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, coughs, and on occasion lost yield. 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, inserts DNA or RNA, and thereby 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 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 host of viruses that cause symptoms in our field crops. Viruses like bean pod mottle of soybean and maize dwarf mosaic in corn. Since wheat, for the most part, is 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 barely yellow dwarf virus (bydv) usually comes up. The fly and the virus that causes bydv are not related. However, by planting wheat after the fly free date we can usually avoid the vector of bydv. A vector is another organism that introduces the virus to the plant it will infect. In the case of bydv the vector is the bird oat cherry aphid. Other vectors for viruses include fungi like Polymyxa graminis which transmits wheat soil borne mosaic virus. Once the disease vector introduces the virus to the host infection occurs.
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 disorders. In the case of bydv symptoms present as yellowing or purpling that progresses from the leaf tip down to the base of the leaf blade. This typically occurs on newer leaf growth but can easily be confused with normal senescence of lower leaves or deficiencies of nitrogen or phosphorus. Wheat soil borne mosaic virus presents as a pale green 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 below 65 F. Symptoms can actually subside when temperatures warm up even though the virus is still present. 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 a specialized lab like AgDia is needed to complete the test.
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 and how early in season the plant was infected. Fall infection of wheat by bydv and soil borne mosaic can reduce yield more severely than a spring infection. Treatment is also virus specific. Treating the vector typically does not work. Treating for aphids won’t prevent bydv and it’s not economical to treat for the fungal infection that transmits soil borne mosaic, nor 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 the fly free date to control bydv. Another option is varietal/hybrid resistance. Viruses are a fairly rare occurrence in the field and also are very rare in performance testing, so it is hard to rate plants on resistance unless we are blessed with an infection during a performance test. If information is available it will be listed in university performance trials or your seed salesman may be able to obtain company data.
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. Jim Noel our state climatologist, said to expect around 2-4 inches of rain over the next two weeks but a dry spell is expected after that.
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