The wheat crop seems to be in very good shape, as far as the current disease outlook is concerned, though there are a few things to keep an eye out for this next week. Given the recent weather, you should begin looking for the presence of fungal diseases in the lower canopy. Powdery mildew and the beginning of Septoria (Stagnospora) leaf spot are showing up in east-central Illinois. |
Varietal characteristics, nutrient imbalances, or viral diseases can cause 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 early in the spring are barley yellow dwarf virus and soilborne wheat mosaic virus. Each can cause damage to the plants, with barley yellow dwarf virus being the most damaging in Illinois.
Barley Yellow Dwarf Virus (BYDV)
Aphids spread BYDV and 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.
Early-season foliar symptoms of BYDV.
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 (SBWMV)
The other most common disease that causes 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, 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.
Early-season leaf mottling from SBWMV.
The mottling will be seen only very early in the season. The second symptom is stunting to the point, in which the wheat plant looks like a rosette when growth begins in the spring.
Early-season stunting from SBWMV.
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 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 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 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: 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; 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 don't 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 viruliferous aphids may have been feeding in, such as grassy areas on field edges. Also BYDV infection is completely dependant 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 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 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. Fresh plant material is needed for serological analysis, because the tests use fresh plant sap.--Suzanne Bissonnette