Pythium leak has been showing up before harvest in a number of Ontario fields this year. Symptoms of Pythium leak at the early infection stage are moist gray or brown lesions around wounds, or near the stem end of tubers.
Cut through the lesions and look for a creamy rot, which will darken from light brown to dark brown, to grey and finally black when exposed to air. The cut surface has a vinegar-like smell, and a clear liquid will ooze out when tubers are squeezed. Cavities often form in the rotten tissue.
The Pythium fungus needs wounds or bruises to penetrate tubers, for tuber damage that usually occurs at harvest. Once infected, tubers rot in transit or in storage.
How does the fungus infect tubers before harvest? The stem end and lenticels may open the door to Pythium when the soil is too warm or too wet, respectively.
Pythium is strictly soil-borne. It is found in most soils and survives a long time, especially in wet areas where it overwinters in debris. Pythium attacks many crops and weeds, so it can’t be eliminated through crop rotation.
Does Ridomil Suppress Pythium Leak?
According to Neil Gudmestad, a professor of plant pathology at North Dakota State University, Ridomil’s active ingredient remains only in the tuber skin. Wounds or bruises allow the fungus to penetrate the tuber flesh without being affected by Ridomil.
Practices that reduce Pythium leak at harvest include minimizing bruising, and not digging the crop in warm, humid weather. High temperatures and high humidity are very favorable conditions for Pythium leak infection.
The article I wrote titled, “Top 5 Storage Diseases” for Spud Smart on management practices to avoid crop losses in storage includes management of Pythium leak.