Issue No. 3, Article 7/April 14, 2006
Winter Wheat Disease Virus Update
Some concerns have trickled in about purpling of the winter wheat in some fields. Could this be virus disease?
Suspect purple leaves on winter wheat.
Early-Season Wheat Virus Disease
Varietal characteristics, nutrient imbalances, and viral diseases can all cause leaf discoloration this time of year. If viruses are going to be a problem, symptoms should be well evident by now. The most common virus diseases early in the spring are barley yellow dwarf virus (BYDV), wheat streak mosaic virus (WSMV), and soilborne wheat mosaic virus (SBWMV). Each can cause damage to the plants, with BYDV being the most damaging in Illinois.
Barley yellow dwarf virus (BYDV) and cereal yellow dwarf virus (CYDV): Aphids spread BYD and CYD disease. Aphids carrying the virus transmit the virus 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 only discolor the flag leaf and do not cause significant yield reductions. There were three strains of BYDV: MAV (mild), PAV (serious), and RPV (more serious). However, 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 that a virus is causing the symptomology and then to determine which one is responsible.
Reddish discoloration of BYDV.
Soilborne wheat mosaic virus: The other most common disease causing leaf discoloration this time of year is SBWMV. It is usually one of the first plant diseases reported in the spring. An unusual aspect of the disease is the mode of transmission to wheat plants. The virus is transmitted to the plant by a soilborne fungus. The virus is carried in the fungus, and when the fungus enters wheat roots it transmits the virus. The fungus, a water mold, favors low, wet areas of the field, which is usually where the disease is first seen. Plants infected with SBWMV can show two types of symptoms. The first is leaf mottling, which appears as a light green and light yellow mosaic on the leaves. The mottling will only be seen very early in the season. The second symptom is stunting to the point where the wheat plant looks like a rosette when growth begins in the spring. Under good growing conditions the infected plants may recover somewhat. SBWMV is not commonly a yield-reducing disease, because higher spring temperatures inactivate the virus and symptoms do not then appear on new leaves. Yield reductions with SBWMV are uncommon except where extremely susceptible plants are present. Most wheat varieties are resistant to this pathogen, although that can vary.
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 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. Plants with fall infection can experience severe yield loss; early spring infection, light to moderate losses; and infection after jointing, minimal losses.
Streaking of leaves from WSMV.
Viral diseases of wheat usually produce symptoms in newer growth. Viruses typically cause stunting of plants as well as discoloration of leaves, with the most common colors 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.
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 the 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 may have caused the symptoms, such as winterkill, 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 or are associated with places where viruliferous aphids may have been feeding, such as grassy areas on field edges. Also, BYDV and CYDV infections depend completely on aphid movement, and symptoms can continue to spread throughout the season. SBWMV, on the other hand, will most typically be associated only with low, wet areas of a field, and symptoms will not continue to spread throughout the season. Rarely do we see virus infection all across a field or the same type of virus symptoms exhibited on both grasses and dicots in the same area.
Testing Tissue for Virus Particles
The Plant Clinic at the University of Illinois and our 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 must 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.
Answering the Question
So what about the fields with the purple leaves? The fields I mentioned at the outset exhibited purpling throughout the field. Weeds in the field and near edges were purple, too. Low areas seem worse and unpurpling doesn't seem to be happening very fast as temperatures warm up. Of our viruses in Illinois that might cause that type of leaf symptom, BYDV and CYDV would be the top suspects. To be absolutely certain a virus isn't involved, a tissue sample would need to be tested. However, the symptomology suggests not virus infection but rather an environmental or nutrient issue.--Suzanne Bissonnette