Without even a chill in the mid-November air, Arkansas wheat fields are crawling with aphids. This is a concern. Not because the aphids are doing much harm to the wheat themselves, but because the pest carries the barley yellow dwarf virus. And the earlier a wheat field becomes infected with barley yellow dwarf, the more severe the symptoms later.
“This year, unfortunately, what would be normally considered late-planted wheat runs the risk of barley yellow dwarf, too,” says Gene Milus, University of Arkansas associate professor of plant pathology. “The warm weather is really setting the virus up nicely.
“Basically, if aphids come in at this time of year and they're carrying the virus, there's little question the virus will be established in a field. With such warm weather, the aphids will multiply and spread the virus around. If you're seeing a lot of aphids, chances are the virus is being spread.”
Aphids usually vector the virus to a wheat plant where it will multiply. Aphids that feed on the infected plant can pick up the virus and then transmit it to other plants.
“That's why you usually see patches of barley yellow dwarf in the field the following spring — the initial infection occurs in one spot and spreads from there,” says Milus. “The plants in the center of those hot spots are more severely stunted than the ones at the outer edges.”
If plants are infected in the fall, there likely won't be any symptoms until the plants begin growing again in the spring. At that time, the plants will be slower to grow, will tiller poorly and appear to be nutritionally needy. The wheat won't have distinctive symptoms of barley yellow dwarf until heading or when the flag-leaf appears. At that point, the typical yellowing beginning at the leaf tips. Infected leaves are usually stiff and upright compared to healthy ones.
“Another thing is the root systems are even more stunted than the tops,” says Milus. “If you see stunting in the foliage, the root system will be affected even worse. As a result, the plants with barley yellow dwarf will run out of water during grain fill. They'll dry up earlier than normal with poor grain fill.”
Once a plant is infected, there's nothing that can be done.
“That's where resistant varieties have potential and why they're needed. We're looking for plants that can become infected but don't allow the virus to build enough to mess up the plant's physiology. We want a normal yield even though the plant is infected.
“I've got a germ plasm enhancement project being funded by the Arkansas Wheat Promotion Board. We're trying to develop some varieties that are resistant to barley yellow dwarf.
“There are some wheat lines already with some moderate levels of resistance. There are also a couple of advanced lines out of the Purdue breeding program that supposedly have a high level of resistance. I'm testing those this year.”
Milus normally plants his lines early (late September) to make sure aphids are attracted. That usually means a good infection of barley yellow dwarf in the early-planted plots.
As for the other plots put out for disease evaluations, Milus wants to avoid barley yellow dwarf.
“To prevent that, we treat seed with Gaucho and usually go back with a foliar insecticide application to try and get plots through the winter without barley yellow dwarf infection.
“If the disease is introduced in the spring, it isn't nearly as devastating as a fall infection.
“This year, we've already sprayed some of our plots. We'd have sprayed even with low levels of aphids, but with the high populations being reported, we needed to go ahead and pull the trigger.”
Among other areas, Milus works test plots in Jackson County, Clay County, and St. Francis County.
“Rhone, a variety out of Virginia, has a pretty good level of barley yellow dwarf resistance. We actually use Rhone as a resistant check in our plots. The lines we're working with are very similar to Rhone.
“This year, we're crossing our lines with Rhone and Coker 9663 — another variety that seems to have some resistance to barley yellow dwarf.
“We're hoping that if these different sources have different genes for resistance, we'll be able to select a line with a higher level of resistance.”