Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Viruses cause a wide range of symptoms in spring wheat and barley. The symptoms that develop will depend upon the strain of virus, crop variety, and cultural environment. Viruses in small grains are mainly transmitted by feeding aphids and mites that vector the virus. Once the virus enters the healthy plant it begins to replicate itself and moves cell to cell. Small grain virus infections can induce stunting, rosetting, mosaics, chlorotic streaking, yellowing, and necrosis. Chlorosis ranges from light green to yellow and in some instances white. Symptoms of virus infection are difficult to distinguish from the signs of malnutrition and adverse weather effects.

Barley Yellow Dwarf Virus
Barley yellow dwarf virus (BYDV) is transmitted by aphids. It is easily confused with a nutritional disorder or the effects of adverse weather. Visible symptoms are contingent upon the time of infection. Leaf discoloration in shades of yellow, red (seen on oat plants), and purple (rare) appear from the tip to the base and along the margins to the midrib. The earliest infection by BYDV is the 4- to 5-leaf stage resulting in slow growth and delayed maturity. Infected barley and wheat plants appear as a conspicuous yellow whereas infected oat plants have a reddish hue to their leaves. This disease is more pronounced during in cool weather (60Ð80¼F) with ample sunlight. Barley yellow dwarf virus is carried by the oat birdcherry aphid and infestation coincides with large migrations of aphids. Infection sites can appear as stunted yellow single plants or as groups or clusters of yellow plants among healthy plants. This disease can be confused with aryloxyphenoxypropionate, benzonitrile, imidazolinone, and sulfonylurea herbicides. See photos 42 and 43.

Photo 42 Aphids found on stunted and discolored plants are a good diagnostic clue of the presence of BYDV; however, this clue is not a definitive diagnosis.

Photo 43 Leaf discoloration in shades of red, yellow or purple that runs from leaf tip to base and from midrib to margins is typical of BYDV.

Wheat Streak Mosaic Virus
Wheat streak mosaic virus (WSMV) occurs on spring wheat, barley, corn, rye, oats, and numerous annual and perennial wild grasses. It is the most serious on winter wheat grown in the Great Plains. The virus is vectored by the wheat curl mite (Eriophyes tulipae, [synonym] Aceria tulipae). Infection sites depend on three factors: 1) the population of the wheat curl mite; 2) proximity of virus infected grass plants, especially volunteer wheat; and 3) proper moisture levels that keep wheat actively growing where mites are maintained at high populations. Symptoms will vary according to the time of infection and the distribution of wheat curl mites. Many times, only margins of fields are infected. Infected spring wheat or barley plants are stunted, with mottled green-yellow streaked leaves. The streaks are parallel and discontinuous. Leaves and sometimes entire plants may become yellow and necrotic. Stunting and yellowing become more obvious as spring temperatures rise. The mite vector prefers to feed on the upper surface and near the margin of leaves. Mites feeding near the leaf margins causes the leaves to curl upward and toward the midvein. Crop injury from WSMV can be confused with injury symptoms of imidazolinone and sulfonylurea herbicide families. See photo 44.

Photo 44 Plants infected with WSMV have green and yellow streaks that run parallel and are discontinuous on the leaf. Healthy leaf appears at the bottom.

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