Viruses are simple particles consisting of a nucleic acid surrounded by a protein. They are totally dependent upon on host metabolism for replication. They have no means of overcoming the physical barrier of the leaf cuticle without assistance of either an insect vector or being physically rubbed into the leaf. Aphids, leafhoppers, plant hoppers, mites and beetles that feed on corn are frequent vectors of corn viruses, each being very specific for the vector species.
After being inserted into the corn cell by the vector, the virus uses its nucleic acid component, a single stranded RNA in many corn viruses, to utilized the host cell’s metabolism to replicate the virus nucleic acid and protein coat. Often this is done near the cellular membrane which is connected by a strand through the cell wall called a plasmadesmata to the adjacent cell. This allows the virus to move cell to cell within the corn plant. These channels function to move materials between cells in plants, including the very small virus particles.
Pathogenic viruses often cause local chlorosis, resulting in yellow streaks and mosaic patterns on the leaves. If the virus reaches the meristem it results in significant stunting of the plant as it interferes with normal cell duplication and cell elongation. This severe reaction usually requires initial infection to occur before the V6 stage of corn development. This timing is dependent upon the timing of invasion by the vector and its infection. For example, late planted sweet corn is more likely to be infected with Maize Dwarf Mosaic Virus because the aphid vector population increased on earlier planted corn now can feed on young corn plants.
Resistance to corn viruses is very specific to each but again the genetic variability in maize results its presence. The more difficult virus diseases such as corn lethal necrosis involves two viruses, maize chlorotic mottle virus, transmitted by rootworm beetles, including their larvae, and additional virus such as maize dwarf mosaic virus, transmitted by corn leaf aphid or wheat streak mosaic virus transmitted by wheat curl mite. Biology of corn virus diseases is often further complicated by alternate plant hosts to the virus. Johnson grass is a perennial host to both maize dwarf mosaic virus (transmitted by aphids) and maize dwarf mosaic virus (transmitted by a leafhopper) the combination of which causing severe damage to nearby susceptible genotypes.
With a little time, corn breeders identify the genotypes with reasonable resistance to each of the new virus diseases, as illustrated with the recent corn lethal necrosis outbreak in East Africa. We can be grateful that corn has broad genetic diversity.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.