Pear Scab, or Black Spot is caused by the fungus Venturia pirina, and is a major disease of apples and pears in various producing regions. 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 Pear Scab Growing Degree Day Model to track ascopore maturation in the spring, based on local weather factors.

Calculating Growing Degree Days

The Pear Scab growing degree model uses a lower threshold of 32F.

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 swell stage. After this, ascopores will reach 50% maturation at about 800 GDD (F) and 100% maturation at about 1620 GDD (F). Within this time frame, fungicides will have the highest likelihood of success in order to prevent infection later in the season.

Source

Spotts, R. A., Cervantes, L. A., and Niederholzer, F. J. A. 2000. Pear scab: Components of potential ascospore dose and validation of an ascospore maturity model. Plant Dis. 84:681-683.