Characterization and Comparative Studies of Mucor Isolates from Stone Fruits from California and Chile. Themis J. Michailides, Department of Plant Pathology, University of California, Berkeley, Kearney Agricultural Center, Parlier 93648. Plant Dis. 75:373-380. Accepted for publication 21 September 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0373.

Twenty-six isolates of Mucor spp. collected from stone fruit, mainly peaches and nectarines, were identified and compared for pathogenicity. Thirteen isolates from California and six from Chile were classified as M. piriformis; the other seven included two M. circinelloides, three M. racemosus, and two M. plumbeus. These four species of Mucor were also recovered from soil of three stone fruit orchards in Parlier, CA. The isolates of Mucor spp. obtained from orchard soil included <1–8% M. piriformis; 21–28% M. racemosus; 1–5% M. circinelloides, M. plumbeus, M. hiemalis, and M. genevensis; and other (59–73%) unidentified Mucor spp. Optimum temperature for mycelial growth was 21 C for M. piriformis (all isolates), 27 C for M. racemosus and M. plumbeus, and 30 C for M. circinelloides. Maximum temperature for growth of M. piriformis was approximately 27 C; 33 C for M. racemosus and M. plumbeus, and 39 C for M. circinelloides. The isolates of M. piriformis from peaches and nectarines grew well and sporulated abundantly at 0 C, but the isolate of M. piriformis from apricot and M. racemosus grew much more slowly, M. circinelloides grew very slowly, and M. plumbeus did not grow at all after 11 days of incubation. Only the isolates of M. piriformis decayed peaches and nectarines at 0 C—the isolates from Chile being the most virulent. At 20 C, however, an isolate of M. piriformis from apricot was the most virulent. M. circinelloides, M. racemosus, and M. plumbeus caused decay of Dixon peaches at 20 C. Single-spore inoculations of M. piriformis caused an infection of wounded peaches and nectarines. Wounding of peach fruit is necessary for infection by ungerminated sporangiospores of M. piriformis but not for infection by germinated sporangiospores of the fungus. Once the fungus is established on fruit, it can spread in storage to surrounding healthy, unwounded fruit during both ripening (20 C) and cold storage (4 C) temperatures.

Keyword(s): Mucorales, postharvest decay.