I. Introduction: Apple scab is of major economic importance in the mid-Atlantic region. If not controlled, the disease can cause extensive losses (70 percent or greater) where humid, cool weather occurs during the spring months. Losses result directly from fruit or pedicel infections, or indirectly from repeated defoliation which can reduce tree growth and yield.
II. Symptoms: Apple scab can be observed on leaves, petioles, blossoms, sepals, fruit, pedicels, and less frequently, on young shoots and bud scales. The first lesions are often found on the lower surfaces of leaves as they emerge and are exposed to infection in the spring. Later, as the leaves unfold, both surfaces are exposed and can become infected. Young lesions are velvety brown to olive green and have feathery, indistinct margins (photo 2-5). With time, the margins become distinct, but they may be obscured if several lesions coalesce. As an infected leaf ages, the tissues adjacent to the lesion thicken, and the leaf surface becomes deformed. Young leaves may become curled, dwarfed, and distorted when infections are numerous. The lesions may remain on the upper and lower leaf surface for the entire growing season; occasionally, the underlying cells turn brown and die, so that brown lesions are visible on both surfaces. The number of lesions per leaf may range from one or two to more than a hundred. The term "sheet scab" is often used to refer to leaves with their entire surfaces covered with scab. Young leaves with sheet scab often shrivel and fall from the tree. Infections of petioles and pedicels result in premature abscission of leaves and fruit, respectively. In late summer or early fall, lesions may appear whitish due to the growth of a secondary fungus on the lesion surface.
Lesions on young fruit appear similar to those on leaves, but as the infected fruit enlarge, the lesions become brown and corky (photo 2-6 and 2-7). Infections early in the season can cause fruit to develop unevenly as uninfected portions continue to grow. Cracks then appear in the skin and flesh (photo 2-8), or the fruit may become deformed. The entire fruit surface is susceptible to infection, but infections early in the season are generally clustered around the calyx end. Fruit infections that occur in late summer or early fall may not be visible until the fruit are in storage. This syndrome is called "pin-point" scab, with rough circular black lesions ranging from .004 to 0.16 inch (0.1 - 4 mm) in diameter (photo 2-9).
III. Disease Cycle: Although research in New York has shown that the scab fungus can overwinter in trees as conidia on bud scales, the pathogen generally overwinters in leaves and fruit on the orchard floor. Ascospores are the major source of primary inoculum and are produced within pseudothecia that develop in leaves during the winter months. In a typical year in most locations, the first mature ascospores are capable of causing infections at about the time of bud break or soon thereafter. Ascospores continue to mature and are discharged over a period of five to nine weeks, with peak discharge during the pink to petal fall phenological stages. The length of time required for infection to occur depends on the number of hours of continuous wetness and the temperature during the wet period (Table 2). Young leaves remain susceptible for five to eight days, but their lower surfaces may become infected in late summer. For fruit, the duration of the wet period required for infection increases with the age of the fruit, which remains susceptible until harvest. Once the fungus is established in the leaf or fruit, conidia form on the surface of the lesion and become the source of secondary inoculum for the remainder of the season. Conidia are disseminated to developing leaves and fruit by splashing rain and wind. Several secondary cycles of conidial infection may occur during the growing season depending upon the frequency of infection periods and the susceptibility of host tissue.
IV. Monitoring: During the dormant period, no monitoring is required by growers. Consult with regional Cooperative Extension Service personnel to determine the onset of ascospore maturity. An awareness of the scab inoculum situation during the previous year and in adjacent abandoned or commercial orchards may influence early-season scab control decisions. During the prebloom period, for both fresh and processing apples, determine apple scab infection periods by observing duration of leaf wetness and average temperatures during the wet period (Table 2, chapter 2). This Mills table provides an indicator of time required for lesion expression. Begin monitoring for first leaf symptoms (photo 2-5) on early-developing cultivars such as Delicious by examining the upper and lower leaf surfaces on a minimum of ten leaf clusters on each sample tree. In monitoring, walk around the perimeter of the tree and examine at least two leaf clusters at each of the four compass directions. Record the total number of clusters with scab lesions. For fresh market production, more than one infected leaf cluster per tree represents potentially damaging levels of apple scab. For processing apples, one to ten infected clusters represents a moderate risk, and more than ten infected clusters represents a high risk.
