Enumeration of bacterial and yeast colonists of apple fruits and identification of epiphytic yeasts on pear fruits in the Pacific Northwest United States

Abstract

A procedure for the isolation of diverse culturable microflora for the estimation of the population size of yeasts and bacteria on the surface of pome fruits is described. Maximum numbers of morpholgically distinct colonies of both yeasts and bacteria were recovered from apple fruit surfaces when fruits were shaken for 5 min in sterile phosphate buffer plus tween, sonicated for 5 min, and aliquots of the buffer plated onto diluted yeast malt agar and diluted nutrient broth agar, respectively. The yeast and bacterial populations on the surface of unsprayed Golden Delicious apple fruits were approximately 8.0 × 10 3 and 9.5 × 10 4 colony forming units (cfu) per cm 2, respectively. The densities of yeasts on the surface of pear fruits collected from Yakima, Wa, Cascade Locks, Medford, and Hood River, OR, were approximately 7.3 × 10 3, 6.4 × 10 3, 4.1 × 10 3, and 9.9 × 10 2 cfu · cm -2, respectively. The highest number of morphologically different yeast isolates were recovered from pear fruits from Cascade Locks and Hood River, Oregon and Yakima, Washington. Aureobasidiumpullulans was present on fruits in all pear orchards sampled whereas Cryptococcus albidus and Rhodotorula glutinis were isolated from 80% of the orchards. Other yeasts colonizing pear fruit surfaces in 20–60% of the orchards were Cryptococcus infirmo-miniatus, Cryptococcus laurentii, Debaryomyces hansenii, Rhodotorula aurantiaca, R. fujisanensis, R. minuta and Sporobolomyces roseus.