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
1). 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 1).
This 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.
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 chemicals 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
pseudothecia formation in overwintering apple leaves would probably eliminate scab as a
serious threat to apple production in the U.S. Unfortunately, complete elimination
of pseudothecia 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 (Table of fungicide
effectiveness). 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 infection period table. Protectant fungicides 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 fungicides 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 (Va./W.Va./Md. Recommendations)
- home orchardists (Virginia Home Orchard Management Bulletin)
Arneson, P. 1998. Apple Scab. Cornell University Department of
Plant Pathology (Ithaca) Web Site.
Ellis, M. A.,
Chatfield, J., and Draper, E. 1997. Scab of Apple and Crabapple. The Ohio State
University Extension Factsheet HYG-3003-94.
Grove, G. G. 1997. Apple Scab. Washington State University
Tree Fruit Research and Extension Center.
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.
L., and Gubler, W. D. 1997. Apple Scab. UC Pest Management Guidelines, University of
California Statewide Integrated Pest Management Project.
Based on original text
prepared by A. R. Biggs and K. D. Hickey for the Mid-Atlantic Orchard Monitoring
Guide, and A. L. Jones and T. B. Sutton in Diseases of Tree Fruits in the East