The 2014 potato production season in Idaho included a couple of surprises. One of these was the heavy rainfall in the early weeks of August; the other was the occurrence of potato late blight, a direct result of all that rain. Most of the positive fields were reported from the eastern production region.
Fortunately, late blight seems to occur only sporadically in Idaho and then only when we are subjected to much higher than normal rainfall. Another reason for the sporadic occurrence is very likely due to the fact that the causal organism (Phytophthora infestans) generally doesn’t survive very well here in our desert climate and the disease must be imported into the system by some means, usually in infected seed potatoes or infected nursery plants, particularly tomatoes.
An exception to this rule occurs during the season directly following one in which late blight was present. This is because there are a number of pathways by which the disease can survive the winter out here in Idaho, meaning we won’t need to import the disease because it could already be present in the system next season.
Late blight requires living host tissue to survive from one season to the next, but there are a number of sources of living potato tissue that provide that vital “green bridge” between seasons. Volunteer potatoes are one source. Cull piles are another. Tubers infected with late blight survive just fine in the friendly atmosphere of a potato cellar. Many of these diseased tubers are easily recognized and are pulled out during grading and shipping. These tubers wind up in cull piles, and cull piles are one of the most important sources of late blight for a new production season.
For this reason, the Idaho State Department of Agriculture (ISDA) has established rules regarding the management of cull potatoes. Remember that late blight is very much a community disease and, to borrow a phrase, “it takes a village” to properly manage late blight. The ISDA rules are as follows:
- Vine and tuber waste debris are hosts for late blight and are primary inoculum for both early and mid-season outbreaks of this disease. Phytophthora spores can be massively produced on the leaves, vines, sprouts and tubers in a cull pile. All cull potato material must be routinely discarded to help prevent spore production and seasonal spread of late blight.
- ISDA regulations require daily disposal of waste potatoes from all operations, including seed cutting operations, after April 15 in the Magic and Treasure Valleys, and after May 15 in the Upper Snake River Valley of eastern Idaho.
- Daily disposal of cull or waste potatoes must continue through Sept. 20 to help protect the actively growing potato crop.
Additional information on the rules and guidelines for cull potato management can be found at the ISDA's website.
Infected seed potatoes are another significant source of late blight. Growers who suspect or know that their seed has been exposed to late blight are encouraged to use a seed piece treatment such as mancozeb that is effective against late blight. Early scouting of fields planted with suspect seed sources is also a good idea.
Growers always face an increased chance of another late blight outbreak during a season following one in which late blight occurred, especially if the weather conditions turn out to again be favorable for the disease during the second year. Along with paying attention to the cull pile regulations, growers and consultants are encouraged to scout areas in the field such as low spots or pivot towers where late blight is likely to occur first.