In-canopy Environment of Sprinkler Irrigated Potato Fields as a Factor for Late Blight Management in the Semiarid Environment of the Columbia Basin

Relative humidity (RH), leaf wetness and temperature were quantitatively characterized within potato crop canopies and their potential effect on late blight development was estimated in commercial potato fields under sprinkler irrigation in the Columbia Basin in 1993, 1994, 1996, 2004, and 2013. Humid periods of relative humidity > 90 % for 10 or more hours per day with mean temperatures favorable of late blight development were not observed prior to canopy closure. However, at and after canopy closure, a total of 34 of 54 (62.9 %) weeks summed over 5 fields during 1993, 1994, 1996, and 2004 had humid periods favorable for late blight development when considering 10 h humid periods, and a total of 47 of 54 (87 %) weeks had late blight favorable periods when considering 12 h humid periods. In 2013, the mean number of days per week after canopy closure from four sites ranged from 0 to 5.5 days when RH was > 90 % for at least 10 h per day, ranged from 0 to 4.3 days when RH was > 90 % for at least 12 h per day, ranged from 0 to 3.0 days when RH was > 90 % for at least 14 h per day, and ranged from 0 to 2.0 days when RH was > 90 % for at least 16 h per day. Daily hours of contiguous RH > 90 % varied between mid- and low-canopy levels, monitoring sites within fields and between fields. Daily humid periods favorable for late blight development were frequent in June at mid- and low-canopy levels and continued intermittently in July and August in 2 of 2 fields in 2013. Mean temperatures after row closure were generally favorable for late blight development during the humid periods and were > 7.2 °C in June and > 12.2 °C in July and August. Number of rainy days per week and weeks with long periods of RH > 90 % within the potato canopy was significantly correlated, indicating that rare rain events in a semi-arid environment promote long periods of RH in irrigated fields. However, favorable late blight periods occurred without rain and were a factor of sprinkler irrigation. Canopy and ambient relative humidity and temperatures were highly associated, and the association could be used to model late blight development from existing, proximal weather stations. The mean hourly RH over the trial season at all sites for potato canopies attained minima between 1500 and 1700 h in 2013 of < 46 %. From the minima the RH increased rapidly to approximately 2200 h then increased moderately until attaining maxima at 0600. Monitoring duration of RH > 90 % was more practical and efficient than monitoring leaf wetness.