No. 8 Article 9/May 18, 2007

Winter Wheat: Discolored Leaves

Some of the winter wheat fields that made it through the late freeze are now showing significant disease problems. Robert Bellm, crop systems educator in the Edwardsville Extension Center, reports some fields with 30% to 50% of flags discolored with typical barley yellow dwarf virus (BYDV) symptoms. He notes that aphids were quite active in the area prior to the freeze and suspects spring as opposed to fall infection. While spring infection is typically not as severe a factor for yield loss, the high incidence within these fields may have a significant yield impact. Loretta Ortiz-Ribbing, crop systems specialist in the Macomb Extension Center, also notes active BYDV in western Illinois fields.

Varietal characteristics, nutrient imbalances, and viral diseases all can be causes of leaf discoloration this time of the year. If viruses are going to be a problem, symptoms should be well evident by now. The most common virus diseases we see now are BYDV and wheat streak mosaic virus (WSMV). Each can cause damage to the plants, with BYDV being the most damaging in Illinois.

BYDV and cereal yellow dwarf virus (CYDV). Aphids spread BYDV and CYDV disease. Aphids carrying the virus transmit it to wheat plants through their saliva when they feed. The most serious yield loss results from fall infection by viruliferous aphids' feeding on wheat seedlings. Fall infections typically result in stunted plants and fewer tillers when spring growth resumes. Leaf discoloration is usually the most notable early-season symptom. Leaves may be varying shades of red to purple, pinkish-yellow to brown. As the plant continues to grow, older leaves typically begin to die back from the tip and may feel somewhat leathery while the new leaves begin to discolor. Spring infections occur as well, but they commonly discolor only the flag leaf and typically do not cause significant yield reductions.

Spring BYD leaf symptoms typical of fall infection. Photo courtesy of L. Ortiz-Ribbing.

Typical red leaf symptoms of BYDV infection. Photo courtesy of L. Ortiz-Ribbing.

There were three strains of BYDV: MAV (mild), PAV (serious), and RPV (more serious). This was probably confusing enough, but for numerous biological reasons the BYDV-RPV strain has been renamed and put in the cereal yellow dwarf group; its acronym is now CYDV-RPV. Both BYDV-PAV and CYDV-RPV are common in Illinois. Testing of plant material for BYDV or CYDV should include tests for both BYDV-PAV and CYDV-RPV (formerly known as BYDV-RPV) to be certain first whether a virus is causing the symptoms, and if so, which one it is responsible.

Wheat streak mosaic. Initial foliar symptoms of wheat streak mosaic virus, also known as yellow mosaic virus, typically show up in the spring, too. The pattern of the disease in the field is tied to the distribution of its vector, the wheat curl mite (Aceria tulipae). Affected wheat plants are typically stunted, with mottled, streaked leaves. The streaks consist of yellow discontinuous dashes running parallel to the veins. Leaves that are heavily infested with mites tend to remain upright, and the margins of the leaf may roll inward. Symptoms tend to get worse as the weather warms up, and severely infected plants may produce sterile heads or die. Yield loss is related to when infection took place: fall-infected plants can experience severe yield losses; early spring-infected plants, light to moderate losses; infection after jointing, minimal losses.

Streaking of leaves from WSMV.

Life cycle. Viral diseases of wheat usually produce symptoms in newer growth. Viruses typically cause stunting of plants as well as a discoloration of leaves, with the most common color being red or yellow. In some viruses, streaking of the leaves or a mosaic pattern also can be seen. Viruses are unusual pathogens because they neither require a food source nor do they have the typical physiological processes associated with other biotic pathogens. Viruses are vectored to plant cells, release their genetic material, and cause the plant cell to replicate more copies of the virus. Most viruses consist of only a genetic and a protective protein outer coat. Once inside plant cells, the virus sheds the protein coat, and the genetic material begins replicating the virus.

Management. The most common method of virus management is to plant resistant wheat varieties. These varieties do not allow virus replication to occur, and the infection is stopped early. Other control measures are directed at reducing the time plants are in the field when vectors are active, which explains the recommendation to plant after the fly-free date, when insect activity is reduced. Systemic insecticide seed treatments have also shown some success.

Putting the pieces together for diagnosis. So which virus may be in the field? First, rule out any other problem that might have caused the symptoms, such as winter kill, nutrient imbalances, or herbicide carryover. This is probably the most important step. Next, find out what virus resistance the variety is supposed to express. There is good resistance to SBWMV in most of our varieties, whereas good resistance to BYDV and CYDV is lacking. If those things don't help, then the pattern may help you decide. BYDV and CYDV usually first show up in the field as a typical insect-type pattern. Infected patches occur randomly in the field or are associated with areas where viruliferous aphids may have been feeding, such as grassy areas on field edges. Also, infection by BYDV and CYDV is completely dependent on aphid movement, and symptoms can continue to spread throughout the season.

Testing tissue for virus particles. The University of Illinois Plant Clinic or Extension's Digital Diagnostic System can only make a visual estimation of the presence of a virus in a wheat plant. We cannot tell you which virus is actually present based on the visible symptoms. To have a virus positively identified, you need to send virus-infected tissue to a lab such as AgDia for serological testing. Fresh plant material is needed for serological analysis because the tests use fresh plant sap.--Suzanne Bissonnette

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