Great care is taken to minimize wounding, bruising and abrasions at harvest, but unfortunately some impact is inevitable. These wounds are entry points for Fusarium dry rot and Pythium leak. Both of these diseases need some access beyond the skin to infect the tuber and cause disease. Pink rot does not necessarily need a wound to infect, but wounding will increase the potential for disease development. Incomplete skin set can also be considered an entry point for these pathogens.
In addition, enlarged lenticels also may be an entry point for pink rot and Pythium leak. Whether disease infection will occur or not due to wounding is often dependent upon pulp temperature of the harvested tuber. Monitoring pulp temperatures and adjusting harvest times during the day may be necessary. Below describes a few things to consider in managing each of the three wound-influenced diseases. Some of these management strategies may need greater adherence if harvesting in a rocky field, growing a susceptible variety or growing conditions in the field increased crop susceptibility such as stress, early-die and over- or under-watering.
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 3-year evaluation is assessed by wounding potatoes, inoculating with Fusarium and storing at 45 degrees F for about 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 varieties that are less susceptible 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.
Avoid harvesting potatoes with pulp temperatures less than 45 degrees F. For some susceptible varieties, such as Premier Russet, harvest at pulp temperatures above 50 degrees F. Cold temperatures increase the potential for potatoes to shatter bruise and thus allow a perfect entry point for Fusarium dry rot. Shatter bruises are also more difficult to wound-heal due to the jaggedness of the wound. Turgid potatoes or potatoes that are well hydrated are also more susceptible to shatter bruising.
Unfortunately there are limited post-harvest products currently available in the U.S. potato industry for suppression or control of Fusarium dry rot in potatoes. Phosphorous acid is used in the industry for pink rot, late blight and silver scurf control, but lacks dry rot suppression capabilities.
A newly registered post-harvest product (Stadium by Syngenta) for Fusarium dry rot and silver scurf control in storage is a three-way mixture of azoxystrobin, difenoconazole and fludioxonil. Post-harvest disease trials have demonstrated its efficacy in Fusarium dry rot control. A recent UI study showed a 20 percent decrease in the incidence and a 12 percent decrease in the severity of dry rot decay after five months in storage. Post-harvest product applications are most effective when integrated into an overall disease management program.
Pulp temperatures above 70 degrees F dramatically increase the chance for potatoes to become infected with Pythium even without major wounding. Ideally, it is best to harvest potatoes with pulp temperatures below 65 degrees F to minimize the potential for tuber decay due to Pythium leak.
Most varieties grown in Idaho are susceptible to Pythium leak disease development. Varietal differences may be seen due to maturity and time of typical harvest (eg. early harvest under warm temperatures) and vulnerability to mechanical injury. UI regularly evaluates storability of new or soon-to-be-released varieties. The variety Teton Russet, in particular, appears to be more susceptible to Pythium leak development in storage compared to Russet Burbank and Russet Norkotah.
At this time, there is limited research suggesting a post-harvest product to control Pythium leak. The best tools are to minimize wounding when handling and harvest at cooler times of the day with cooler pulp temperatures to significantly reduce the potential for Pythium leak development.
Although pink rot does not need a wound to infect and decay a tuber, the chance for infection increases dramatically as the tuber is wounded, especially at pulp temperatures above 60 degrees F. Even if you are gently handling your potatoes, pulp temperatures of 70 degrees F increase the chances of pink rot development tremendously if the inoculum is present. Some varieties may be more susceptible to pink rot development and should be handled more carefully at harvest and pulp temperatures should be monitored closely.
Irrigation management is very important during the growing season, but minimizing wounding during handling and lower pulp temperatures at harvest are great tools, along with post-harvest application of phosphorous acid. A post-harvest product application will significantly reduce the potential for diseased potatoes to further infect healthy potatoes during the handling process.