Influence of spore dose and interrupted wet periods on the development of pear scab caused by Venturia pirina on pear (Pyrus communis) seedlings

The influence of spore dose and interrupted wet periods on pear scab infection was studied under controlled conditions by assessing disease development on pear seedlings. Under optimal conditions (20°C and continuous wetness), the severity of leaf scab (lesions/cm2) increased with conidial concentrations ranging from 102 to 105 conidia/mL. Pear seedlings inoculated with conidia were also exposed to an initial short wet period (3 and 5 h) and a final 24 h wet period which was interrupted with 1, 6, 8, 12, 24, 48 or 90 h of dryness under low (<70%) and high (>90%) relative humidity. The length of the dry period reduced disease severity. During interrupted wet periods under high relative humidity (>90%) at 18" and 20°C, the number of lesions per cm2 of leaf tissue decreased from 2.7 to 0.24 and 2.7 to 0.15, respectively as the dry period ranged from 1 to 90 h. In the same two experiments with similar wet/dry/wet regimes under low relative humidity (<70%), disease severity was also reduced with no scab lesions occurring if leaves were dry for more than 12 h.