Source: Justin Petrosino, Pierce Paul, Ohio State University
Virology is the term used to describe the study of viruses. In Latin virus refers to a slime or poison. Modern science has given us a better look at this microscopic organism responsible for many sniffles and coughs in humans and animals, and on occasion lost yield in plants. Viruses are nothing more than a protein case that houses genetic material, either in the form of DNA or RNA. The virus latches onto a cell and inserts genetic material that basically hijacks the cells production machinery. Organelles inside the cell that used to produce proteins and enzymes for cellular function now become a veritable virus factory. Cellular function breaks down, the cell dies and in a graphic explosion of the cell, amazing when viewed under a microscope, the manufactured viruses are released into the organism, colonizing more cells.
Most folks are familiar with viruses that cause the common cold, influenza, and if you are a hog farmer, the virus that causes PRRS. There are a plethora of viruses that cause symptoms in our field crops. In soybeans there are viruses like bean pod mottle and maize dwarf mosaic in corn. Since wheat, for the most part, is currently the only actively growing grain crop in the state let's discuss a few viruses of wheat.
Whenever the Hessian fly free date is discussed barley yellow dwarf virus (BYDV), a disease caused by seven different species of virus, usually comes up. The fly and the viruses that cause BYDV are not related. However, by planting wheat after the fly free date we can usually avoid the vector of BYDV, hence the association between the two. A vector is another organism that introduces the virus into the plant it will infect. In the case of BYDV, more than 20 different species of aphids may serve as vectors. However, BYDV is not the only viral disease that affects wheat in Ohio. Other common diseases are wheat soilborne mosaic (WSBM) and wheat spindle streak mosaic (WSSMV). Unlike BYDV both of these are caused by soilborne viruses transmitted by Polymyxa graminis, a soilborne protist (a single celled relative of a fungus) that produces motile zoospores (the mobile single celled reproductive stage of a protist).
Diagnosis of a viral infection can be difficult to make because visual symptoms may or may not be present. If visual symptoms are present they can be easily confused with nutrient deficiencies or physiological symptoms. In the case of BYDV symptoms appear as yellowing, purpling or reddening of the leaf, progressing from the tip down to the base of the leaf blade. This typically occurs on newer leafs (the flag leaf especially) but can easily be confused with normal senescence of lower leaves or deficiencies of nitrogen or phosphorus. As the name of the disease suggests, severe infection may result in dwarfed plants. Symptoms of wheat soilborne mosaic virus appear as a pale green to yellow mottling of leaves early in the spring with the possibility of stunted plants. Symptoms typically show up in wet areas of the field when temperatures are between 50 and 68 F. Symptoms can actually subside when temperatures warm up even though the virus is still present. For wheat spindle streak mosaic virus, symptoms usually appear in early spring as yellow-green mottling, dashes and streaks on the leaf. Streaks are parallel to the leaf veins and tapered at the ends to form spindles. Symptoms of both WSSMV and WSBM may be uniformly distributed in the field, but the distribution of WSSMV tends to be more uniform than WSBM.
So how do we accurately diagnose a viral infection? The Wayne C. Ellet Plant and Pest Diagnostic Clinic at Ohio State can identify most common insects and diseases in Ohio for the great price of $20 (http://ppdc.osu.edu/). However, for viral infections of wheat a specialized lab is needed to complete the test. Labs like Agdia or plant pathology labs at land-grant universities in the Great Plains will perform tests for viruses for a fee. Samples submitted can be tested for a single strain of a virus or for multiple viruses with fees increasing with the number of strains and viruses tested.
The two main questions when a virus is diagnosed are: "will it impact yield?" and "how do I treat it?" Viruses can reduce yield but the severity of yield reduction depends on the strain of the virus, how susceptible the variety is, and how early in season the plants are infected. In terms of yield loss, fall infections are most important for all three of the viruses mentioned above. Fall infections of wheat by BYDV, WSSMV and WSBM may reduce yield more severely than spring infections. Treatment is also virus specific. For BYDV, treating for the vector typically does not work. By the time aphids are found in the field and a control measure is applied, wheat has already been infected by BYDV. For WSSMV and WSBM, it is not economical to treat the soil to control Polymyxa graminis, neither is it effective. There are no products registered to control viruses. Fungicides control fungi, insecticides control insects, and herbicides control weeds. We can use cultural practices like planting after the fly free date to manage BYDV. Another and by far the best option is varietal resistance. Viruses are rarely detected in the field and also are very rare in variety performance testing, so it is hard to rate plants for resistance unless we are blessed with an infection during a performance test. A quick search online using a site filter 'site:edu wheat virus resistance' (which filters out all the junk and leaves educational sites) can locate a few websites that will help with resistance ratings. Seed salesman may also have resistance ratings. Finally planting certified seed as opposed to bin run seed will help insure resistance traits are maintained in the seed stock.
So to answer the question in the title, no, wheat can't get a cold, but it can get a viral infection. In the grand scheme of yield determination Mother Nature is still king, followed by soil fertility, insects, diseases, and weeds (in varying order). This spring wet weather is going to play an important role in yield determination.
Source: Justin Petrosino, Pierce Paul, Ohio State University