Potato Grower

WWW.POTATOGROWER.COM 83 157023CSIChe12h.indd 1 11/13/15 9:04 AM effect on late blight development under sprinkler irrigation in the Columbia Basin. For the study, relative humidity and temperature were quantitatively characterized within potato crop canopies in Columbia Basin commercial potato fields under sprinkler irrigation in 1993, 1994, 1996, 2004 and 2013. Humid periods of relative humidity greater than 90 percent for 10 or more hours per day with temperatures favorable for late blight development were not observed prior to canopy closure. However, at and after canopy closure, a total of 34 of 54 weeks monitored over five fields had humid periods favorable for late blight development when considering 10-hour periods. Forty-seven of 54 weeks had late blight-favorable periods when considering 12-hour humid stretches. Temperatures after row closure were generally favorable for late blight development during humid periods. It was observed that rare rain events in a semi-arid environment promote long periods of relative humidity in irrigated fields. However, favorable late blight periods occurred even without rain and were a factor of sprinkler irrigation. The microenvironment near potato plants did not favor potential late blight development before canopy closure, which explains why late blight has not been observed in potato fields before closure in the Columbia Basin. This result was expected, since air movement and solar irradiance are not restricted by a dense plant canopy and ambient relative humidity is relatively low in the semiarid atmosphere. However, the microenvironment within the canopy at and after canopy closure was frequently favorable for late blight development. Potentially favorable late bight periods after canopy closure occurred up to seven days per week, and 87 percent of the weeks monitored had late blight-favorable days over five fields in 1993, 1994, 1996 and 2004. Temperatures during humid periods were mostly favorable for late blight after canopy closure. Humid periods were longer in June than in July and August. Fungicides for late blight management may be justified in June, especially after relatively wet weather in April and May, which has been associated with late blight epidemics in the Columbia Basin. Favorable periods for late blight were often accompanied by rain in a study in Minnesota where potatoes were not sprinkler- irrigated. Rainfall, when it did occur in the semiarid environment of the Columbia Basin, was highly associated with long durations of humid periods throughout the trial period. The longest periods of relative humidity higher than 90 percent were highly correlated with rainy days. Rainy days are often accompanied with cloud cover and reduced solar irradiance, which is associated with a high incidence of late blight in the Columbia Basin. However, favorable late blight periods occurred in this study without rain and were a factor of sprinkler irrigation. Favorable periods for late blight in the Columbia Basin would be extended with rainfall, increasing the potential for late blight development and promoting the disease's spread within and between fields. Fungicide application just before a major rainfall would be important in the Columbia Basin if inoculum

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