Growers and consultants can map root-knot nematode variations in their fields for about $40 an acre by examining cotton roots for galls and recording their location with a GPS, according to Allen Wrather, a plant pathologist at the Delta Center in Portageville, Mo.
Wrather, speaking at the Conservation Systems Cotton and Rice Conference in Tunica, Miss., said the maps can then be used to apply nematicides only in the parts of the field infected with nematodes. Nematodes can be controlled with Temik, the fumigant Telone or Avicta, a seed treatment.
Root-knot nematodes enter cotton roots at the growing point of the root and establish a homesite, notes Wrather. This causes the cells of the root to swell, which interferes with the movement of water and nutrients into the plant. In severe infestations, this results in stunted plants. Galls are easily spotted on the roots of infected plants.
Research has shown that root-knot nematodes are rarely distributed evenly across a cotton field. “We have found that often the root-knot nematodes are restricted to half-acre- to acre-size areas. In many cases, only 10 percent of a field may be infested with root-knot nematodes. So when you put out a nematicide uniformly, you're wasting product. You're putting it on 90 percent of the field and only needing it on 10 percent. We need to put out these products site specifically because they are expensive.”
The traditional way to map nematode variation is to walk the field, take soil samples and have them analyzed. “But in Missouri, that is going to cost about $100 an acre, including a $20 cost to analyze the soil sample. And to get an accurate picture of where the nematodes are, you have to sample no more than every 100 feet. Sampling on 2.5-acre grids doesn't even begin to work. You're often going to miss where the nematodes are.”
The gall-mapping method employs a scoring system based on damage to roots on a scale of 1 to 6. For example, a rating of 1 would be free of nematode damage while a 6 would have every root infested.”
The system was developed in California and adapted for the Mid-South by University of Arkansas nematologist Terry Kirkpatrick and University of Clemson plant pathologist John Mueller.
One advantage of this method is that growers would not have to send a sample to a lab, according to Wrather. “Just walk out to the field with a GPS, grid sample, dig up 10 roots where you stop and evaluate them. If there is root-knot there, there will be galls on the roots.
“It's not going to be cheap because you need to sample every 100 feet. If you pay a consultant to do it, it's going to cost around $40 an acre, which is still quite expensive. But we're confident that root-knot nematodes stay in certain parts of the field. So if you've mapped a field, you may not have to map it again. So the $40 mapping cost could be a one-time expense.” The grower can then use the geo-referenced map to site specifically apply nematicides.
Researchers are also investigating whether aerial imagery could indicate the presence of root-knot nematodes, according to Wrather. “In two fields, one in northeast Arkansas and another one is southeast Missouri, we took an InTime (a precision agriculture provider) image of the field and grid sampled a portion of it to see if mid-season plant growth could help us determine if plants were stressed due to root-knot nematode.”
The results were not promising, according to Wrather. “In the Arkansas field, the remote image was only able to predict about 33 percent of the variability in root-knot nematodes in the field. In the southeast Missouri field, we were able to predict 55 percent of variability in the field, still not enough to develop an accurate nematode map. Something else is needed.”