By David O. Tebeest, 
Plant Pathology Department, University of Arkansas

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Powdery mildews are among the most common and most easily diagnosed fungal diseases of plants. The diseases are characterized by the white powdery patches that occur on the surfaces of the leaf or stem. The patch is actually the fungus itself.  Click image for an larger view, more information

From time to time, new viruses of animals and plants are discovered or outbreaks of common and uncommon viruses occur. Each of the viruses that cause the outbreaks has to be identified in order to assess its potential impact on agriculture in the United States or world.

A team of scientists in the United States Department of Agriculture Agricultural Research Service and cooperators at Ohio State University that receives and identifies viruses in samples of corn from around the world have discovered and reported two previously unidentified corn viruses. One of the viruses was found in samples obtained from Georgia, and another one was found in samples from Arizona.

Figure 1.  Lesions caused by infection of corn with maize necrotic streak virus.	Figure 2. An electron micrograph of the spherically shaped virus which causes maize necrotic streak virus.

The Arizona virus has been named "maize necrotic streak." It appears to be related to a family of viruses that was previously known to attack only broadleaf plants such as tomatoes and peppers, not crops like corn. Symptoms of the disease caused by the virus first appear as white- or cream-colored streaks on leaves (Figure 1). Eventually it kills the leaves, turning them papery and translucent, with brown spots. Maize necrotic streak virus is in a family of spherical shaped viruses (Figure 2) that tend to spread by soil rather than by insects, and this trait makes them less likely to spread widely. And, this virus doesn't seem to spread from plant to plant. Nevertheless, the experts have suggested that the virus should be taken seriously because lab tests indicate that a cornfield infected by the new virus would be unlikely to yield any corn.

Figure 3. Lesions caused by infection of corn with an un-named virus.	Figure 4. Symptoms of infection of corn by a newly discovered and unnamed virus.

The virus found in Georgia is in a family of viruses that are bullet-shaped (Figure 3) and which includes maize mosaic virus, a major corn disease in the tropics. This virus causes long chlorotic lesions on leaves (Figure 4). Viruses in this family tend to be transmitted by any of a number of insects, including aphids and thrips, as well as mites. A collection of insects is being tested to determine which of them can transmit the Georgia virus. This new virus may be limited to the southern United States.

Readers searching for additional information on these two diseases can contact Dr. P. Redinbaugh, by email at redinbaugh.2@osu.edu. A technical article describing maize necrotic streak virus has been published in Plant Disease. An interpretive summary of that article can be found at www.apsnet.org under journals, plant disease, October 2000, pages 1133-1139.

Two additional websites may be of interest to many readers searching for information on emerging diseases of plants and animals. At the website, www.promedmail.org, readers will find announcements of outbreaks of toxins and diseases caused by both animal and plant pathogens. There is also an APSnet Feature called "New and Emerging Plant viruses" that describes new and emerging virus diseases of plants in the US and world at http://www.apsnet.org/online/feature/NewViruses/Top.html.