Exclusion of the Inoculum Source of Brown Spot (Stemphylium vesicarium)

Stemphylium vesicarium is the causal agent of brown spot of pear. The disease causes severe damage especially in southern Europe. In recent years, the disease has also been found in the Netherlands and in Belgium. The pear cultivar commonly grown in the Netherlands is 'Conference', which is highly susceptible. This cultivar is one of the most susceptible cultivars. S. vesicarium overwinters and survives on organic material such as dead grasses or pear leaves. On this dead leaf material the fungus produces spores that can infect pears. To examine the importance of dead leaf material on the ground as an inoculum source, the orchard soil was covered with plastic. In total 24 rows (3500 m(2)) of a pear orchard were covered after full bloom in 2006 until the end of September. It was assumed, that spores present on dead leaf material (in the ground, cannot reach the canopy through this plastic cover. There were 4 rows without plastic on the ground, to serve as a source of inoculum on one side of the plastic cover with dead leaf material on the ground. Four Burkhard spore samplers were installed at four distances from the expected source in July and August 2006. Aerial concentrations of conidia above the source and the plastic were recorded to find correlations between the distance from the source and the spore concentration. At the end of September pears were harvested and assessed on symptoms of brown spot:. It was found that the trees without plastic underneath were more severely infected than trees with the plastic cover. At all four distances spores were found in the spore traps.