Cowpea stunt: a disease caused by a synergistic interaction of two viruses.

PIO-RIBEIRO, G. , S. D. WYATT, and C. W. KUHN. 1978. Cowpea stunt: a disease caused by a synergistic interaction of two viruses. Phytopathology 68: 1260-1265. Severely stunted cowpea plants have been found in similar to those observed on field plants. When inoculated Georgia fields in each of the last 4 yr, 1974 to 1977. Leaves of singly, each virus caused a relatively mild disease; leaves had the stunted plants were small, mottled, blistered, and a mild mottle, and plants displayed moderate stunting during malformed. The causal agent was sap-transmitted to the early infection period and almost no stunting at cowpeas. Several other hosts were susceptible and back senescence. In a greenhouse study, CMV reduced yield of inoculation from Cucumis sativus and Cassia obtusifolia to California Blackeye seed by 14.2% and BICMV by 2.5%. cowpeas established the presence of two viruses: cucumber Yield on doubly infected plants, however, was reduced mosaic virus (CMV) from C. sativus and a potyvirus, later 86.4%. Furthermore, the double infection reduced leaf identified as blackeye cowpea mosaic virus (BICMV), from weight, stem weight, and root weight by 94.3, 89.3, and C. obtusifolia. When seedlings of California Blackeye 87.3%, respectively. Seed and aphid transmission studies cowpeas were inoculated simultaneously with CMV and showed that the viruses can be transmitted from doublyBICMV, a strong synergistic reaction occurred; the primary infected plants to cause single or double infections in and first two trifoliolate leaves became necrotic within 5-10 cowpeas. The name cowpea stunt is proposed for the disease days and usually abscised. Subsequent leaves were severely caused by the synergistic interaction of CMV and BICMV. diseased but free of the necrosis; the symptoms then appeared Additional key words: serology, nonpersistent transmission. Virus diseases of plants caused by simultaneous (1%) and rubbing primary leaves of 8to 9-day-old infection of two or more unrelated viruses have been cowpea plants with a cheesecloth pad soaked in the recognized since the 1920's (6, 17, 19, 20). More recent prepared inoculum. Viruses in either single or mixed examples include corn lethal necrosis caused by maize infections were maintained in California Blackeye chlorotic mottle virus in combination with either maize cowpeas (previously named Early Ramshorn by the seed dwarf mosaic virus or wheat streak mosaic virus (18), and source, W. Atlee Burpee Co., 300 Park Avenue, a disease complex of cowpeas, Vigna unguiculata (L.) Warminster, PA 18974). Test plants were kept in a Walp. subsp. unguiculata, caused by Moroccan strains of greenhouse (24 to 30 C) in 10or 15-cm diameter pots cucumber mosaic virus (CMV) and cowpea aphid-borne containing a mixture of soil, sand, and vermiculite (2:1: 1, virus (CAMV) (8, 9). v/v) provided with a complete fertilizer. In a preliminary report (22), we noted a severe stunting Identification studies. -Both CMV and B1CMV were disease of cowpeas caused by CMV and an unknown partially purified for identification studies. For CMV, virus now identified as blackeye cowpea mosaic virus diseased tissue (1 g/ml of buffer) was homogenized in 0.5 (B1CMV). This paper is concerned with the M phosphate buffer (pH 8) containing 0.5% mercaptoecharacterization and naming of the disease, identification thanol, and the resulting sap was clarified with butanol of the specific viruses involved, synergistic effects of the (8%); the virus was concentrated with one cycle of two viruses on plant growth and seed yield, and ultracentrifugation, and test virus was fractionated after transmission of the viruses via seed and aphids. centrifugation on sucrose gradients (10-40%) in 0.01 M phosphate buffer (pH 8). Purification of B1CMV was MATERIALS AND METHODS achieved by the method described by Lima et al. (13). Sedimentation coefficient values were estimated (4) in Virus maintenance. --Sap inoculations were made by sucrose gradients by using cowpea chlorotic mottle virus grinding diseased leaf tissue (I g /9 ml) in neutral (CCMV) (S20,w = 88) and southern bean mosaic virus potassium phosphate buffer (0.01 M) containing celite (SBMV)(S20,w =115) as markers. Virus preparations were negatively stained with phosphotungstic acid (2%) for 00032-949X/78/000 227$03.00/0 observation with the electron microscope. For serological Copyright © 1978 The American Phytopathological Society, 3340 tests, CMV and B1CMV antisera were furnished by the Pilot Knob Road, St. Paul, MN 55121. All rights reserved. Plant Virus Laboratory, University of Florida,