Global: Leak and Pink Rot Risk Rises When Potatoes Are Lifted Above 18°C Pulp Temperature
Two water-borne tuber rots can destroy a warm, wet harvest within days. Extension guidance from Maine and California sets the same simple harvest limit: tuber pulp below 65°F, or about 18°C.
Leak and pink rot are two tuber diseases that often start at harvest. Leak is caused mainly by the soil organism Pythium ultimum. Pink rot is caused by Phytophthora erythroseptica. Both enter tubers in wet soil and both do most damage in warm crops.
University of Maine Cooperative Extension advises growers to avoid harvesting when tuber pulp temperature is 65°F or higher. That is about 18.3°C. The UC Statewide IPM Program in California gives the same limit for pink rot. It advises against harvesting tubers with pulp temperature above 65°F.
How fast leak develops
Leak symptoms appear when tuber pulp temperatures reach 65°F, according to the Maine bulletin. The fungus grows rapidly above 77°F, or 25°C. At high pulp temperatures, infected tubers can show leak in as few as 36 hours. The disease is halted below 50°F, or 10°C.
Weekdays. Unsubscribe in one click.
Leak needs a wound to enter. Brown, water-soaked lesions form around wounds. Inside, the flesh turns into a dark, watery rot. In late stages only a hollow shell remains, and pressure releases a clear to brownish liquid. The spores can survive in soil for many years. The Maine bulletin notes that Pythium does not spread well from tuber to tuber in storage.
Pink rot and wet soils
The pink rot pathogen becomes active when soil is saturated with water, according to UC IPM. Growers should avoid prolonged saturation during irrigation. In store, the fungus is inactive below 40°F, or 4.4°C.
UC IPM lists fungicide timing for pink rot at flowering and again 14 days later. It also describes applications 2 and 4 weeks before harvest. Fungicides in groups 1, 4, 9, 11 or 17 should get no more than one application before rotating to another mode of action. Phosphite can be applied after harvest at 0.1 gallon per ton of tubers.
Why it matters
Many Northern-hemisphere crops are still being lifted in late September. Warm afternoons and wet fields create the exact conditions for these rots. Lifting in the cool morning hours can keep pulp temperature below the limit.
A few rotting tubers can wet their neighbours and open the door to bacterial soft rot. A probe thermometer in the field is a low-cost tool. It helps decide when to start and stop the harvester each day.
Photo: Potato tubers inoculated with the pink rot pathogen in a laboratory study (illustrative). He Jiang, Hye Weon Hwang, Tongling Ge, Barbara Cole, Brian Perkins, Jianjun Hao / Wikimedia Commons, CC BY 4.0
