Avoiding Pythium Leak

Watch out for high pulp temperatures

Published in the June 2009 Issue Published online: Jun 05, 2009 Phillip Nolte, University of Idaho Extension Professor

In the storm of events that surround our harvest activities, it's sometimes hard to keep track of all the details. But, as we all know, the devil is in those details. One devil that requires particular attention is harvest pulp temperatures.

The fall of the year in Idaho and many other potato-producing areas is often marked by weather conditions that are unsuitable for harvesting potatoes. Trouble is, you often don't know if conditions will get better or even worse if you wait any longer. This much we do know: harvesting potatoes above the recommended upper pulp temperature limit of 65°F is risky business.

Growers pay the price for harvesting too warm somewhere almost every year. As you might expect, the main problem is disease.

Just about all of the tuber diseases you find in your storage are made worse by warmer temperatures. Adding to the problem is the fact that many of these diseases gain entry into the tuber and actually start their infection cycle during the activities associated with harvest, particularly wounding.

This is one reason why so many crops that "looked perfect in the field" can wind up with serious problems in storage.

The diseases that seem to cause the most problems in storage are Pythium leak, pink rot, late blight and Fusarium dry rot. No list would be complete without mention of bacterial soft rot as well, as this dreaded storage disease often follows these other diseases and is usually responsible when really severe rotting problems are encountered. Mix any of these storage diseases with a little excess heat, and disaster is often the result. Pythium tuber infection is characterized by a soft, watery rot that usually affects the tissues inside the tuber, inward from the vascular ring. This pattern of rotting often leaves hollowed-out tubers, a condition referred to as "shell rot."

Infected tissues may be brown, gray or even black and are extremely soft and wet. If you gently squeeze a tuber that has Pythium, it will readily leak a clear watery fluid, hence the name `leak'.

This symptom is so common that Pythium is often referred to as "watery wound rot." The disease is usually confined to storage and is rarely found in the field beforehand. Another hallmark of Pythium is the incredible speed with which it can rot a potato tuber or take down a pile of potatoes.

Pythium is caused by soil-borne organisms that are probably present just about anywhere that potatoes have been produced. The causal agent of leak can be one of several species of Pythium, with Pythium debaryanum and Pythium ulitimum being the most common. These organisms require a wound in the tuber-they cannot penetrate tuber tissues through intact periderm. As I mentioned earlier, the disease is greatly favored by high pulp temperatures during harvest and handling with anything above 65°F being risky and anything above 70°F being downright dangerous.

The disease can be managed by avoiding pulp temperatures over 65 degrees when harvesting and doing as much as you can to avoid bruising. Ridomil (mefanoxam) application during the season can help but can also be easily overcome by anything more than very minor tuber damage. Unfortunately, the phosphorus acid products, which seem to work so well for pink rot and late blight, are not nearly as effective on Pythium.

If you wind up with a leak problem in storage, the best way to handle it is to cool and dry the tubers down as rapidly as possible. Early marketing of the crop may be the best option in many cases.

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