During mid-season and the preharvest period, no monitoring is required for processing apples at this time. For fresh market production, continue monitoring for lesions on leaves of vegetative terminal shoots (photo 2-5) and on fruit (photos 2-7, 2-8). Walk around the perimeter of each sample tree and examine at least two terminals at each of the four compass directions and 25 fruit per tree. Record the total number of terminals and fruit with scab lesions. More than one infected fruit per tree is a potentially damaging level for the fresh market. After harvest, for both fresh and processing apples, determine the percent of leaves infected and number of lesions per infected leaf (photo 2-5) on six terminal shoots from each sample tree after harvest and before natural defoliation begins. Greater than 0.5 percent leaves infected with an average of one lesion per leaf represents significant risk of early scab infection next season.
V. Management: Management of apple scab is multifaceted, with resistant cultivars, sanitation, and chemical all being used to some degree depending on the orchard system being used and the goals of the grower.
Resistant cultivars: Most of the major apple cultivars are susceptible to the fungus, although this varies somewhat (Table of apple cultivar susceptibility to V. inaequalis). Breeding programs to develop high quality disease-resistant apple cultivars are underway at the New York State Agricultural Experiment Station in Geneva, and cooperatively among Purdue University, Rutgers University, and the University of Illinois. In addition, several foreign countries have active apple breeding programs with disease resistance as an objective. More than 25 scab-resistant cultivars have been released, included Prima, Priscilla, Jonafree, Redfree, Liberty, Freedom, Goldrush, and Pristine. Most are adapted to the more northern apple-growing areas of the U.S. All scab-resistant cultivars vary in their susceptibility to other early-season diseases; and all are susceptible to the summer diseases (Table of scab-resistant cultivar susceptibility to other diseases). Some recently released apple cultivars that have not been bred specifically for resistance to scab show varying levels of scab susceptibility, also (Table of foliar disease susceptibility of new apple cultivars in West Virginia).
Sanitation: Prevention of pseudothcial formation in overwintering apple leaves would probably eliminate scab as a serious threat to apple production in the U.S. Unfortunately, complete elimination of psuedothecia is not possible under orchard conditions with current methods. The potential for severe scab can be reduced with several methods that vary in practicality, depending on orchard size. Leaf pickup and destruction in late autumn can be employed if orchard size is extremely small; although diseased leaves from neighboring areas can be blown in. Flail mowing in late autumn to chop litter can help reduce numbers of pseudothecia. Applications of 5% urea to foliage in autumn can hasten leaf decomposition, thus reducing formation of pseudothecia. Applications should be made just prior to leaf fall to avoid stimulating tree growth and predisposing the trees to winter injury.
Chemical control: Apple scab is controlled primarily with fungicide sprays. A variety of fungicide sprays with differing modes of action are available. When and how they are used depends upon their mode of action. Protectant fungicides prevent the spores from germinating or penetrating leaf tissue. To be effective, they must be applied to the surface of susceptible tissue before infection occurs. Occurrence of infection can be predicted with an accurate weather forecast and the Mills table above. Protectant fungicdes are applied routinely at 7 to 10 day intervals or according to anticipated infection periods.
Postinfection fungicides control the scab fungus inside leaves and fruit. These chemicals can penetrate plant tissues to eliminate or inhibit lesion development. The ability of these fungicides to stop infections is limited to a few hours, or up to few days (depending upon the specific fungicide), and often varies with temperatures during the first 24 to 48 hours after infection. Some fungicides can inhibit the fungus even later into the incubation period (the time between infection and the appearance of symptoms). Eradication of scab lesions after they appear does not usually occur, but can be achieved with the proper rate and timing of certain fungicides. The selection of fungicdes for management of scab is based on several factors, including the entire spectrum of other diseases that must be managed at that time, the potential for resistance in the scab fungus to the selected chemical, the history of the disease in a particular orchard, the final market for the fruit, and other social and economic factors. Good horticultural practices, such as proper site selection, tree spacing and annual pruning, facilitates chemical control by improving spray coverage and reducing the length of wet periods.
Chemical control - commercial growers
Chemical control - home orchardists (pdf file - Acrobat Reader required)
Ellis, M. A., Chatfield, J., and Draper, E. 1997. Scab of Apple and Crabapple. The Ohio State University Extension Factsheet HYG-3003-94.
Ritchie, D. F., Sutton, T. B., and Sorensen, K. A. 1997. Disease and Insect Management in the Home Orchard. North Carolina State University Cooperative Extension.
Teviotdale,B. L., and Gubler, W. D. 1997. Apple Scab. UC Pest Management Guidelines, University of California Statewide Integrated Pest Management Project.
Text prepared by A.R. Biggs and K.D. Hickey
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