Effects of temperature and leaf wetness duration on infection of pear leaves by Venturia pirina

The effects of temperature and wetness duration on the infection of pear le aves (Pyrus communis L.) by Venturia pirina were studied by inoculating pla nts with ascospores and conidia under controlled conditions and in the fiel d. Under controlled inoculations, minimum wetness durations that lead to le af infections by ascospores were 27, 15, 13, 11, 10, 9, and 9 h at 4, 8, 10 , 12, 15, 20, and 25 degrees C, respectively. In parallel inoculations with conidia, the minimum wetness durations that lead to leaf infections were s imilar to ascospores at temperatures between 12 degrees C and 25 degrees C, but at lower temperatures (4, 8, 10 degrees C), conidia infected leaves on ly after an additional 2 h of leaf wetness. The relationship between minimu m wetness times and temperature was best described using an exponential reg ression. In field experiments, leaf infection on plants inoculated with asc ospores and conidia under various naturally occurring wetness and temperatu re conditions was in close agreement with those under controlled conditions . Disease severity (percent of leaf area infected) increased with increasin g leaf wetness duration at all temperatures. The optimum temperature for in fection was 20 degrees C. Analysis of variance with orthogonal polynomial c ontrasts was used to define the relationship of the angular transformation of disease severity to temperature and leaf wetness duration.