Fig. 1. Yellow or purple leaf tips caused by barley yellow dwarf. Photo by Bill Willis.

Barley yellow dwarf (BYD) is caused by barley yellow dwarf virus (BYDV). BYD attacks a wide range of grass hosts, including wheat, oats, and barley. Oats are usually considered more susceptible than wheat. BYD is frequently serious in southeast Kansas and it is rarely serious in extreme northwest Kansas. In the rest of the state, incidence is quite variable.

SYMPTOMS

BYD often occurs in patches that are 1-5 feet in diameter. Plants in the center of the patch are typically stunted, and plants on the edge of the patch are typically normal in height. However, BYD can also occur uniformly across the field. The primary symptoms of the disease are stunting, and leaf tip yellowing, reddening, or purpling of leaf tips.

The color of the symptoms depends on the variety. For example, wheat varieties like Karl and 2137 express the disease as mostly purple leaf tips. Newton, Scout, and TAM 107 express the disease primarily as yellow leaf tips. In most cases, the discoloration of the leaf tips increases over time until eventually the entire leaf is discolored. The midrib of the leaf often remains green longer than the edges of the leaf. Typically, there is no mosaic pattern on the leaf, but sometimes there is some striping at the border between the discolored leaf tip and the green leaf base. In addition, leaves affected with barley yellow dwarf often have small black spots or streaks spaced randomly over the discolored part of the leaf tip. These are presumably opportunistic infections by bacteria. Infection by BYD is often associated with the occurrence of dark heads with shriveled grain. These occur in small patches similar to BYD patches. It has not been conclusively proven, but we suspect that the dark heads were caused by BYD.

BYD can be confused with other maladies such as wheat streak mosaic or nutrient deficiency. Accurate serological tests for BYDV are available from the KSU Plant Diagnostic Lab for a $10.00 fee. There is a surcharge for out of state samples.

LIFE CYCLE

BYDV has a very broad host range among the grass family, including many perennial weeds and forage grasses. Therefore, the reservoir of BYDV is fairly large. BYDV is carried to small grains by several species of aphids, including greenbugs, English grain aphids, and oat birdcherry aphids.

There are five described strains of BYDV, and each is defined by preferences for different aphid species as vectors. In Kansas, the vector-nonspecific PAV strain is most common. The source of aphids may be local, or aphids may migrate great distances from southern states. Infection may take place in the fall or the spring.

LOSSES

Losses depend on the percent of plants showing symptoms. Often this percentage is overestimated by a "windshield survey." Random samples taken along a transect give a more accurate percentage. Losses also depend strongly on time of infection. If plants are infected in the fall, losses can exceed 35%. If plants are infected after heading, losses are minimal.

CONTROL

The primary control is to avoid early planting. Early planting allows maximum time for aphids to infect the plants in the fall. Plant after the Hessian fly-free date to minimize the risk of BYD infection. The Hessian fly-free date works well against BYD unless there is a very mild late fall which allows aphids to survive longer than usual.

No varieties have high resistance to BYD, but some are more tolerant than others. Under severe BYD pressure, a tolerant variety (rating 4 or 5) might have a loss around 15% while a susceptible variety (rating 8 or 9) should have more than a 30% loss. See "Wheat Variety Disease and Insect Ratings", MF-991, for details on tolerant varieties. BYD is difficult to rate, so variety ratings may be less reliable than with other diseases.

Chemical control of the aphid vectors can suppress BYD. Unfortunately, chemical control has not proved practical. First, multiple applications would be required to achieve satisfactory control. Second, it is not possible to wait for obvious aphid populations prior to spraying because by the time they are detected, significant virus transmission would already have occurred. Therefore, applications would have to be made on a preventive schedule. Given the unpredictable nature of BYD epidemics, it is not economical to make several preventive sprays in the fall and early spring.

A new seed treatment systemic insecticide called Gaucho is labeled for BYDV control. It has shown fair to good suppression of BYD in university trials. The variability in effectiveness is probably due to the timing of aphid infestation. If aphids arrive after the 6-8 week period of protection provided by the chemical, then Gaucho will have no effect. Gaucho is more expensive than other seed treatments, so its use has been limited.

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Web updated 9/01/06
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