This article appears in the September 2018 issue of Potato Grower.
Leak is a storage disease that can occur after tubers undergo mechanical damage at harvest, especially at warmer temperatures. Pythium ultimum is the most common leak-causing species; however, other Pythium species have been found to cause symptoms. Oospores are the main source of inoculum, infecting via openings or wounds of the periderm. The tuber periderm may appear dark, sunken and water-soaked while the flesh is a dark brown to gray color in infected tubers. If squeezed, water will readily “leak” or drip out of the tuber. These symptoms can develop within days of infection.
Cultural practices such as tuber pulp temperature and early storage management play a role in leak development, and specific relationships need to be studied further. In addition, potato cultivars developed or grown in the Pacific Northwest may differ in susceptibility. Management practices should therefore be tailored to minimize risk. As a result of these ongoing questions, research was designed to determine how harvest pulp temperatures and early storage temperature manipulation affect leak disease incidence among different russet-skinned cultivars. Bannock Russet, Clearwater Russet, Russet Norkotah, Russet Burbank, Teton Russet and Umatilla Russet were used in these trials.
In the first trial, tubers with pulp temperatures of 55, 60, 65 and 70 degrees Fahrenheit were wounded, inoculated with P. ultimum, and then maintained at the same temperatures until evaluated four days later. Russet Norkotah tubers developed less leak than Russet Burbank across the four temperatures. When the study was expanded to other cultivars, results showed leak susceptibility varied among cultivars. The least susceptible cultivars were Ranger Russet and Russet Burbank. Having knowledge of relative susceptibility to leak development helps identify cultivars of greater risk, and management can be intensified for these cultivars to minimize that risk.
Tuber pulp temperature at harvest is an important management consideration because temperature can determine the initial risk of leak development and how quickly tubers can be cooled down in storage. Our study showed that the risk of leak decreases with each decrease in pulp temperature and associated early storage temperature. This demonstrates the importance of avoiding harvesting tubers at warmer temperatures, especially with cultivars more susceptible to leak.
Harvest pulp temperatures can dictate early storage temperatures. Warm pulp temperatures are often a sign of warm outside air temperatures, which results in less available cooling air. If cooling air or refrigeration is available, growers have the ability to manipulate or lower the temperature of the incoming crop. In the case that temperatures are manipulated to be cooler, leak incidence will also decrease. However, the rate of cooling depends greatly on the pulp temperatures of incoming tubers, cooling capacity of the refrigeration system, and temperature of available outside air.
This study shows that disease incidence is affected by storage temperature more than harvest pulp temperatures. However, pulp temperature is important for determining how quickly tubers can be cooled in storage.
Cultivars should be managed differently based on their susceptibility to leak. Bannock Russet is a very susceptible cultivar. To manage its leak, harvest may need to be delayed until pulp temperatures are cooler, making it easier to immediately cool tubers to 55 degrees. Umatilla Russet and Russet Burbank are less susceptible to leak and could be held at 60 degrees, whereas Russet Norkotah is the least susceptible and could be held at 65 degrees. In all situations, the ability to cool tubers in the first few days of storage will decrease the likelihood of leak development.
Research is ongoing to determine why cultivars differ in susceptibility to leak. One observation from our research is that shatter bruise susceptibility is positively correlated with leak susceptibility. This means that cultivars that tend to shatter bruise also are more susceptible to leak. Other current research has focused toward chemical control for better management of the disease. However, there is good evidence that either harvesting or immediately cooling tuber pulp temperatures to 55 degrees is one of the most effective management procedures. If cooling is not available, then delay harvest of more susceptible tubers until pulp temperatures are lower.
Andrew Hollingshead is a Ph.D. student at the University of Idaho. Nora Olsen and Mike Thornton are extension potato specialists at UI’s Kimberly and Parma R&E Centers, respectively. Jeff Miller is with the independent firm Miller Research LLC, based in Rupert, Idaho.
This article was originally distributed as a Potato Progress newsletter for Washington, Oregon and Idaho on May 30, 2018.