Barley yellow dwarf virus (BYDV) is an annual threat to wheat throughout the U.S. and wheat growing areas of the world. It can routinely cut yields 10-20 percent and double that in moderate to severe cases.
Though it has been around a long time, there are still few remedies for the viral disease.
The virus is only known to be transmitted by aphids, and Virginia Tech Entomologist and IPM Coordinator, Ames Herbert says managing the aphids offers growers the best opportunity to manage the virus.
Symptoms of BYDV vary from crop to crop. In wheat, the most common symptoms are yellow to red discoloration that begins at the leaf tip and progresses to the base. The last leaf on the plant, or flag leaf, often becomes permanently discolored.
When wheat is infected by BYDV at an early growth stage it often becomes severely stunted and reduces head size.
Barley yellow virus is yellow when it infects barley, but when wheat is infected the leaf tips tend have a reddened coloration. An undulating stand of wheat is a tale-tale sign of barley yellow virus. The low areas of the field represent areas that are infected by barley yellow dwarf virus.
In the upper Southeast four species of aphids are known to vector BYVD. These include: green bug, English grain aphid, cherry-oat aphid, and corn leaf aphid. Herbert points out that each of these aphids has its own life cycle and biology. Any of these aphids can vector the virus.
BYDV is a complicated virus. Five different strains are known to infect wheat. Any of the vector aphids can carry any of the strains or combinations of strains.
The two most prevalent BYDV-carrying aphids in the upper Southeast are the English green aphid and the cherry-oat aphid — sometimes called great cherry-oat aphid or bird cherry-oat aphid.
Herbert says these aphids typically infest crops like fescue. When these crops begin dying out in the late summer and early fall, aphids move on to other crops. In the upper Southeast, he says October is a critical month for aphid migration into wheat fields. Spraying foliar insecticides in late October and November is critical to management of the insects and hence BYDV management.
BYVD is a disease that growers can manage by managing aphids, Herbert says. “Our research over several years and several sites, show that November application of Warrior almost eliminated BYDV. We tell gowers to spray for BYDV-carrying aphids by Turkey Day or by late November.
“By comparison, if Warrior, at the same rate, is used in February, we saw very little control of the virus. And, by waiting until March to spray was a waste of time and money,” he adds.
“If you spray in February, you are spraying dirt, and you have to spray insecticides on green plants to get aphid control and BYDV management. It is an amazing thing that we can spray wheat in November and get aphid control through April,” Herbert stresses.
Herbert notes excellent results from seed treatment using Gaucho. This treatment, he says did a good job of eliminating aphid buildups and subsequent yield data indicated it had a positive effect on BYDV management.
Since the original research was conducted several more options have become available to growers to control fall aphids in wheat.
Baythroid and Mustang Max are new pyrethroids that have shown to be excellent in fall aphid control. In addition, a new seed treatment, Cruiser, has also shown good control of aphids in wheat.
Growers can also decrease risk to BYDV by planting wheat later in the fall.
Herbert explains that early planted wheat is green and growing when aphids are at peak movement. It only takes a few aphids to spread BYDV in the fall, making scouting and control essential to managing the disease.
Though growers aren't likely to pick a variety based on BYDV resistance, Herbert says it is important to know which varieties are most susceptible and manage aphids more intensely in the more susceptible varieties.
Low seeding rates and/or low plant populations make wheat more susceptible to BYDV simply because the aphids can move around the plants more easily and pick and choose feeding sites.
A late, warm fall allows aphid populations to stay in the field longer and build up higher populations. Likewise, a mild winter allows aphids to survive cold weather, as does a warm spring.
Herbert says fields with a history of virus problems are more susceptible to future outbreaks because the virus is surviving in the summer in summer host plants. So, unless adjacent pastures and other host sites are removed, the virus is likely to reoccur in these fields year after year.
Little can be done to save diseased plants after infection. Losses to BYDV will depend on variety susceptibility, growth stage at the time of infection, and persistence of cool weather conditions that favor disease development.
Scouting and identifying aphids in the fall can give growers some insights on insect management. The primary aphids vectoring BYDV in the upper Southeast are easily identified and distinguishable from one another.
The English grain aphid is green, yellowish-green, red, or purple. In the adult stage these aphids are 2.5 to 3 mm long. Cornicles and antennae are black, legs mostly black.
These aphids infest leaves early but suck sap from grain heads as soon as heads emerge, causing stunting. Among the many host plants are cereals, roses, cattails and wild grasses.
The cherry-oat aphid is olive-green with a red-orange patch surrounding the base of each cornicle. Old, wingless, over-wintering adult aphids may appear black.
BYDV is one problem that growers can definitely manage by managing the aphids that carry the disease. The primary weapons in aphid management are either seed treatment or foliar sprays. And, the key to foliar sprays is to get these insecticides on the plant after the plants are up and growing, but before aphids can transmit BYDV.