|Mike Thornton||University of Idaho|
|Nora Olsen||University of Idaho|
|Tina Brandt||JR Simplot Co.|
Clearwater Russet was released by the Pacific Northwest Potato Variety Development Program in 2008. In the past several years there has been a large increase in production of this variety due to a high proportion of marketable tubers, low reducing sugar content and acceptance by the quick service restaurant industry. Clearwater Russet has been described as medium-late in maturity, but relatively little is known about its growth and development profile. Therefore, field trials were conducted during 2014 and 2015 to evaluate the key foliage and tuber growth parameters of maximum foliar growth, timing of 50% harvest index, vine senescence rate, tuber bulking rate, specific gravity and reducing sugar content compared to Russet Burbank, the industry standard. Four rates of in-season N (0, 90, 180, 270 kg/ha) were applied in 8 weekly increments, starting after tuber initiation. Destructive sampling was initiated at 56 days after planting (DAP) and performed approximately every fourteen days until vine kill. Compared to Russet Burbank, Clearwater Russet exhibited a slower rate of vine development, which was also reflected in lower vine senescence ratings throughout the growing season, indicating later plant maturity. Tuber bulking also started later in Clearwater Russet compared to Russet Burbank. In-season N rate had relatively large influences on both foliage and tuber growth parameters, but the response differed between the two varieties. For example, high rates of N shifted the date of 50% harvest index by about 10 days in Russet Burbank compared to 40 days in Clearwater Russet. In terms of tuber processing quality, Russet Burbank exhibited a decline in specific gravity with increasing N rate while Clearwater Russet did not. Increasing N rate was also associated with a reduction in glucose content at final harvest in Russet Burbank but had no effect in Clearwater Russet.