’ll probably be changing things in the next few years.”
(Additional information on peanut disease control can be found at http://southeastfarmpress.com/peanuts/disease-control-still-critical-peanut-production. To see what a disease-resistant peanut variety is worth to a grower, see http://southeastfarmpress.com/peanuts/placing-value-disease-resistant-peanut-varieties-0).