While cotton growers are trying to determine the best combinations of transgenic seed varieties to ease pressure from weeds and most insects, soil-borne diseases accompanied by root-knot nematodes should also be considered as major threats to yields.
Verticillium wilt, Fusarium wilt and nematodes can easily reduce returns per acre, says Terry Wheeler, Texas A&M University plant pathologist in Lubbock. She works in a region that grows about 25% of the nation's cotton crop, including a record production of 5.9 million bales in 2005.
But that region, as well as the Mississippi Delta and other production regions, is seeing increased invasion of diseases along with nematodes. For nematodes, the best prevention comes from a seed treatment or application of Temik 15G or AVICTA at planting. For Verticillium, Wheeler suggests planting varieties that have proven tolerance to the disease.
Nematodes are microscopic bugs. More than 250 bugs per pint of soil can cause problems, says Wheeler.
Eggs survive in roots or soil during the winter and hatch in the spring to produce second-stage juveniles. They can move to cotton roots, infect them and cause the root to swell and form galls around the nematodes as they mature. Galls can reduce plant performance if formed in high numbers on young cotton plants. They make it more difficult for the plant to transport water and nutrients from the soil.
“If you get into a dry summer, the lack of the ability to pull water up hurts the crop,” says Wheeler. “Nematodes keep the root system from fully functioning.”
Soil samples from three areas of a field should be taken in the fall after cotton harvest, or harvest of corn, soybeans, peanuts or grain sorghum, all of which are host crops.
If treatment is needed for the following year, Wheeler recommends Temik 15G from Bayer CropScience, a long-time successful pesticide that is proven against a host of insects, including nematodes. Another treatment available this year is AVICTA from Syngenta Crop Science. It can be part of an AVICTA Complete Pak to help fight disease as well.
Wheeler says the input costs of Temik or AVICTA can pay for themselves many times over by preventing major crop losses.
“A typical application of 3-5 lbs. of Temik will cost about $11-15/acre,” she says. “For a 120-acre field of cotton that's about $1,320-1,800. But if the field isn't treated and suffers a bad nematode infestation, losses could be from $3,000 to $12,000.”
She says the cost of AVICTA Complete Pak is about $15-19/acre. Similar control results can be seen with that treatment.
Craig Rothrock, University of Arkansas plant pathologist, says seed treated with AVICTA Complete Pak can enhance planting along with providing nematode control.
“I think farmers are always looking for a way to speed up the planting operation,” he says. “Any time you have to apply an additional product, such as an in-furrow treatment, it means an additional stop for the tractor driver. AVICTA Complete Pak, with nematicide, fungicide and insecticide on the seed, will speed up the planting operation tremendously.”
Other diseases often come with nematode problems, such as Fusarium wilt. Resistance traits in some varieties may decrease the amount of problems associated with Fusarium in cotton. However, when the Fusarium wilt is severe, yield loss and plant mortality may still be unacceptable, says Rothrock.
Fumigation with Telone II from DowAgroscience before planting can control nematodes and thus reduce other disease problems such as Fusarium. However, fumigation is expensive and Wheeler says she isn't sure if it can control Verticillium.
Early symptoms of Verticillium include yellow leaves, cracked stems and discoloration. Following the seedling stage, older plants exhibit a chlorotic mottling on the leaf margins and between the major veins.
Yellow progresses inward, followed by brown, and the leaf finally dies. Severely affected plants shed all their leaves and most of their young bolls. These plants may survive throughout the growing season and send out young sprouts or shoots from the base of the plant.
“Although no variety is immune to the disease, variety choice is important,” says Wheeler, who conducted extensive Verticillium wilt variety resistance tests in 2005.
She measured the degree of wilt infestation for each variety along with overall yield. The top 10 performing varieties within Verticillium infected fields were: FiberMax 960BR, FiberMax 989B2R, FiberMax 989BR, DeltaPine 455BR, FiberMax 960RR, Paymaster 2379RR, DeltaPine 5690RR, FiberMax 960B2R, Paymaster 2167RR and All-Tex Atlas RR.
“Disease is generally worse in the wettest area of the field, however, once the pathogen builds up in the soil, any part of the field can be affected,” says Wheeler.
She says crop rotation may slow down its buildup, “but once there are high levels in the soil, then crop rotation will not help manage the disease.”
Verticillium wilt is difficult to distinguish from Fusarium. Leaf symptoms are similar and the internal tissues of the stems are discolored in both diseases. “One difference is that Fusarium is usually seen about two weeks earlier than Verticillium,” says Wheeler.