| Apple -- Scab | |||
| See Also: Crabapple -- Scab | |||
| Cause:
Venturia inaequalis, a fungal disease that is common and destructive west of the Cascade range. Each year there can be an average of 8 infection periods lasting an average of 28.5 hours (as calculated for Western Oregon from 1989 to 2001). Economically important scab outbreaks can also occur in central Washington, Hood River, and eastern and southern Oregon. In these more arid districts, the disease is more of a problem when overtree irrigation is practiced.
The apple scab fungus overwinters on dead apple leaves or fruit on the ground. During winter, black structures (pseudothecia) develop in the dead tissue. During the spring, in the presence of moisture and over a wide range of temperatures, the pseudothecia produce sexual spores (ascospores) which are forcibly discharged and moved by air currents to infect young developing foliage and fruit. Infections are visible 8 to 15 days later. They in turn produce asexual spores (conidia) in 15 to 18 days whenever weather conditions are favorable. Conidia cause new infections, continuing the disease cycle. All outer parts of unopened fruit buds are highly susceptible to infection when exposed after cluster buds break. Bud or blossom infection often leads to shedding of blossoms or to severe infection of developing fruit. Preventing infection of flower stalks and sepals is one of the most important steps in successfully controlling scab. Mature apple fruit are much less susceptible to apple scab than immature fruit. However, infections can occur near harvest which will not develop until apples are in storage. It can take as little as 6 hours of wetness at 70oF for infection of fruit around bloom, but it takes almost 48 hours of wetness at the same temperature to infect mature fruit. |
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| Symptoms:
The first visible symptoms on the leaves in spring are pale, chlorotic, water-soaked spots the size of a pinhead. These enlarge, becoming darker and smoky in appearance, later taking on an olive shade and ultimately a brownish black color. Spots may be any shape but are frequently circular. Young infections often show a radiating spread of fungus tissue through the leaf, and such areas later appear as irregular, brown-colored infections. Diseased leaves can be curled and distorted and often drop early, thus defoliating and weakening the tree.
Scab spots on fruits are at first small, raised, brown or black circular areas with no skin break. Later the skin ruptures, and the exposed tissue has a brown or black velvet-like appearance surrounded by a whitish ring or torn cuticle tissue. As fruit develops, scab spots enlarge and the central areas become brown and corky, but the dark marginal zones remain. Scabs may be scattered over the fruit surface and, when numerous, coalesce into larger irregular infections. Such fruit often cracks, allowing fruit-rotting organisms into the apple. Late-season infections may not show on fruit until they are in storage where rough, black, circular lesions develop. The lesions, usually varying in size up to 0.25 inch in diameter, are sometimes referred to as pinpoint scab or storage scab. |
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| Cultural
control:
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| Chemical
control: The key to successfully controlling scab is
to apply fungicides early and thoroughly to protect new growth. The first susceptible tissues exposed in opening cluster buds are the tips of the leaves and sepals. The most critical period for scab development is from the breaking of the cluster buds until leaves are fully expanded. Apply sprays at prepink, pink, calyx, and first cover. If the disease was a problem last year, begin applications at green-tip. Forecasting also can be used to time fungicide sprays.
Scab resistance to certain fungicides can occur if these materials are used exclusively. To minimize the development of resistant fungi, alternate or tank-mix fungicides from different families that have different modes of action. For example, use a tank-mix of a group 3 fungicide at full rates plus a different fungicide at lower rates. Some products used in the tank mix will also aid in Bull’s eye rot control.
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| Notes: Although registered, the commercial efficacy of Messenger (a bacterial protein) is unknown in the PNW and not recommended. | |||
| Forecasting: Apple scab infection periods can be determined by measuring temperature and leaf wetness with weather monitoring equipment. Although infection periods may start with rainfall, significant ascospore release will not occur in low inoculum orchards until day break. Identifying infection period means fungicide treatments can be planned. If a protective spray has been used recently, another spray will not be needed. On the other hand, if the time between the last protective spray and an infection period has been too long, choosing a material with curative or kickback properties would be wise. The system works well to reduce fungicide use in eastern states where many more fungicide sprays are used. Using the system in the Pacific Northwest may increase or decrease the number of fungicide sprays needed, depending on local weather conditions. In either case, better scab control will be realized. All ascospores will have matured and be ready for dispersal once 865 degree days (base 32oF) have accumulated since bud break. | |||
Biological
control:
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References: MacHardy, W.E. 1996. Apple Scab. Biology, Epidemiology and Management. St. Paul, MN: APS Press. Koller, W. and Wilcox, W.F. 1999. Evaluation of tactics for managing resistance of Venturia inaequalis to sterol demethylation inhibitors. Plant Disease. 83:857-863. Lennox, C.L. and Spotts, R.A. 2003. Sensitivity of populations of Botrytis cinerea from pear-related sources to benzimidazole and dicarboximide fungicides. Plant Disease 87:645-649. |
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| Content edited by: Jay W. Pscheidt on January 1, 2009 | |||
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