Single-Leaf Resolution of the Temporal Population Dynamics of Aureobasidium pullulans on Apple Leaves

Abstract

The abundance of phylloplane microorganisms typically varies over several orders of magnitude among leaves sampled concurrently. Because the methods traditionally used to sample leaves are destructive, it has remained unclear whether this high variability is due to fixed differences in habitat quality among leaves or to asynchronous temporal variation in the microbial population density on individual leaves. We developed a novel semidestructive assay to repeatedly sample the same apple leaves from orchard trees over time by removing progressively more proximal ϳ1-cm-wide transverse segments. Aureobasidium pullulans densities were determined by standard leaf homogenization and plating procedures and were expressed as CFU per square centimeter of segment. The A. pullulans population densities among leaves were lognormally distributed. The variability in A. pullulans population densities among subsections of a given leaf was one-third to one-ninth the variability among whole leaves harvested concurrently. Sequential harvesting of leaf segments did not result in detectable changes in A. pullulans density on residual leaf surfaces. These findings implied that we could infer whole-leaf A. pullulans densities over time by using partial leaves. When this successive sampling regimen was applied over the course of multiple 7-to 8-day experiments, the among-leaf effects were virtually always the predominant source of variance in A. pullulans density estimates. Changes in A. pullulans density tended to be synchronous among leaves, such that the rank order of leaves arrayed with respect to A. pullulans density was largely maintained through time. Occasional periods of asynchrony were observed, but idiosyncratic changes in A. pullulans density did not contribute appreciably to variation in the distribution of populations among leaves. This suggests that persistent differences in habitat (leaf) quality are primarily responsible for the variation in A. pullulans density among leaves in nature.

Dispersal, Density Dependence, and Population Dynamics of a Fungal Microbe on Leaf Surfaces

“…We showed previously that Ap populations typically vary from 10-to 100-fold among apple leaves (Woody et al 2003), analogous to epiphytic bacterial populations (Hirano et al 1982). First, what is the basis for variation in microbial population densities among leaves?…”

“…This approach is justified by two principal findings detailed in Woody et al (2003). This approach is justified by two principal findings detailed in Woody et al (2003).…”

“…First, we developed a technique for serially sampling a given leaf for Ap density through time (Woody et al 2003). First, we developed a technique for serially sampling a given leaf for Ap density through time (Woody et al 2003).…”

Yeasts colonizing the leaf surfaces

“…The habitat of A. pullulans is quite broad; it has been isolated from different substrates and samples (bark, roots, marine sediments, waters). This "black yeast" is from an ecological point of view the ubiquitous species, found mainly on the phylloplane (Inácio et al 2002, Woody et al 2003. A. pullulans was the dominant species of water samples taken in autumn from the fish-pond (situated also in the forest park Železná Studnička) (Sláviková and Vadkertiová 1995) and artificial lake waters (located in the Lowland of Záhorie) (Sláviková and Vadkertiová 1992) when the water contained many fallen leaves.…”

Role of Microbial Immigration in the Colonization of Apple Leaves by Aureobasidium pullulans

“…This work further demonstrates the advantages of being able to repeatedly sample the same leaves over time (31), thereby uncovering potentially subtle leaf-specific changes that otherwise would be obscured in experimental noise. Similar patterns tended to develop among individual leaves, with densities becoming significantly higher on veinal sites than on interveinal sites.…”