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Historical Significance

A century ago, oats were an important crop in the U.S. because they were "fuel" for the horses that were our principal means of transportation. In 1890 a mysterious disease of oats was seen from New England south to Georgia and west to Indiana and Illinois. The oats were stunted, sometimes turned red, tillered poorly, and often set little seed. Poor soil, lack of nutrients, and cold, wet weather were considered likely causes of this epidemic.

The next epidemic struck the U.S. oat crop in 1907. During this epidemic, an abundance of "plant lice," that we now call aphids, was noted on the affected oat plants. This led Ohio scientist T. F. Manns to study the possible relationship between aphids and the disease. Manns removed some aphids from diseased oats and caged them on healthy oats. He found that "Ten to twelve days after the caging the blight began to show, (while) the check outside the cage remained free (of disease)." Manns' observations and experiments led him to conclude that the aphids did indeed spread the disease, which he believed was caused by a bacterium.

Reports of red oats were common from the Pacific Northwest in 1918 and following years. Again, different abiotic (e.g. low temperatures, water-logged soils) and biotic (e.g. bacteria, fungi) agents were considered as possible causes. Most of these causes, however, could not explain why the red leaf symptoms persisted in spring oats from May through to maturity, or why symptomatic plants were found in a wide range of temperatures and soils.

It wasn't until 1951 that a virus was proposed as the cause of these epidemics in small grains. In April of that year, many barley fields in California turned brilliant yellow within a single week. By early May, severe yellowing symptoms were found in almost every barley-producing county of the state. Young barley plants were either killed or stunted so severely that they did not head. It was soon noticed that plants in nearby wheat fields were turning yellow (Figure 18), and that many nearby oats were turning red.

Figure 18

Because virus diseases do not usually affect crops so uniformly, a virus cause was probably not the first possibility considered. Again, the roles of climate, nutrition, fungi, and bacteria were examined. However, the presence of large aphid populations led Oswald and Houston to study the role of these insects in the disease. They found that five aphid species were capable of transmitting the disease. Oswald and Houston attributed the disease to a virus, but pointed out that this virus was not similar to most other plant viruses then known. The yellowing symptoms, transmission by aphids, and lack of transmission by rubbing (mechanical inoculation) differentiated it from other plant viruses. Barley yellow dwarf became one of the first and most widely studied of the "yellowing" virus diseases of plants.

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