Nature, man combine to reduce yellow dwarf The drought conditions that have delayed wheat planting in the Mid-South may be something of a blessing in disguise - if Mother Nature finally relents and allows some rainfall to occur before December.
In general, university and Extension plant pathologists recommend that growers delay planting wheat to reduce the potential for yellow dwarf virus. (Scientists dropped the barley that used to precede yellow dwarf some years ago.)
"We're not saying you should plant in December," said Charles Pless, professor of entomology and plant pathology at the University of Tennessee. "But, if you delay planting until the first of November, the numbers of aphids and the disease potential decreases."
Yellow dwarf virus is not as much of a household word among wheat growers as rust or powdery mildew. But, it can infect up to 40 percent of the plants in some fields in some years, according to Pless. Yield losses of 5 to 20 percent may occur.
Aphids transmit the disease to the wheat in the fall, but the symptoms - yellowing or reddening of the leaves - do not begin to show up until the following spring. No fungicide will control yellow dwarf after the plant becomes infected.
"If you grow wheat, you have the potential for yellow dwarf - it goes hand-in-hand," said Pless, who along with UT professor Albert Chambers spoke on yellow dwarf at this year's Milan No-Till Field Day. "The only way to control yellow dwarf is to control the aphids that vector the disease."
Farmers can spray for aphids, but achieving control may be difficult because of the way the insects move in and out of the fields and because not all aphids carry the disease.
From 20 to 25 aphid species transmit yellow dwarf, and identifying the species and the type of insect can be challenging. "You may see a clump of aphids, but it may have only one winged aphid that actually carried the virus," said Pless.
Pless says University of Tennessee research indicates that seed treatments of imidacloprid - Gaucho - insecticide can provide control of aphids and, thus, increase wheat yields in years when yellow dwarf is a problem.
"Gaucho is proving to be an effective seed treatment," said Pless. "With an investment of about $8 an acre, we're seeing yield increases of seven to 10 bushels an acre."
In one study at the Milan Experiment Station, applications of Gaucho 480 F at 0.75 ounce per hundredweight of seed produced an average of 55.8 bushels of wheat per acre, compared to 45.4 bushels for plants that did not receive a seed treatment.
In another, 0.5 ounce of Gaucho produced an average of 78.8 bushels of wheat per acre vs. 70.1 bushels for the no-insecticide-treatment plots and 76.6 bushels per acre for plots that received 0.75 ounce of Gaucho per hundredweight of seed.
Applications of thiamethoxam - Adage 5FS, an experimental compound - produced yields of 79.8 bushels per acre in the same test.
Researchers planted FFR 555 variety wheat in early October in both studies.
Natural predators, such as lady beetles, can also help reduce the risk of yellow dwarf infections, but the amount of activity can vary with weather conditions, says Pless.
Now, if Mother Nature will just sending some rain....