Cooperative
Extension
Service


University of Illinois
at
Urbana-Champaign


No. 1/March 19, 1998

Wheat Disease Update

Due to the unusually warm weather, wheat plants have broken dormancy and begunto grow throughout southern Illinois. Along with this early season growth,some virus problems have begun to appear.

The most commonly found viruses in the early season are wheat soilborne mosaicvirus (WSBMV) and barley yellow dwarf virus (BYDV). Each can cause damage toplants, with BYDV being the more damaging of the two. BYDV can infect in thefall if wheat is planted early (before the fly-free date) when the insectvectors are still active; or it can infect in the spring. Fall infectionstypically result in stunted plants and fewer tillers after spring growthresumes. Spring infections commonly only discolor the flag leaf and do notcause significant yield reductions.

WSBMV is among the first plant diseases reported every spring. An unusualaspect of this disease is the mode of transmission to wheat plants. The vectoris a fungus that enters the roots and transmits the virus. Most viral diseasesare vectored by insects or insect relatives such as mites. The fungal vectorfavors cooler and wetter areas of the field, and this is where the disease isusually seen. Plants infected with WSBMV are stunted; leaves are eitherreddish or yellowish in color and, typically, twisted. WSBMV is not commonly ayield-reducing disease because the virus is inactivated by higher springtemperatures and symptoms do not appear on younger leaves as the temperatureswarm. Yield reductions with WSBMV are uncommon except where extremelysusceptible plants are present. Most wheat varieties are resistant to thispathogen, although that can vary.

Viral diseases of wheat, as is the case for most viruses, produce symptoms inthe newer growth. Viruses cause stunting of plants, as well as a discolorationof leaves, with the most common color being either dark red or dark yellow. Insome viruses, a streaking of the leaves or a mosaic pattern also can be seen.

Viruses are considered living pathogens although they do not require a foodsource, nor do they have the typical physiological processes associated withother biotic pathogens. Viruses simply enter cells, release their geneticmaterials, and force the plant cells to produce more copies of the virusrather than producing normal plant cellular components. Most viruses consistof only a genetic core containing the "blueprint" for additional viruses and aprotective outer coat. Once inside plant cells, the virus sheds the outercoat, and the genetic materials begin their work of replicating viruses. Thesimplicity of this system makes it difficult for plant pathologists to devisesuccessful control strategies. There are few options for fighting an organismas simple, yet efficient, as a virus.

The most common method of virus management is to plant resistant wheatvarieties. These varieties do not allow replication to occur, and theinfection is stopped early. Other control measures are directed at reducingthe time the plants are in the field when vectors are active--thus therecommendation to plant after the fly-free date, when insect activity isreduced.

The Plant Clinic at the University of Illinois can make only a visual estimateof the presence of a virus in a wheat plant. We cannot tell you which virusbased on the visible symptoms. To have a virus identified, it is necessary tosend it to a lab for serological testing. The cost of this procedure is $40per specimen. If you desire to know exactly which virus is present, pleaseindicate this on any specimen forms sent to the Plant Clinic. Fresh plantmaterial is needed for serological analysis because the tests use fresh plantsap.
H. Walker Kirby (kirbyw@idea.ag.uiuc.edu), Crop Sciences, (217)333-8414