Tar spot ‘bogeyman’ knocking on Virginia border
PAINTER, Va. — The “bogeyman” of corn diseases for Virginia and the Mid-Atlantic region is tar spot which has caused big yield losses in the Midwest.
It has not been seen in Virginia yet, but it is on the border, according to Dr. David Langston, plant pathologist and director of the Tidewater Agricultural Research and Extension Center in Suffolk, Va.
Langston discussed several minor and major diseases affecting corn at the March 31 Corn Specialist Day at the Eastern Shore Agricultural Research and Education Center.
Most fungicides work against tar spot and “in some years, a second application of fungicide did pay for itself against tar spot in the Midwest,” he said.
“All fungicides are preventive,” he stressed. “Unlike insects which you spray when you see them. You suppress disease. You don’t want to get it.”
He recommended timing fungicide application at the VT/R1 stage for the best return on investment. “Earlier sprays typically don’t have that return on investment even though it’s easier to spray earlier.”
To minimize corn diseases, he recommended rotating with non-grass crops every two-to-three years, planting the most disease-resistant varieties, spraying at VT/R1 stage and scouting for tar spot through the season.
“Nematodes are problematic” but the damage will not be standard across the field, Dr. Langston said. “It’s going to be up and down – very variable.”
Sting nematodes, which occur only in “almost beach sand-type soils,” are “absolutely the most damaging nematode there is,” he said. “If I find 10 sting in a sample, you’ve got trouble.”
Root knot nematodes are “hard to rotate away from. Most everything we grow is susceptible to root knot.”
He said soil samples, best taken right after harvest, can determine which nematode is present. He recommended that samples be kept moist and cool. The samples need to be fresh, so overnight shipping is best to assure a live sample and more accurate numbers.
Trials testing nematicides Counter, a granular product, and Velum, a liquid have so far generated inconsistent results and more trials are planned.
Soil sampling is essential to nematode management, he said. In addition, he advised cultural practices such as good sanitation, crop rotation, avoiding drought stress and maintaining good fertility.
As to chemicals, he said, “You have got to have a bad nematode problem to even think about pesticides. Seed treatment, both chemical and biological, may have a very subtle effect on nematodes but they are not anything to write home about.”