Potato storage rots: a potential problem in wet seasons

Potatoes in Canada
January 11, 2018
By Potatoes in Canada

Usually wet seasons favour crop development, but incidence of storage rots is a concern, especially if rainfall occurs late in the growing season, advises Eugenia Banks, Ontario potato specialist.


Tubers from risky areas such as low spots, compacted areas, or any area where drainage was poor and/or you suspect the crop was under water stress are prone to soft rots. Serious economic losses are caused by the following storage rots:

Late blight
Recommendations to protect your crop from late blight rot in storage are:

  • Harvest the crop at least two weeks after topkilling. This interval minimizes the chance of tubers getting contaminated with late blight spores during harvest and allows previously infected tubers to decompose in the field.
  • Spray the dying canopy with copper hydroxide to kill any late blight that may still be active in the green areas of dying stems.
  • Apply phosphorous acid when potatoes are being stored. This helps to reduce the risk of late blight in storage.
  • Make sure that tubers go into storage dry. Manage the air temperature and humidity in storage to keep the tubers dry. Warm, moist air circulating over cold tubers will result in condensation. Wet tubers infected with late blight will start to produce spores, and the disease may spread in the pile. 
  • Hold the potatoes at the lowest temperature consistent with their ultimate use (table or processing). Most fungi do not grow much at temperatures of 38 F or lower, but some development will occur at higher temperatures.

Pink rot is a late-season rot that is most common in wet soils.

  • Avoid wounding tubers at harvest.
  • Do not harvest when tubers are wet. 
  • Apply phosphorus acid when the tubers are being stored. Phostrol or similar products reduce the spread of pink rot in storage.

Pythium leak usually develops when potatoes are harvested under hot, humid conditions but occasionally it can be also found on tubers in wet areas of fields before harvest.

  • Do not harvest the crop when tuber pulp temperatures are above 65 F, especially when the weather is warm and it would be difficult to remove field heat from stored tubers.
  • Minimize bruising.

Soft rot
A number of bacteria cause soft rots; the most common is Pectobacterium carotovorum (formerly Erwinia carotovorum). This bacterium is often found on the tuber surface or in the lenticels and wounds of harvested tubers. If moisture levels during storage are too high, leading to condensation on the tuber surface, then rapid multiplication of the bacteria leads to soft rotting, accompanied by an increase in temperature of the pile which itself is favourable for further decay. These “hot spots” can rot healthy tubers because soft rot will spread from tuber to tuber wherever tubers are in contact. Tubers affected by late blight, pink rot or pythium leak often develop soft rot as well.

Soft rot can rapidly get out of control in storage and create hot spots that can rot healthy tubers. Practices that reduce soft rot in storage include:

  • Do not harvest under wet conditions and do your best to store dry potatoes.
  • Minimize tuber bruising. 
    Increase air circulation and lower humidity if stored potatoes are at risk of soft rot. 
  • Monitor storage closely, and market the crop promptly if soft rots begin to develop

(Hydrogen peroxide (Storox) is registered for soft rot treatment to post-harvested potatoes in storage. According to the label, Storox is applied as a direct injection into humidification water. Apply the diluted product for at least 20 minutes per day, based on a humidification airflow rate of 0.6 cfm.)

The risk of tuber rot in storage is high after a wet season. Check the tubers from risky areas – low spots, compacted areas, areas of poor drainage and areas that were flooded – before harvesting. It may not be worth it to mix these tubers with healthy tubers. This preventative practice will help you to maintain a good quality crop in long-term storage.

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