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 infect ion was studied under controlled conditions by assessing disease developmen t on pear seedlings. Under optimal conditions (20 degreesC and continuous w etness), the severity of leaf scab (lesions/cm(2)) increased with conidial concentrations ranging from 10(2) to 10(5) conidia/mL. Pear seedlings inocu lated 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 degrees and 20 degree sC, the number of lesions per cm(2) 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. I n the same two experiments with similar wet/dry/wet regimes under low relat ive humidity (<70%), disease severity was also reduced with no scab lesions occurring if leaves were dry for more than 12 h.