Control strategies based on the development of resistant varieties may be based on resistance of the plant to feeding by the virus vector. Identification of germplasm that is resistant to plant viruses and incorporation of this resistance into crop cultivars appropriate for our geographical area will continue to be part of our cooperative efforts with other researchers. Incorporation of genetic resistance to viruses into crop plants is the most effective way to control many diseases caused by plant viruses. Diseases caused by seed-borne viruses that depend upon re-introduction via infected seed are easily controlled through the use of virus-free seed. Techniques for indexing for virus in these various crops will be established and improved. In vegetatively propagated crops control may be achieved by avoiding the introduction of virus-infected nursery stock, especially if the virus vector or wild hosts of the virus do not occur here. Control strategies will be assessed based on a thorough knowledge of all possible means of virus spread in the field.
Attempts will be made to locate overwintering reservoirs of viruses by indexing to include inoculation of indicator hosts, serological testing and use of the electron microscope. In the case of some virus diseases transmitted by mites and aphids, it may be necessary to utilize the virus vector for manipulations of the causal viruses since some these cannot be transmitted mechanically by sap inoculation or grafting. Healthy, non-viruliferous colonies of vectors will be established and used, if deemed necessary, to identify the vectors of undiagnosed When an unknown virus is the causal agent, the particle morphology of the virus will be determined using the electron microscope. Surveys for viruliferous vector species associated with the occurrence of virus diseases will be made through collection of plant and soil samples and by sweeping fields for insects or use of insect traps. Emphasis will continue to be placed upon the development of antiserum specific for each plant virus. Overwintering strategies and vector relationships will be studied, and resistant varieties will be sought. Viruses which cannot be identified will be fully characterized. Samples of tissues thought to be virus-infected will be collected and processed through host range and preliminary serological tests and leaf-dip electron microscopy. Project Methods Virus occurrence will be monitored in various crops and associated weeds.
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