Reports of viruses are that they are mild and scattered this season. Primary symptoms are light mottling of the leaves accompanied by an overall light yellowish to lime green discoloration of affected areas. Although these symptoms, particularly if they wane in the next week or so, are characteristic of soilborne wheat mosaic virus infection, the only way to know with certainty is to have the live tissue tested for virus.
Early-Season Wheat Virus Disease
Varietal characteristics, nutrient imbalances, and viral diseases can all be causes of leaf discoloration this time of the year. If viruses are going to be a problem, then symptoms should be 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 virus can cause damage to the plants, with BYDV being the most damaging in Illinois.
Barley yellow dwarf virus: Aphids spread BYD 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 commonly only discolor the flag leaf and do not cause significant yield reductions.
Soilborne wheat mosaic virus: The other most common disease causing leaf discoloration this time of the year is SBWMV. It is usually one of the first plant diseases reported in the spring. An unusual aspect of this 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 is a water mold and favors low, wet areas of the field, which is where the disease is usually 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 then symptoms do not 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 virus: 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. We did see the "leaf rolling" symptom in Illinois in 2000 as well. 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 loss; early-spring infection, light to moderate losses; and infection after jointing, minimal losses.
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 either 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.
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 an important step. Next find out what virus resistance the variety is supposed to express. Most of our varieties have good resistance to SBWMV, whereas good resistance to BYDV is lacking. If those things don't help, then the pattern may help you decide. BYDV usually first shows up in a typical insect-type pattern. Infected patches occur randomly in the field or are associated with areas in which viruliferous aphids may have been feeding, such as grassy areas on field edges. Also, BYDV infection is completely dependent on aphid movement, and symptoms can continue to spread throughout the season. SBWMV, on the other hand, most typically will be associated only with low, wet areas of a field, and symptoms will not continue to spread throughout the season.
The Plant Clinic at the University of Illinois or our Digital Diagnostic System can make only 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, it is necessary to send virus-infected tissue to a lab such as AgDia (http://www.agdia.com) for serological testing. Fresh plant material is needed for serological analysis because the tests use fresh plant sap.--Suzanne Bissonnette