Phytophthora and irrigation water

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Many Michigan vegetable crops such as squash, cucumber, eggplant, pepper, beans, watermelon, melon and tomato are suscepti­ble to Phytophthora capsici. This pathogen causes disease on plant crowns, roots and fruit. Phytophthora can move in surface wa­ter, which is often relied upon for irrigation in Michigan. Research conducted by Michi­gan State University from 2002 to 2005 iden­tified Phytophthora in a river, ponds and ditches in several Michigan counties.

Milk crate with cucumber baits
Milk crate with cucumber baits.

In standing water, Phytophthora produces swimming spores which can be spread with moving water. Swimming spores can sur­vive and cause infection for 8 to 10 hours. Therefore, when surface water is contami­nated with Phytophthora, the pathogen can be spread to a field through irrigation.

Milk crate floating in a pond
Milk crate floating in a pond.

Pear and cucumber baits, along with tem­perature sensors, were placed in plastic milk crates kept afloat by attaching polyethylene foam pool noodles to the lid. Floating the baits was important because swimming spores always move to the surface of the wa­ter.

Phytophthora capsici spores
Phytophthora capsici swimming spores.

The crates were then suspended in the water for three to seven days before baits were removed for examination.

Pieces of infected pear and cucumber baits were placed onto media plates which pro­mote Phytophthora growth. After three days, the plates were examined for Phytophthora. Some of the Phytophthora collected from sur­face water was found to be resistant to the fungicide mefenoxam (Ridomil).

Graph 1
Ponds fed by deep wells appeared to offer a safer
alternative to surface water for irrigation. Provided
that run-off from nearby fields does not enter the
pond, there is a low risk of pathogen infestation in
well-fed ponds.

Phytophthora was detected in several surface water sources in multiple years, even when non-host crops were planted nearby. Most of the infestations occurred when the water temperature was between 59°F and 77°F.

It is important to take precautions when ir­rigating crops. If irrigation is necessary, do not use surface water sources. Drip irriga­tion from well water or ponds fed by well water is recommended to reduce the risk of Phytophthora spread on susceptible vegetable crops. Irrigate conservatively, and try to limit irrigation close to harvest. 

2005 River Sites
Graph 2
The pathogen did not appear to overwinter in surface water sources. Phytophthora was
detected in water at times when rainfall was low and the likelihood of irrigating was high.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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