Apple Scab Control
APPLE SCAB CONTROLApple scab is a serious disease of apples in California, resulting in severe crop loss due to damaging blemishes on the fruit. It is most severe in coastal and foothill areas where spring and early summer weather is cool and moist. However, it can be a problem wherever apples grow when conditions are favorable for pathogen development. Apple scab also is a problem on ornamental crabapple.
HOST PLANTS: Apples and Crabapples.
SYMPTOMS: Apple Scab first appears as yellow spots on foliage. As the disease matures, dark, olive-colored spots form on leaves, fruit, and sometimes stems. Spots on the underside of leaves sometimes appear to have a velvet-like texture due to fungal growth. Affected leaves might twist or pucker; in minor cases, this will affect only a few, irregularly scattered leaves, but if the disease is severe, all foliage could show signs of infection. Severely affected leaves often turn yellow and drop.
When scab affects flower stems, it can cause flowers to die and fall off. Scabby spots can appear on fruit later in the season. These begin as fuzzy or sooty, gray-black (and sometimes oily looking) lesions that sometimes have a red ring. The lesions later become sunken and tan and can have areas of olive-colored spores around their edges. Severely infected fruit becomes distorted and usually drops from the tree. Fruit also can crack, which allows entry of secondary organisms.
FAVORED ENVIRONMENT: Apple Scab overwinter primarily in diseased foliage on the ground. Rainfall or irrigation is necessary to release the spores. In spring, wind or splashing water carry these primary spores (ascospores) from the infected leaves to flowers, leaves, or fruit where they germinate and cause disease or infection.
APPLE SCAB CONTROL: Fungicide sprays are necessary only if the weather is rainy and leaves are likely to remain wet for 9 or more hours. Fungicide applications require careful attention to timing, as preventing early infection is the most important step toward successfully controlling later fruit infections. It is difficult to prevent secondary fruit infections once primary infections occur.
Several fungicides are available for controlling apple scab. These include fixed copper, copper soaps (copper octanoate), sulfur, mineral or neem oils, and myclobutanil. All these products except myclobutanil are considered organically acceptable.
Generally copper or Bordeaux sprays should be used only from green tip to full bloom. Later applications increase the risk of fruit russetting, a chemical burning of the fruit skin, although in some years this occurs even if you’ve used these materials only before full bloom. You can apply wettable sulfur through bloom and early fruit set. When using sulfur-containing compounds such as wettable sulfur, never apply them within 3 weeks of an oil application or when temperatures are near or higher than 90°F.