Potato Grower

June 2010

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UNIVERSITY OF IDAHO by Phillip Nolte, University of Idaho extension professor Pay Attention Tuber bruising and pulp temperature during harvest MANY OF THE STORAGE DISEASE problems that growers face actually begin during harvest. These problems occur because several important storage diseases gain access to the tuber through wounds that are inflicted at harvest. Not only are wounds readily accessed by several important pathogens, storage problems can be made considerably worse in the presence of high or low tuber pulp temperatures. Careful attention to reducing or eliminating bruising and wounding and limiting harvest activities to periods when tuber pulp temperatures are within the recommended range can go a long way toward eliminating these problems. We’ll focus on three of these diseases: Fusarium dry rot, early blight tuber lesions and pythium leak. All three of these diseases exploit wounds to infect the tuber, and all three will be a great deal more serious if tuber pulp temperatures are high. When I say “high” I mean anything over 65 degrees F. If potatoes are harvested at temperatures above 65 degrees, then steps should be taken to lower pulp temperatures immediately after tubers are placed in storage. On the other end of the temperature scale, it is best to avoid harvesting when tuber pulp temperatures are below 45 degrees F. Tubers are more prone to all forms of bruising when they’re cold but “shatter bruising” tends to be more severe when tubers are cold. These large, open wounds are easily invaded by pathogens. EARLY BLIGHT Early blight tuber lesions can affect most types of potatoes, but the disease is far more likely to be a problem in certain varieties. Ranger Russet and Western Russet as well as a many of the thin- skinned specialty and chipping varieties are all very susceptible to early blight in the tubers. Extra care is recommended when producing these varieties to avoid damage and high pulp temperatures. Of course, early blight management begins in the field with a strong program to prevent early blight from getting out of control in the foliage of these varieties. Keeping the disease out of the field drastically lowers the amount of early blight inoculum available and can greatly reduce the opportunity for tuber infection. Unlike the other two diseases discussed here, early blight can infect tuber eyes as well as invade wounds. FUSARIUM DRY ROT Fusarium dry rot has no effect whatsoever on the foliage of the potato. It is strictly a seed decay and storage pathogen. The main method for managing dry rot in storage is to avoid creating the wounds that are required for infection to occur. For a number of years, we were able to avoid significant damage from dry rot by using a post-harvest fungicide like Mertect 340F. This product is still effective for some producers but, with the advent of widespread resistance to this chemistry during the 1990s, this product may be ineffective in many parts of the country. New post-harvest materials are currently being developed, but it may still be several years before they are available. For the time being, bruise and tuber damage avoidance is your best management policy. PYTHIUM LEAK Pythium leak is also associated with tuber wounding, but it is greatly favored by high pulp temperatures (70 degrees and above). In-field applications of Mefanoxam (Ridomil and other products) may help to reduce the potential for problems, but any benefit can be easily negated if tuber damage is severe enough. The most important recommendation for pythium is to avoid high pulp temperatures. Most of the time when I diagnose a pythium problem, the tubers have been harvested at tuber pulp temperatures well in access of 70 degrees. As we begin to plan ahead for the harvest, make bruise reduction and frequent tuber pulp temperature checks a part of your harvest routine. Avoiding the problem in the first place is the best form of management. PG Early Blight Dry Rot Pythium Leak Early Blight. Wounds on these Western Russets were invaded by the early blight fungus during harvesting and handling. Courtesy photo. Dry Rot. Wounds created on this Russet Burbank tuber were also created and invaded during harvest and handling. Courtesy photo. Pythium Leak. Pythium leak infection in tubers harvested at pulp temperatures over 75 degrees F. This level of damage was visible less than a week after the tubers were placed in storage. Courtesy photo. www.potatogrower.com 27

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