Fusarium dry rot of potato, caused by Fusarium sambucinum or F. coeruleum, is a disease that develops in storage and is universal to all market sectors of the industry. The presence of dry rot decay in the tuber has significant impact on overall quality, product and trim losses and the potential for other disease development such as bacterial soft rot.
Identification of dry rot in storage can be fairly distinctive, although sometimes the wet phase can be deceptive to identify. Dry rot typically has a final tuber decay that is dry, crumbly and brown in color, with collapsed tissue often infected with secondary white or other colored fungal growth. These infected tubers often shrivel up and become mummified.
Initial stages of dry rot can look very different. For instance, F. sambucinum infected tubers initially have a dark brown-black color tunneling through the tuber. The infected tissue has the consistency of healthy tissue and has not broken down into the dry or papery rot yet. Early infections of F. coeruleum appear tan in color and spread uniformly throughout most of the tuber.
Disease development of Fusarium dry rot can be amplified depending upon variety, harvest and handling conditions, tuber characteristics and storage temperatures. New variety susceptibility to Fusarium dry rot decay can influence market acceptance. There is a wide range in susceptibility to dry rot decay between varieties as seen in Table 1.
Varieties are evaluated at the University of Idaho for three storage seasons. This three-year evaluation is assessed by wounding potatoes, inoculating with Fusarium and storing at 45 degrees F for approximately five months. For example, Russet Burbank has a moderate dry rot potential classification, whereas Clearwater Russet has a high dry rot potential. Identifying and growing less susceptible varieties to dry rot development is a solid management strategy to begin the storage season. Unfortunately, you may not always have the market option to select less-susceptible varieties, but having awareness of the potential for dry rot development will help modify harvest, handling and storage management strategies to lessen the impact of dry rot.
Some strategies to lessen dry rot development may impact other disease development or quality parameters. Preliminary (one-year data only) research at UI evaluated the impact of vine-kill timing of Premier Russet. Results indicated dry rot incidence and severity was lowered when potatoes were harvested under four-week dead vines, compared to two-week dead vines or green-dug.
Conversely, processing quality was better in the green-dug potatoes. In addition, literature indicates that the longer potatoes remain in the ground under dead vines, the greater the potential for development of tuber silver scurf and black scurf (Rhizoctonia). Additional research needs to be performed to ascertain whether altering vine-kill timing can be used to mitigate dry rot without sacrificing quality.
Additional dry rot mitigation strategies may be implemented at harvest and in storage, such as harvesting at proper pulp temperatures, creating wound healing conditions and lowering storage temperatures. Preliminary research with Premier Russet indicated a lower severity of dry rot decay with warmer pulp temperatures (65 versus 45 degrees F) at harvest and lower holding storage temperatures (42 versus 45 degrees F). Scientific literature indicates Fusarium has greater disease development above 59 degrees F. Additional research is needed on ideal storage temperatures to minimize dry rot development and to see if varieties respond differently based upon their susceptibility.
Unfortunately, there are limited post-harvest products currently available in the U.S. industry for suppression or control of Fusarium dry rot in potatoes. For many years, the industry used TBZ or Mertect (thiabendazole), but due to resistant populations of Fusarium to the product, there is currently limited use. The industry has been utilizing bio-control agents or general biocides for dry rot suppression in storage. Phosphorous acid is used in the industry for pink rot, late blight and silver scurf control, but lacks dry rot suppression capabilities. Multiple years of testing phosphorous acid on several varieties have shown that a post-harvest application of phosphorous acid will not aid in controlling dry rot in storage (Table 2).
Dry rot management in storage is an integrated approach that includes variety selection, handling and harvesting conditions and storage management. Additional research is in progress to look at if vine kill management and storage temperatures can be integrated into the overall dry rot control strategy.