Just as in weed management the adage “start clean and stay clean” applies to tackling the guava root knot nematode which has become a problem pest for North Carolina sweet potato farmers.
The guava root knot nematode has been found on sweet potato farms in eight counties in eastern North Carolina prompting the North Carolina Department of Agriculture to issue an internal quarantine of the invasive pest last October in all 100 of the state’s counties.
The quarantine, which remains in effect one year later, requires sweet potato seed roots to be inspected and certified free of guava root knot nematode before leaving the state. Sweet potato slips are required to have no soil or root on them before they can leave the state. The quarantine does not affect fresh sweet potatoes leaving the state.
The North Carolina sweet potato industry has remained diligent in managing the guava root knot nematode to limit its spread and impact on the crop. North Carolina State University has worked closely with sweet potato farmers on seeking ways to control the pest.
At the North Carolina Sweet Potato Field Day Oct. 3 at the Cunningham Research Station in Kinston, Dr. Adrienne Gorny, a nematologist at North Carolina State, offered guidelines for managing the root knot nematode in sweet potatoes. The main point she emphasized was to start clean and stay clean and to use soil tests to determine if the guava root knot nematode is present.
Site selection is critical with Gorny encouraging farmers to start clean and stay clean by planting in fields with no history of guava root knot nematode. “To determine if guava root knot nematode is present, soil sampling is essential,” Gorny said.
“To sample a field, we’re recommending 20 soil cores per five acre spread. Collect from about the four to eight-inch depth. If you collect them in zig zag or W shaped pattern, across the field, that’s going to be best. The guava root knot nematode is usually not evenly distributed over the entire field. They generally occur in hot spots or patches of high populations so if you sample a field in a zig zag pattern that’s going to increase your odds of crossing over a hot spot,” Gorny explained.
The best time to sample is late summer or early fall because populations will be higher at this point and easier to detect.
Once the soil samples are selected, Gorny said it is critical to keep them cool in a refrigerator, but don’t freeze the samples because this will kill the nematodes and the lab won’t be able to detect them.
Send the soil sample to the North Carolina Department of Agriculture nematode assay laboratory that will detect the nematode through a DNA-based test to determine if nematodes are present. Gorny said this test is above and beyond the standard nematode analysis and you need to specifically request the test on the submission form.
Gorny said planting in seed beds with no history of guava root knot nematode is the best defense. That’s why soil sampling is so important. Planting certified seed with no symptoms of root knot nematode such as bumps, and galls is also critical.
“When cutting slips, cut those above the soil line with no soil and no roots because that’s where the nematode lives. It can help to disinfect your cutting knives in a solution of 10 percent bleach and hot water before beginning work and periodically throughout the day.
“If your workers take home sweet potatoes at the end of the day, try to discourage them from discarding any they don’t want back into the field, especially if those sweet potatoes came from a different field,” Gorny advised.
Limiting the spread of infested soils is critical. Gorny noted that the nematode is very small and can’t move far on its own (studies suggest it only moves three feet per year on its own). The way it moves long distances is travelling by people who carry infected soil or infected plant material.
Gorny advises sanitizing equipment after visiting infected fields. She suggests washing down tractor and truck tires with a soliton of 10 percent bleach and hot water from a power washer.
“Even a strong blast from a hose is going to be better than no washing at all,” she said.
The infected crop should be removed from the field. Removing sweet potatoes and as much plant debris as possible is important because debris left in the field gives the place for the nematode to call home in the winter and provide a food source they need to survive. “The guava root knot nematode can also feed on many weeds, so good weed management will help remove a food source,” she said.
If the nematode assay report shows the guava root knot nematode is present in the soil, Gorny recommends a fumigant or a non-fumigant nematicide if the pest is present or if populations are high.