Dynamics of pear-pathogenic Stemphylium vesicarium in necrotic plant residues in Dutch pear orchards

Brown spot disease on pear caused by Stemphylium vesicarium may affect leaves and fruits. Inoculum sources present on orchard floors play an important role in the epidemiology of pear brown spot. The pathogen can overwinter on plant residues and multiply and spread on the residues during the growing season. In the Netherlands, brown spot characteristically occurs only in a fraction of the orchards per season. Until now, no tools are available for Dutch pear growers to predict the risk of brown spot in specific orchards. As a consequence, preventive fungicide sprayings are common. The concentration of DNA of pear-pathogenic S. vesicarium was quantified by a specific TaqMan-PCR assay for various types of plant residues present on orchard floors to evaluate their importance as potential inoculum source. The pathogen was often found in residues of pear leaves, grasses and weeds, but only occasionally in mummies and prunings. Studies of the population dynamics showed that S. vesicarium decreased in dead pear leaves during early winter whereas pathogen populations developed with irregular pattern during the growing season on residues of weeds and grasses. Based on DNA concentrations of S. vesicarium in plant residue samples taken in 78 to 106 orchards in the springs of 2010, 2011 and 2012, the risk of brown spot development could be predicted for individual orchards. Such a risk prediction will allow growers to adapt their fungicide spray schedules to avoid unnecessary sprays in low-risk orchards. © 2013 KNPV.