Populations dynamics of Stemphylium vesicarium causing brown spot of pear and Stemphylium spp. non-pathogenic in pear on plant residues in Dutch pear and apple orchards

Stemphylium vesicarium causes brown spot of pear. Pear-pathogenic isolates of S. vesicarium and S. vesicarium isolates not able to cause symptoms on pear leaves and fruits has been reported. Stemphylium spp. have a necrotrophic lifestyle and crop debris is an important niche for Stemphylium spp. populations to survive, grow and multiply. We hypothesized that density and dynamics of populations of non-pathogenic S. vesicarium have an impact on the occurrence and dynamics of pear-pathogenic S. vesicarium populations during its saprotrophic phase. A genus-specific Taqman PCR assay for the quantification of Stemphylium spp. and a Taqman PCR assay specific for the pear-pathogenic S. vesicarium populations was applied to approximately 2000 litter samples from Dutch orchards. Co-occurrence of both pear-pathogenic S. vesicarium and Stemphylium spp. regarded as non-pathogenic were found in approximately 25% of the samples from pear orchards collected in an earlier study on brown spot of pear. Fluctuations for both Stemphylium spp. and pear-pathogenic S. vesicarium show the same pattern. No cases were found were increased concentrations of Stemphylium spp. coincided with a decrease in concentrations of pear-pathogenic S. vesicarium. The hypothesis that high colonization by saprophytic Stemphylium spp. may outcompete pear-pathogenic populations can thus not be supported.