Potato Pointers: The Big Three: Fast Moving Potato Storage Diseases
August 29, 2011 
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Pink rot (Phytophthora erythroseptica), late blight (Phytophthora infestans) and pythium leak (Pythium ultimum) all have several things in common. One is the fact that the causal organisms are no longer considered to be fungi. For some time now, they have been classified as members of the Kingdom Protista which includes such diverse members as the algae and the organism that causes malaria in humans. We always knew these organisms were somewhat different from the “true” fungi that cause so many of the other diseases of potato. For one thing, these three organisms produce self-motile spores, called “zoospores,” a factor that can be very important in the disease cycle of both late blight and pink rot.
One other important factor that all three diseases have in common is that they often don’t show up until after potatoes have been placed in storage and, once established in storage, they can very rapidly rot your potatoes. Leak must have a wound to infect the tuber and, while mechanical damage to tubers may not be absolutely mandatory for infection to occur with late blight and pink rot, both of these diseases are greatly favored by tuber wounding, as well. Not only does wounding favor infection by all three of these pathogens, wounds can also be exploited by other storage pathogens such as the true fungi that cause early blight (Alternaria solani) and dry rot (Fusarium spp.).
First Hours Of Storage Most Critical
Unfortunately, the first 12 to 24 hours in storage, a time when most storage managers have very limited options for control of temperature or ventilation parameters because the storage facility is still being loaded, may be the most critical time. This is because the wounding and simultaneous inoculation of the tubers occur during harvesting and handling and the infection activities of the various pathogens (spore germination, penetration of the plant tissues, and establishment of the organism within the host) probably occur within the first 12 hours or so for all of these diseases. A bad situation can become even worse if night time temperatures remain too warm to provide cool air to reduce tuber temperatures. Starting the ventilation system as soon as you have the first few duct pipes covered is highly recommended. Some storage managers, particularly those out here in the West under our low humidity conditions, may be able to provide some relief with the use of evaporative cooling, if their storage is so equipped.
Probably the most important management tool for all three of these diseases is prevention. For late blight and pink rot there are foliar fungicides and other materials, such as phosphorous acid, that do a very good job of reducing the level of disease. Late blight requires frequent fungicide applications during the growing season because it is the only member of this trio that is both an aggressive foliar and tuber pathogen. Pink rot can often be adequately managed with two to three foliar fungicide applications.
For late blight and pink rot, phosphorous acid can also be applied to tubers as they are being loaded into storage. Fungicide applications in the field and at harvest for leak have been considerably less successful. For all three diseases it is best to avoid bruising and high temperatures during harvest. Leak, in particular, is almost exclusively associated with tuber wounding during harvest in combination with high pulp temperatures.
Damage Control Is Key
Bottom line: Practice prevention during the growing season, avoid bruising and other types of mechanical damage that breach the tuber skin during harvest, and don’t harvest when pulp temperatures are approaching 70°F.
Careful attention to pulp temperatures may be the most important management tool available. Cooling tubers as rapidly as possible is also a good practice when you know they are too warm or you are aware there is a disease problem.
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