Several calls have come in from around the state concerning leaf symptoms on the wheat crop. Most are reporting that reddish to purple leaves, as well as leaf dieback, are showing up in patches in wheat fields. Varietal characteristics, nutrient imbalances, or viral diseases (which are very common this time of the year) can cause leaf discoloration. The most common viral diseases early in the spring are barley yellow dwarf virus (BYDV) and soilborne wheat mosaic virus (SBWMV); both can cause damage to plants. BYDV is the most damaging virus in Illinois.
Barley Yellow Dwarf Virus. Aphids carrying the virus spread BYDV disease by transmitting 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 various shades of red to purple or pinkish-yellow to brown. As an infected 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 these commonly discolor only the flag leaf and do not cause significant yield reductions.
Soilborne Wheat Mosaic Virus. The disease caused by SBWMV is usually one of the first plant diseases reported in the spring. An unusual aspect of this disease is its mode of transmission to wheat plants: The virus is transmitted to the plant by a soilborne fungus. When the fungus enters wheat roots, it transmits the virus. The fungus is a water mold and favors low, wet areas of the field, and this 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 be seen only 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.
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 either red or yellow. With some viruses, streaking of the leaves or a mosaic pattern also can be seen.
Viruses are unusual pathogens because they do not require a food source and lack 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 genetic material 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 simplicity of this system makes it difficult for plant pathologists to devise successful control strategies.
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 activethus the recommendation to plant after the fly-free date, when insect activity is reduced. Systemic insecticide seed treatments have also shown some success.
Diagnosis. So which virus is 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; the samples that have been sent in so far this season have been negative for BYDV. 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 dont help, then the pattern may help you decide. BYDV usually shows up first in 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. In addition, 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 at the University of Illinois 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 identified, it is necessary to send it to a lab for serological testing. The cost of this procedure is $25 per specimen if you desire to know exactly which virus is present; please indicate this on any specimen forms sent to the clinic. Note that fresh plant material is needed for serological analysis because the tests use fresh plant sap.SB