Dr Eugenia Banks, potato specialist and consultant for the Ontario Potato Board in Canada, reports that Pythium leak has been showing its ugly face before harvest in a number of fields this year. She says that symptoms of Pythium leak at the early infection stage are moist gray or brown lesions around wounds or near the stem end of tubers.
Dr Banks advises growers to cut through the lesions and look for a creamy rot which will darken from light brown to dark brown, then to grey and finally to 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, tuber damage that usually occurs at harvest,” she points out. Once infected, tubers rot in transit or in storage. How then 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,” Dr Banks says.
According to Dr Banks, Pythium is strictly soil borne. “It is found in most soils and survives a long time specially 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? (Ridomil contains the active ingredient Metalaxyl-M and S-isomer). According to Dr. Neil Gudmestad, a professor of plant pathology at North Dakota State University, the active ingredient of Ridomil remains only in the tuber skin. Then, wounds or bruises allow the fungus to penetrate the tuber flesh without being affected by Ridomil.
Dr Banks says 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,” she says.
Pythium leak is one of the serious diseases of potatoes going into long term storage, according to Dr Banks, and requires continuous vigilance during the storage period. She advises growers to adhere to the basic principles of sensible storage practices, such as ensuring that harvested potatoes go into thoroughly sanitized storage facilities.
Proper curing is a must to heal cuts and bruises produced during harvest, to reduce pathogen spread, and to keep shrinkage losses at a minimum. Growers should monitor storages daily if possible at all, Dr Banks says.
Thermometers suspended at various depths in the pile provide a good indication of the average temperature. Infrared guns are helpful in locating hot spots before they begin to sink and spread.
Dr Eugenia Banks can be reached at EugeniaBanks@onpotato.ca for more information.
Source: Global Potato News magazine