Symptoms: Scab is similar to a large leaf spot, but the affected area looks dull, is usually dark olive green and has indefinite margins. The fruit symptoms are usually sunken, corky, dark olive areas.
Cause: Scab is a fungal disease that attacks many species, including crabapple, apple, quince and pear. The disease in some years can completely cover leaves of susceptible cultivars. Such severe infections, referred to as "sheet scab," will defoliate an entire tree. The loss of infected leaves weakens the tree. Scab can defoliate a tree or shrub several times in one season.
The scab fungus forms two types of spores. The first type is blown about in the wind and causes the first infections in the spring. These infections form a second type of spore that is spread by rain splash and infects only the same plant or other plants of the same species. The more rainy the season is, the more the disease spreads throughout adjacent plants.
If you can prevent the first type of spore from infecting the plant in early to midspring, the plant will escape further infections that season. The first type of spore is formed on fallen leaves of the previous year.
Control: Rake and burn fallen leaves before spring. If your plant has a history of serious scab infections, apply a fungicide when swollen flower buds show a half inch of green tissue. Follow with applications, as needed, to protect the plant throughout the blooming period, whenever dew or rain threatens to wet leaves for a 6-hour period. Replace highly susceptible cultivars with resistant cultivars.
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