Pythium leak infects Ontario potatoes

Practices during harvest that help reduce Pythium leak include minimizing bruising and not digging the crop during warm, humid weather.
Potatoes in Canada
September 26, 2018 | Ontario
By Potatoes in Canada
The entry point for Pythium leak.
The entry point for Pythium leak. Photos courtesy of Eugenia Banks.
Pythium leak has been observed in a number of fields in Ontario this harvest season, according to Eugenia Banks, potato specialist with the Ontario Potato Board.

The risk of Pythium leak increases when potatoes are harvested under hot, humid conditions or found in potatoes that are located in a wet area of a field.

The Pythium fungus penetrates tubers through wounds or bruises - tuber damage that usually occurs at harvest. Once infected, the tubers continue to rot in transit or in storage. Hot and humid conditions are responsible for the fungus infecting tubers before harvest happens. Banks says the stem end and lenticels of a potato open up the door to Pythium when the soil is too warm or too wet.

Moist grey or brown lesions around wounds or near the steam end of tubers indicate early infection of Pythium leak. Banks says to cut through the lesions and look for a creamy rot, which will darken from light to dark brown, to grey, and finally to black when exposed to air. The cut surface will have a vinegar-like smell and clear liquid will ooze out when tubers are squeezed. Cavities will also often form in the rotten tissue.



Banks says Pythium is strictly soil borne and attacks many crops and weeds, so it can’t be eliminated through crop rotation. The disease is found in moist soils and can survive a long time, especially in wet areas where it overwinters in debris.

Practices during harvest that help reduce Pythium leak include minimizing bruising and not digging the crop during warm, humid weather. It is encouraged not to harvest the crop when tuber pulp temperatures are above 65 F, especially when the weather is warm and it’s difficult to remove field heat from stored tubers. Potato storages rots are a potential problem in wet seasons but there are many practices that help minimize rot.

The active ingredient of Ridomil fungicide remains only in the tuber skin. Wounds or bruises still allow the fungus to penetrate the tuber flesh without being affected by the fungicide, according to Neil Gudmestad, professor of plant pathology at North Dakota State University. As a result of the fungicide only staying on the tuber skin, the best prevention methods for pythium leak still occur at harvest.


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