Diurnal patterns in dispersal of Monilinia fructigena conidia in an apple orchard in relation to weather factors

The aerial concentration of Monilinia fructigena (causing brown rot in pome fruit) conidia was sampled during 1997 and 1998 in an apple orchard and was related to weather conditions. The highest hourly concentration measured in 1997 was 230 conidia/m3, in 1998 concentrations were lower than in 1997 throughout the season. In both years concentrations were highest in the afternoon hours. Generalised linear models, employing a Poisson distribution for the spore counts and a logarithm link function, were used to study the relationships between spore counts and lagged weather variables. In 1997 the best fitting model had variables temperature lagged at 100 h, an east–west component of wind direction, and wind speed; whereas in 1998 the best model included in addition an effect of relative humidity. Temperature and wind direction had consistent effects on hourly spore counts in both years, but whereas temperature has a biologically relevant effect on spore production and maturation, the effect of wind direction is likely to reflect the purely local effect of orchard layout. Results are compared with observations made in stone fruit orchards where Monilinia laxa and Monilinia fructicola are the predominant species, and differences in epidemiology between these systems are discussed.