The presence, size and importance of bacterial communities on plant leaf surfaces are widely appreciated. However, information is scarce regarding their composition and how it changes along geographical and seasonal scales. We collected 106 samples of field-grown Romaine lettuce from commercial production regions in California and Arizona during the 2009-2010 crop cycle. Total bacterial populations averaged between 10(5) and 10(6) per gram of tissue, whereas counts of culturable bacteria were on average one (summer season) or two (winter season) orders of magnitude lower. Pyrosequencing of 16S rRNA gene amplicons from 88 samples revealed that Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the most abundantly represented phyla. At the genus level, Pseudomonas, Bacillus, Massilia, Arthrobacter and Pantoea were the most consistently found across samples, suggesting that they form the bacterial 'core' phyllosphere microbiota on lettuce. The foliar presence of Xanthomonas campestris pv. vitians, which is the causal agent of bacterial leaf spot of lettuce, correlated positively with the relative representation of bacteria from the genus Alkanindiges, but negatively with Bacillus, Erwinia and Pantoea. Summer samples showed an overrepresentation of Enterobacteriaceae sequences and culturable coliforms compared with winter samples. The distance between fields or the timing of a dust storm, but not Romaine cultivar, explained differences in bacterial community composition between several of the fields sampled. As one of the largest surveys of leaf surface microbiology, this study offers new insights into the extent and underlying causes of variability in bacterial community composition on plant leaves as a function of time, space and environment.

We used the culture-independent method of high-throughput pyrosequencing of 16S ribosomal RNA (rRNA) gene amplicons to analyze the diversity of bacteria associated with the leaf and berry surfaces of 'Chardonnay' grape in a vineyard close to harvest. Combined, more than half of all bacterial sequences were classified as Proteobacteria. Other well-represented phyla were the Firmicutes, Bacteroidetes, and Actinobacteria. The most abundantly represented genera were Sphingomonas, Hymenobacter, Bacillus, Pseudomonas, Skermanella, Leuconostoc, Massilia, Methylobacterium, Cellvibrio, and Curtobacterium. Together, these top 10 genera accounted for almost one-third of all sequences. We observed differences in the composition of bacterial communities between leaves and berries. Members of the genera Sphingomonas, Hymenobacter and Methylobacterium were abundant on foliage, whereas Achromobacter, Flavobacterium and Cellvibrio were typical for the fruit. Moreover, certain Pseudomonas species appeared to associate exclusively with leaf surfaces, while others were found more frequently on berries. Unanticipated was the discovery of a novel Proteobacterium that dominated the bacterial community on berry surfaces. Our results form the basis for future investigations into the relationship between the structure of microbial communities on grape leaves and berries and the ecosystem (dis)services that these communities provide to grape growers at different pre-and post-harvest stages of production.

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