Eight phylloplane yeasts were isolated from backyard apple trees in Corvallis, OR. Yeast isolates were classified to genus or species level. All isolates were tested in vitro for antagonistic activity against the postharvest pathogens Botrytis cinerea and Penicillium expansum. Of these isolates, Aureobasidium pullulans, Sporobolomyces roseus Rhodotorula sp., consistently reduced mycelial growth of B. cinerea and P. expansum in nutrient yeast dextrose agar (pH 4.5 or 7.0) incubated for 8 or 30 days at 24 or 1 C, respectively. These three yeasts also were evaluated for their ability to suppress spore germination of B. cinerea and P. expansum in a gradient of apple juice concentrations and to suppress development of gray and blue mold lesions in inoculated fruits of Golden Delicious apple. Germination of B. cinerea and P. expansum was reduced significantly (P���0.05) when incubated with the yeast isolates in 100 or 50% apple juice, but not in 0, 1 or 10% apple juice. S. roseus and A. pullulans reduced significantly (P���0.05) the size of gray mold lesions in wounded fruit stored at 5 C and 24 C by 63 to 72 and 81 to 90%, respectively, when compared to the nontreated control. Size of blue mold lesions in fruit stored at 5 and 24 C also were reduced significantly (P���0.05) by 66 to 38 and 74 to 63%, respectively, when pre-treated with S. roseus and A. pullulans. In general, fruit rot suppression by some yeasts isolated in this study was similar in magnitude to suppression obtained by Cryptococcus laurentii isolate 87-108, a yeast with commercial potential to suppress postharvest rots of pome fruits. Pretreatment of apple wounds with washed cells of A. pullulans, S. roseus, Rhodotorula sp., resulted in disease suppression, but treatment of wounds with cell-free culture supernatant of these isolates did not affect lesion development. Population size of A. pullulans, S. roseus, and C. laurentii increased in apple wounds incubated at 5 or 24 C for up to 25 days, indicating that they colonized the wound site. Data collected in this study support the hypothesis that yeast isolates antagonize fruit pathogens by competing for nutrients in wounds on fruit surfaces. The isolates of A. pullulans and S. roseus show promise for commercial development. / Graduation date: 1997