Apple Scab is caused by the Venturia inaequalis, and is a major disease of apples in the Pacific Northwest. The fungus overwinters on dead leaves or old fruit on the ground. During winter, black structures called pseudothecia develop in the dead tissue. In spring, the pseudothecia produce sexual spores ( called ascospores), which are discharged and moved by the wind to infect young developing foliage and fruit.

To help control this disease, researchers have developed the Apple Scab Growing Degree Day Model to track ascopore maturation in the spring, based on local weather factors.

Calculating Growing Degree Days

The Apple Scab growing degree model uses a lower threshold of 32F and an upper threshold of 110F.

It can be calculated with daily high and low temperatures, using the sine model to estimate growing degree day units, or the Pest Prophet app can be used to calculate GDD accumulation from hourly temperature data for a specific field.

The “biofix” date, or date from which to start accumulating growing degree days, should be set at bud break. After this, ascopores will reach 60% maturation at about 500 GDD (F) and 100% maturation at about 900 GDD (F). Within this time frame, fungicides will have the highest likelihood of success in order to prevent infection later in the season.

Source

D. M. Gadoury, R. C. Seem, A. Stensvand, and S. P. Falk. 1995.