Disease reports are at a minimum on the wheat crop this season. Robert Bellm, crop systems educator at the Edwardsville Extension Center, reports that he has seen little or no viral or fungal disease to date. Robert noted that some fields that started off the season looking fairly yellow have greened up well following nitrogen application. The time is right, however, to scout for virus diseases, paying special attention to those fields that seem a bit off color. With the recent rains, this should also be a good week to look for the presence of fungal diseases in the lower canopy as well.|
Early-Season Wheat Virus Disease
When scouting, keep in mind that varietal characteristics, nutrient imbalances, or viral diseases can each or all be causes of leaf discoloration this time of the year. If viruses are going to be a problem, symptoms should be evident now. The most common virus diseases that you will see early in the spring in Illinois are barley yellow dwarf virus (BYDV) and soilborne wheat mosaic virus (SBWMV). Each of these viruses can cause damage to the plants, with BYDV being the most damaging.
Soilborne wheat mosaic virus. SBWMV is a very common disease, causing leaf discoloration in the spring. It is usually one of the first plant diseases reported in the growing season. 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, where the disease usually is seen first. 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. If temperatures continue to quickly rise, this symptom disappears quickly. 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 consideration in variety selection is still important.
Barley yellow dwarf virus. Aphids spread BYDV 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. We had a long fall, which was a good situation for the aphids. 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.
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 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.
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 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.
Diagnosis. If you suspect virus infection when you are scouting, how do you know 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. Next find out what virus resistance the variety is supposed to exhibit. There is good resistance to SBWMV in most of our varieties, 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 on 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, will most typically be associated only with low, wet areas of a field, and symptoms will not continue to spread throughout the season.
The Plant Clinic or our Distance Diagnostic online service at the University of Illinois 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. Frankly, it is not always necessary to know specifically which virus may be present. However, if you want to have a virus positively identified, either you or we have to send virus-infected tissue to a lab such as AgDia for serological testing (http://www.agdia.com/). The cost of this procedure starts at about $50 per specimen if you desire to know exactly which virus is present. Fresh plant material is needed for serological analysis because the tests use fresh plant sap.--Suzanne Bissonnette