Water rots of potato are a problem that many growers deal with to some degree every year. Tubers that have water rot appear discolored and may have a waterlogged appearance that will often be dotted with white tufts of fungal growth. There are actually two different types of water rot, pink rot and Pythium leak, each caused by a different fungus.

Probably the most prevalent of the two water rots is pink rot caused by the soil-borne fungus Phytophthora erythroseptica. This disease can be found in the field before harvest and is characterized by rotted tuber tissues that turn pink after exposure to air for 20 to 30 minutes. Another important diagnostic trait for pink rot is that the rot will usually appear to start from the stem end of the tuber and will then progress through the tuber in a very uniform manner, often with a nearly straight line between the healthy and the diseased portions of the tuber. Pure pink rot is not a slimy soft rot, but infected tissues are easily, and often invaded by soft rot bacteria which will cause this symptom. In a tuber that is infected with the pink rot fungus alone, the rotted tissues will retain some structure and firmness but not nearly as much as the healthy portions of the tuber. The texture of the infected tuber tissue is much like that of a boiled potato. We refer to this as a "cooked potato" texture. Another feature of pink rot is the characteristic smell of ammonia that is given off by pink rot infected tubers. This odor can frequently be defected in potato storages prior to the development of visual symptoms.

The other water rot is Pythium leak, often referred to as simply "leak". This disease is caused by fungi of the genius Pythium and may be either P. debaryanum or P. ultimum. These fungi are also soil-borne. The Pythium and Phytophthora genera are closely related and belong to a class of fungi known as the "water molds." This group also includes the late blight fungus, Phutophthora infestans, although the disease cycle for late blight is much different that either pink rot or leak. Leak is characterized by a rot that starts from an infection site on the surface of the tuber and generally rots out the entire central portion of the tuber while leaving the portion of the tuber from the vascular ring out to the skin of the tuber intact. This results in a condition that is often described as "shell rot." The rotted tissues are brown to black in color and may have cavities within them. The texture of the rotted tissues is not a slimy soft rot but is much more watery with a slightly granular appearance. When a diseased tuber is squeezed a clear fluid can be expressed from the tuber. This is the origination of the name "watery wound rot," another name for leak. Like pink rot, the diseased tubers can be easily invaded by soft rot bacteria. The net result with both diseases are wet spots in storage that frequently 'sink' as a result of rotting tubers being crushed from the weight of tubers above them

As previously stated, both of these fungi are soil-borne and can survive for long periods of time in the soil. The disease cycle for the two diseases is somewhat different, however. Pink rot infections are usually associated with wet conditions, low spots in the field, near wheel tracks or simply in over-watered areas or fields. Infections usually occur in the soil prior to harvest through the lenticels, eyes, or through the stolons of the tubers. This disease can rot a tuber quite rapidly, usually within just a couple of weeks. Some recent evidence obtained at North Dakota State University indicates that infection of wounds made during harvest and handling may also occur. Usually pink rot involves only the tuber but symptoms of the above-ground portions of the plant sometimes occur. Infected plants may wilt, with the leaves becoming chlorotic and eventually drying up and falling off. Occasional, aerial tubers will form as well.

In contrast, Pythium invades the tuber wounds that occur during harvest, especially when tuber pulp temperatures are high (>68^o F). For this reason, leak is usually more prevalent in years characterised by unseasonably warm temperatures during harvest. Wounds are necessary for these fungi to infect. Because of this requirement, Pythium will not be found in the field before harvest like pink rot. Pythium can be responsible for seed-piece decay in cut seed if conditions are warm and wet immediately after planting. There are no vine symptoms associated with Pythium leak.

Management recommendations:

Pink rot and leak can be managed efficiently using applications of Ridomil® during the growing season. One well timed application, when the largest tubers are the size of a nickel to a quarter, is generally sufficient. However, water rot susceptible varities such as Russet Norkotah, Red Lasoda and FL1533 may require a second application 14 days after the first.

Avoid excess watering late in the growing season particularly if temperatures stay above 75 ^oF. Look for this disease in low-lying, water stagnated areas of the field, especially around the pivot shaft. If detected, handle these potatoes separately, after the rest of the field is harvested. Delay harvest of these potatoes to confirm the presence of pink rot. If confirmed, you may wish to simply avoid harvesting problem areas. If problem areas are harvested, sort and discard infected tubers since these tubers can act as sources of inoculum causing infection of tuber wounds during harvest. Potatoes suspected of having a Pink rot problem should be placed in storage last and closer to the access doors so that they can be removed first, or removed if they begin to deteriorate. Alternatively, fields with a history of pink rot should be marketed from the field

If the disease is detected after the potatoes are in storage, provide adequate airflow through the pile. The early curing condition should be as cool as possible for the length of time it takes to dry the potatoes. If only a small portion of the pile is affected, healthy potatoes should be cured at 50 ^oF. Subsequently, a rapid cooling to the holding conditions is advantageous. Continuous airflow is a must during this period, to minimize secondary bacterial soft rots from developing.

A primary management strategy for leak is to generally avoid harvesting potatoes under extremely warm conditions ( >68 ^o F ). At these temparatures, leak development is very rapid. Since leak fungi require wounds for infections, growers should avoid mechanical injury to potato tubers during harvest.