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Cowpea Stunt: A Disease Caused by a Synergistic Interaction of Two Viruses. G. Pio- Ribeiro, Department of Plant Pathology and Plant genetics, University of Georgia, Athens, GA 30602; S. D. Wyatt(2), and C. W. Kuhn(3). (2)(3)Department of Plant Pathology and Plant Genetics, University of Georgia, Athens, GA 30602. Phytopathology 68:1260-1265. Accepted for publication 27 March 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1260.

Severely stunted cowpea plants have been found in Georgia fields in each of the last 4 yr, 1974 to 1977. Leaves of the stunted plants were small, mottled, blistered, and malformed. The causal agent was sap-transmitted to cowpeas. Several other hosts were susceptible and back inoculation from Cucumis sativus and Cassia obtusifolia to cowpeas established the presence of two viruses: cucumber mosaic virus (CMV) from C. sativus and a potyvirus, later identified as blackeye cowpea mosaic virus (BlCMV), from C. obtusifolia. When seedlings of California Blackeye cowpeas were inoculated simultaneously with CMV and BlCMV, a strong synergistic reaction occurred; the primary and first two trifoliolate leaves became necrotic within 5-10 days and usually abscised. Subsequent leaves were severely diseased but free of the necrosis; the symptoms then appeared similar to those observed on field plants. When inoculated singly, each virus caused a relatively mild disease; leaves had a mild mottle, and plants displayed moderate stunting during the early infection period and almost no stunting at senescence. In a greenhouse study, CMV reduced yield of California Blackeye seed by 14.2% and BlCMV by 2.5%. Yield on doubly infected plants, however, was reduced 86.4%. Furthermore, the double infection reduced leaf weight, stem weight, and root weight by 94.3, 89.3, and 87.3%, respectively. Seed and aphid transmission studies showed that the viruses can be transmitted from doubly-infected plants to cause single or double infections in cowpeas. The name cowpea stunt is proposed for the disease caused by the synergistic interaction of CMV and BlCMV.

Additional keywords: serology, nonpersistent transmission.