Effect of Relative Humidity and Interrupted Wetness Periods on Brown Spot Severity of Pear Caused by Stemphylium vesicarium.

ABSTRACT Field observations in four pear orchards during 5 years from April to October indicated that days with uninterrupted wetness of variable length represented 83.9% of the total days studied. However, days with surface wetness interruptions and with high relative humidity (RH) (>/=90%) without wetness occurred with a frequency of 7.1 and 6.2%, respectively. Accordingly, the effect of interruption of 24-h wetness periods by dry periods of high or low RH on infections caused by Stemphylium vesicarium on pear was determined. Pear plants inoculated with conidia of S. vesicarium were exposed to a 12-h wet period followed by a dry period of variable length (0, 3, 6, 12, 18, or 24 h) and a second wet period of 12 h. The dry period consisted either of low (60%) or high (96%) RH. The infection process was irreversibly stopped under low RH during dry periods between wetness, but continued at high RH. The effect of high RH on disease severity in the absence of wetness was also determined. Pear plants inoculated with S. vesicarium were exposed to periods of variable length (3 to 24 h) either at high RH (96%) in the presence of wetness or at high RH (96%) without wetness. No infections were observed on plants incubated under high RH without wetness, indicating that conidia of S. vesicarium require the presence of a water film in the plant surface to develop infections on pear.