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The disease is caused by the fungus Sphaerotheca macularis and is an "obligate parasite." This type of pathogen needs to reside in a living host for its survival and can be found overwintering in old, but living leaves. The pathogen can affect the flowers and fruit in all stages of development. Flowers are deformed and killed, immature fruit becomes hard and does not ripen normally, and mature, ripe fruit is soft, pulpy, and may fail to color. In the spring, the fungus sporulates on the leaves, causing the leaflets to curl upwards along the edges. The lower leaf surface may turn reddish, and a powdery, "frosty" growth of the fungus is often seen. Disease development is influenced primarily by rainfall and temperature, with dry weather and cool temperatures being more favorable.
The use of resistant varieties and adequate plant and row spacing aid in the control of this disease. The removal of overwintering leaves may be of some benefit.
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