Red rot is an important storage disease of potatoes. It is caused by the soil-borne pathogen Phytophthora, Phytophthora, and is found in potato growing areas around the world.

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This pathogen reproduces in saturated soil, so the disease is usually associated with low-lying fields or poorly drained areas. The incidence of disease is highest at temperatures between 70°F and 85°F.
You may not notice the pink rot before harvest or tuber storage, but it starts in the field. Infections usually originate from foot attachments, but they can also occur in the eyes or wounds. Pink rot can also spread from tubers to tubers during storage.
Like the pathogens of late blight (Phytophthora infestans) and leakage (Pythium lethal), the pink rotting pathogen is a fungus-like oomycete, not a “real” fungus.
Why should we care? Because chemical control of fungal pathogens is generally not applicable to oomycetes. This limits the chemical control options.
The most commonly used oomycete fungicides for the treatment of pink rot are mefenfloxacin (such as Ridomil Gold from Syngenta, Ultra Flourish from Nuffam) and metalaxyl (such as MetaStar from LG Life Sciences). Metalaxyl is also known as metalaxyl-M, which is chemically similar to metalaxyl.
The label of phosphoric acid implies various application times and methods. In the Pacific Northwest, we recommend three to four leaf applications, starting with the size of the tuber and the size of the corner.
Phosphoric acid can also be used as a post-harvest treatment after the tubers enter storage. Other fungicides used to control pink rot are fentrazone (for example, Ranman from Summit Agro), oxatipyrine (for example, Orondis from Syngenta), and flufentrazone (for example, Valent USA Presidio).
Read the product label carefully and consult local experts about the best price and schedule in your area.
Unfortunately, some Rhodopseudomonas are resistant to metalaxyl. Drug resistance has been confirmed in potato growing areas in the United States and Canada. This means that some growers may need to consider other methods to control pink rot, such as the application of phosphoric acid.
How do you know if there are metalaxyl-resistant pink rot isolates on your farm? Submit the tuber sample to the plant diagnostic laboratory and ask them to perform a metalaxyl sensitivity test-the tuber should show symptoms of pink rot.
Some areas have been surveyed to determine the prevalence of drug-resistant pink rot. We will conduct a survey this year in Washington, Oregon and Idaho.
We ask growers in the Pacific Northwest to look for symptoms of pink rot when harvesting or inspecting storage, and if found, send it to us. This service is free, because the cost of the test is paid from a grant from the Northwest Potato Research Association.
Carrie Huffman Wohleb is an associate professor/regional expert in potato, vegetable and seed crops at Washington State University. View all author stories here.