Brown spot control on pear: infection models versus the inoculum pressure in Belgium.

Stemphylium conidia and Pleospora ascospores were monitored in pear orchards in the region of Haspengouw in Belgium during the infectious periods of brown spot disease (end May - end August) in the years 2002, 2004 and 2005. The seasonal and daily dynamics of the captured fungal spores are discussed and a correlation analysis was performed to determine possible correlation with weather parameters. Furthermore the fungicide cover obtained by spraying upon climatological infection risk is compared with the inoculum pressure in the orchards. Pleospora ascospores were found until the end of May and discharge took place during rain events at the same time-points when Venturia inaequalis ascospores were detected. Although the fungi ejected during the same time period, the relative importance of the different spore peaks differed. The first conidia were detected at the end of May. After that date conidia were found almost every day. Seasonal dynamics of the conidia clearly differed between the years and also the number of spores retrieved differed. No spores were found below 7.5 degrees C and only at a temperature above 12.5 degrees C an increase in conidia was observed. When looking at the daily dynamics, a significant negative correlation was found between the aerial spore concentration and relative humidity and leaf wetness, and a significant positive correlation with wind, temperature and water vapor pressure deficit. When spraying upon a BSP-cast CR threshold value of 0.4, all days with spore discharges above 3 conidia/m3/day were covered in 2002 and 2004. In 2005, a year with very low infection risk and infestation, two periods with high spore discharge were not covered. The observations made show that incorporation of the inoculum pressure into the infection models will probably not lead to a big improvement of the infection models.