The Postharvest Information Network scald model has been replaced by the WSU-TFREC Decision Aid System (DAS) scald model. The DAS utilizes data from the new AgWeatherNet, which includes 99 monitoring stations throughout Washington. In addition to reporting accumulated hours below 50ºF, the DAS scald model provides a graph of scald hours, a weather forecast, and orchard management activities based on current and projected conditions for each station. Access to the DAS is free; however you will need to set up a user account the first time you visit the site (http://das.wsu.edu/).
Because the DAS utilizes new weather stations, there is no historical scald data available at this time. However, values from the past three years at WSU-TFREC, Wenatchee shows some the potential year to year variation. The model threshold of 150 is usually reached in early to mid October.
Scald Prediction Model
The appearance of storage scald has been related to orchard temperature during the final weeks of fruit maturation. As the fruit is exposed to increasing periods of temperature below 50°F, the susceptibility to scald decreases. Dr. Eric Curry of the USDA-ARS has shown that when starch content is less than 3 (0 to 6 scale) and 150 hours below 50°F have NOT accumulated, scald potential is likely to remain high. If the starch progresses beyond 3, then maturity will begin to override the requirement for cool temperature and scald susceptibility will decrease. Generally, an uninterrupted accumulation of 150 or more hours below 50°F will result in fruit with low scald potential. Dr. Curry provided the following figures.
The University of Massachusetts Fruit Advisor has interactive calculators for determining scald risk of New England Delicious apples, based on harvest date, temperature and starch index.