Potato Grower

28 POTATO GROWER | JUNE 2020 Diggin' In Diggin' In Taking a closer look at Clearwater Russet Clearer Vision Over the past several years, Clearwater Russet has seen a large increase in production due to its high proportion of marketable tubers, low reducing sugar content, and acceptance by the quick-service restaurant industry. Interestingly, it is currently the fastest-growing variety in the U.S. in terms of seed acreage. Given its sudden surge in popularity, we thought it would be a good time to review some of the key characteristics of Clearwater Russet and contrast it with that old stand-by, Russet Burbank. GROWTH AND DEVELOPMENT Compared to Russet Burbank, Clearwater Russet exhibits a slower rate of emergence and vine development early in the season. In southwestern Idaho, it reached maximum foliar growth at around 110 days after planting, compared to 98 days for Russet Burbank. The vines of Clearwater Russet also senesce more gradually at the end of the season, indicating later maturity compared to Russet Burbank. Tuber bulking starts about 10 days later in Clearwater Russet than in Russet Burbank. This characteristic, along with a higher tuber set, can lead to a smaller tuber size profile. Therefore, a 1-inch wider seed spacing compared to Russet Burbank is recommended in situations where large tuber sizes are desirable. RESPONSE TO NITROGEN Due to its later vine and tuber development, it is possible to reduce yields by overfertilizing Clearwater Russet, especially in short-season areas. In eastern Idaho, the total seasonal nitrogen requirement for Clearwater Russet can be as much as 25 percent less than Russet Burbank. However, it is important to match the total season nitrogen rate to the length of the growing season and yield potential for your area. The amount of in-season nitrogen can also have large influences on plant growth and specific gravity. For example, high rates of in-season nitrogen delayed the date of 50 percent harvest index (an indicator of the midway point of tuber bulking) by about 10 days in Russet Burbank compared to 40 days in Clearwater Russet. In terms of specific gravity, Russet Burbank exhibits a relatively large decline in this tuber attribute with increasing nitrogen rate, while Clearwater Russet is impacted much less. PROCESSING QUALITY Clearwater Russet tends to have better fry color and texture compared to Russet Burbank regardless of harvest timing. However, it is still important to harvest Clearwater near physiological maturity to avoid problems with reducing sugars, as both immature and over-mature tubers can have high sugar levels. Research in both central Washington and southwestern Idaho has shown that Clearwater Russet reaches physiological maturity around 140 to 145 days after planting. Rick Knowles of Washington State University has reported that this coincides with 60 percent green vines and approximately 3,000 growing-degree days in the Columbia Basin. He recommends harvesting within 10 to 18 days of physiological maturity to ensure the best quality out of long- term storage. Clearwater Russet can be stored at cooler temperatures compared to Russet Burbank and still maintain acceptable fry color. However, Clearwater tubers are sensitive to cold curing temperatures, so standard curing temperatures of 50 to 55 degrees Fahrenheit are still recommended. Clearwater Russet may be slower to wound heal and can have a higher overall weight loss in storage compared to Russet Burbank. Therefore, factor in early storage temperature and duration to promote good wound healing and to reduce the risk of higher weight loss and disease development. BRUISE SUSCEPTIBILITY Clearwater Russet has a higher risk for shatter and blackspot bruising compared to Russet Burbank. Warmer pulp temperatures can reduce susceptibility to shatter bruise for Clearwater Russet, but not to the extent as seen with Russet Burbank. SUSCEPTIBILITY TO DECAY IN STORAGE The severity and incidence of Fusarium dry rot decay in Clearwater Russet can be two to three times greater than in Russet Burbank. There have also been observations of pink eye development in Cleawater Russet, which led to the invasion of Fusarium and subsequent decay. Clearwater is also more susceptible to Pythium leak than Russet Burbank, so greater caution should be used when harvesting under warm pulp temperatures. Visit www.pvmi.org for more in- depth information and management recommendations on Clearwater Russet. VARIETIES | By Mike Thornton & Nora Olsen, University of Idaho

• Cover