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Etiology

Evidence for Viruslike Agents in Avocado. Ramon L. Jordan, Department of Plant Pathology, University of California, Riverside 92521, Present address of senior author: Horticultural Sciences Institute, U.S. Department of Agriculture, Beltsville Agricultural Research Center, Beltsville, MD 20705; J. Allan Dodds(2), and Howard D. Ohr(3). (2)(3)Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 73:1130-1135. Accepted for publication 1 March 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1130.

Double-stranded RNA (dsRNA), an indicator for the presence of viruses in plants, has been detected in all except 22 of >280 individual avocado plants tested. Three different dsRNA patterns have been detected. DsRNA pattern 1 has three segments. The smallest segment has a molecular weight (MW) of 0.55 x 10⁶ daltons. The larger segments are resolved as a “doublet” by gel electrophoresis with a MW of 6.0- 6.5 x 10⁶. DsRNA pattern 2 has one major segment with a MW of 3.0 x 10⁶. DsRNA pattern 3 has three major segments with MWs of 2.0, 1.9, and 1.7 x 10⁶. The three dsRNA patterns have been detected both singly and in all possible combinations in different individual plants. Three different viruses (avocado viruses 1, 2, and 3) are proposed to explain the variation in the dsRNA patterns detected. All three putative avocado viruses appear to be latent in some cultivars (symptomless carriers). Avocado viruses 1, 2, and 3 are all seed transmitted at a high rate. Avocado viruses 1 and 2 are graft-transmitted across the bud union. Avocado virus 3 is present in all plants of the Hass cultivar tested, both healthy and avocado blackstreak-affected (ABS). Virus 3 alone appears to cause no symptoms. Avocado virus 2 has been detected in tissues from ABS-affected cultivar Hass trees, but has not been detected in healthy Hass trees in the field. It is possible, but not proven, that avocado virus 2, either alone or in combination with avocado virus 3, is involved in the ABS disease. Several avocado cultivars had no detectable dsRNA, including the Phytophthora root rot resistant clonal rootstocks Duke 6 and Duke 7. This is the first report of the presence of viruslike agents other than tobacco mosaic virus in avocado.