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The Distribution and Electron Microscopy of Viruses of Cacti in Southern Arizona. G. M. Milbrath, Department of Plant Pathology, University of Arizona, Tucson 85721, Present address of senior author: Department of Plant Pathology, University of Illinois, Urbana 61801; M. R. Nelson(2) and R. E. Wheeler(3). (2)(3)Department of Plant Pathology, University of Arizona, Tucson 85721. Phytopathology 63:1133-1139. Accepted for publication 17 March 1973. DOI: 10.1094/Phyto-63-1133.

Extensive surveys for naturally occurring, virus-infected cacti within the region of southern Arizona resulted in the detection of only two of the five viruses reported to infect members of the Cactaceae. Symptomatology, host assays, light and electron microscopy, and subsequent analyses of particle morphology revealed the presence of the Sammonsí Opuntia virus (a rod-shaped particle with a normal length of 321 nm and a width of 18 ± 1 nm) and the saguaro virus (a spherical particle with a diam of 32 ± 1 nm). Both viruses were found to occur naturally. Examination of a cultivated specimen of Platyopuntia monacantha was the single occasion in which an elongated virus with a size identical to that reported for cactus virus X was detected. A mixed infection (consisting of the Sammonsí Opuntia virus and cactus virus X) was noted only once, and this was in a cultivar of Platyopuntia chlorotica. Neither of the remaining two filamentous viruses (cactus virus 2 and zygocactus virus) was encountered during this study.