Patterns of airborne conidia of Stemphylium vesicarium, the causal agent of brown spot disease of pears, in relation to weather conditions

Seven- day volumetric spore samplers were installed in pear orchards of northern Italy, in the years between 1993 and 2002, and operated continuously during the development of brown spot epidemics ( mid- April - mid- August), caused by Stemphylium vesicarium. Aerial concentration of conidia was recorded at 2 h intervals to study their diurnal and seasonal patterns and the influence of weather conditions. The diurnal periodicity of aerial conidia showed a peak around midday and low counts in the dark. The increase in spore concentration was significantly correlated with the reduction of relative humidity and wetness in early morning, and the increase of wind in late morning and afternoon. Conidia of S. vesicarium became easily airborne to form a regular component of the air- spora in pear orchards, while ascospores were caught only sporadically. Differences between years concerned total spore counts and numbers of peaks ( defined as days with more than 30 conidia/ m 3 air per day). Periods with highest spore counts occurred in late- May to early- June ( in 2 years), mid to end of June ( 5 years), or after mid-July ( 3 years). There was a significant correlation between spore peaks and days with favourable weather conditions, defined as days with air temperature between 15 and 25 degrees C and high humidity, particularly a wet period longer than 10 h. Occurrence of one or more consecutive days with favourable weather conditions determined an increase in the airborne concentration of conidia, which usually lasted some days and then decreased.