This article appears in the January 2017 issue of Potato Grower.
The Columbia Basin of Washington and Oregon is similar to other regions of the world where potato late blight, caused by Phytophthora infestans, is dependent on the physical environment for development. Foliage and tubers of potato cultivars grown in the region are susceptible to infection, and aggressive genotypes of P. infestans are often present. Once initial inoculum of P. infestans is present, only a favorable environment and sufficient time are needed for epidemic development.
The effect microenvironments within potato canopies have on late blight development has not been fully understood and needs to be better characterized to more efficiently manage the disease. Quantitative information for temperature and relative humidity in potato canopies is needed to augment decision support and disease forecasting systems. The purpose of a study conducted by Washington State University researchers was to quantitatively characterize canopy environment and estimate its potential 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 is present between canopy closure and harvest. Forecasts for rain are beneficial in scheduling fungicide applications for late blight management in the Columbia Basin.
The management strategy in the Columbia Basin is, naturally, to initially keep late blight out of fields. Once inoculum is in a field, the disease is extremely difficult to manage. For example, late blight incidence increased from 0.6 to 70 percent in 35 days in a field of Russet Norkotah even with nine fungicide applications being made during that time period. Initial inoculum for this field originated from infected seed tubers.
ultimately, fungicide applications for late blight before row closure in the Columbia Basin are likely not warranted unless a prolonged rainy period is expected. In that case, fungicides can be effective in protecting foliage from infection by sporangia that could be potentially produced on latently infected potato shoots arising from infected seed pieces, and possibly from other inoculum sources such as infected refuse tubers and infected volunteer potatoes.