Positional variation in phylloplane microbial populations within an apple tree canopy.

Variation in density of epiphytic yeasts, filamentous fungi, and bacteria on apple leaves collected from eight trees at nine dates for two seasons was determined with respect to three positional factors: height, compass direction from the center of the tree, and lateral proximity to the canopy periphery. Univariate analyses of variance were performed on each of the microbial classes for each date according to a model that excluded tree effect but accounted for the positional factors with interactions. The assumption of no tree effect was explored by residual analysis and examination of the seasonal pattern of microbial densities for each tree. No persuasive evidence was obtained to invalidate this assumption. For filamentous fungi and yeasts, height and lateral position were the most significant factors withp<0.05 for yeasts at several periods. The two factors appeared to be of equal importance. Trends were less clear for bacteria, but all three positional factors and some two-way interactions seemed of some importance. For filamentous fungi and bacteria, frequently no factors were significant at a level of 0.10, but at almost all sampling dates certain positional factors and interactions were significant at a level of 0.25. Inspection of partial correlation coefficients indicated no apparent linear association between densities of most pairs of microbial classes. Implications of these results for experimental design and for the microbial ecology of the phylloplane community are discussed.