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Cultural practices important for managing pink rot and leak

Environmental conditions can influence the appearance from year to year

Manitoba growers know there are plenty of differences between “wet” rots and “dry” rots in storage — but both can cause devastating losses each year.

According to Neil Gudmestad, a distinguished professor at North Dakota State University’s department of plant pathology, important rots in storage include dry rot, late blight tuber rot, pink rot, leak and bacterial soft rot.

Gudmestad told this year’s Manitoba Potato Production Days that there has not been an increase in the occurrence of these diseases in the United States and Canada, but grower practices in combination with environmental conditions can influence the appearance of pink rot and leak in storages from year to year.

“Pink rot and leak are chronic diseases and like all diseases they are influenced by the environment and by grower practices that any increase in one particular year is likely due to one or both of these drivers,” he says.

Caused by the pathogen Phytophthora erythroseptica, pink rot infects tubers through stolons, eyes and wounds made at harvest. According to Gudmestad, high soil moisture increases the likelihood of infection. Although there is some cultivar resistance, chemical control measures are important, particularly mefenoxam (the active ingredient in Syngenta’s Ridomil Gold) or phosphonates such as Phostrol.

If growers stay on top of management, pink rot can be controlled through a combination of varietal resistance and phosphonate applications.

Gudmestad also emphasized leak as a key storage disease. Primarily caused by the plant pathogen Pythium ultimum, leak is a soil-borne fungus that infects tubers primarily through wounds made at harvest. Tuber pulp temperatures of less than 65 F favour infection. For leak, control options are limited; mefenoxam provides moderate control but only with in-furrow application at planting. Phosphorus acid, Gudmestad said, does not control leak.

Soil moisture

“High soil moisture conditions that promote soil-borne disease development are those where soils are less than 90 per cent of field capacity,” Gudmestad stated during his presentation. “A combination of either rain and/or irrigation can provide the suitable environment for infection and disease development.”

High rainfall in the growing season and significant rainfall after irrigation can also contribute to the development of disease. Gudmestad says high soil moisture conditions at the end of the season in particular promote disease development.

Soil moisture has a significant impact on both pink rot and leak. As both pathogens are primitive “fungus-like” organisms, they are highly dependent on high soil moisture for survival. Both pathogens produce zoospores with flagella, and Pythium ultimum also lives off decaying plant matter in the soil, its population increasing over the growing season with high soil moisture conditions.

Cultural practices are also important in managing pink rot and leak. Growers should avoid wet soil conditions in August and September. In addition, Gudmestad emphasized, growers should take care that tuber pulp temperatures do not exceed 65 F at harvest. “High bruise-free scores will also minimize infections that occur in storage post-harvest,” he said.

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