Excerpts taken from ONvegetables in the Grower, April 2014. A full pdf version of this article is available at 2014_Infosheet_Phytophthora-in-irrigation-water
Phytophthora blight (Phytophthora capsici) is a serious and complicated disease of peppers and cucurbit crops. Under the appropriate environmental conditions, infections can quickly spread and completely destroy a crop in a matter of days.
Dr. Mary Hausbeck, Michigan State University, conducted an extensive study on the presence and potential impact of phytophthora in irrigation water sources. The study did indicate that irrigation from surface water is a potential source of phytophthora infection in cucurbit and pepper crops.
In this study, they placed cucumbers and pears in floating milk crates. These were placed in various different types of surface water, including: rivers, ditches, naturally fed ponds and ponds fed from a deep well. The fruit baits were left in the water for 3-5 days at which time samples of infected fruit tissue were analyzed in the lab for the presence of P. capsici.
P. capsici was most commonly found at the river and ditch monitoring locations, especially when the sampling location was adjacent to a host crop. However, the pathogen was present even in years when the adjacent crop was not a host crop. It was rarely found in ponds fed by deep wells.
The spores do not appear to overwinter in the water sources. The assumption is the spores enter water courses in the runoff from infected or infested fields. However, the study did not show a clear relationship between significant rain events and the presence of P. capsici in the water source.
Should Ontario growers conduct similar tests on their own water sources?
Not necessarily. In the Michigan study, the presence of P. capsici varied greatly over the sampling periods. Even a high-risk water source may test negative one week and positive the next. Each test really only provides a snap shot in time and not a full picture of the relative risk. By the time the test results have been analyzed, the in-field situation will undoubtedly have changed.
The lab procedure used by Dr. Hausbeck is labour intensive and expensive. It is possible that other methods of testing, such as immunostrips or water sampling, may be less expensive; but these methods have not been thoroughly investigated or proven comparable to the lab results.
The take home message from Dr. Hausbeck’s study was: avoid using surface water to irrigate susceptible crops, especially if there is a history of P. capsici in the area. Deep wells, or irrigation ponds fed by deep wells are the safest option.
The Michigan study did not investigate ponds filled from surface water during spring runoff. If the pond does not receive additional surface water or runoff water during the growing season, it can be assumed that the risk would be low; however there is no research to support this assumption.