This is the first part of a series on spring tree diseases. This article examines anthracnose, and apple scab.
In spring, as the growing season commences, trees become vulnerable to infection from a bevy of diseases. While some of these diseases are relatively benign, many of them can cripple their hosts. The following describes some of the most common diseases to afflict plants in spring, and how they impact their hosts.
Anthracnose is a group of fungal diseases that affect many deciduous and evergreen tree species. Infections often occur in spring. Dark sunken lesions appear on the leaves, stems, twigs, flowers, and fruit of infected trees. Prolonged infection can lead to severe leaf blighting, and foliar dieback.
Anthracnose is most common on trees in landscapes, but it may also occur on trees growing in forested settings. Ash, beech, birch, black walnut, buckeye, butternut, dogwood, elm, hornbeam, linden, maple, oak, sycamore, and walnut trees are the most vulnerable to infection.
Symptoms of Infection
Anthracnose diseases cause aesthetic problems for trees. Symptoms of anthracnose vary by tree species, and the causal fungus. General symptoms range from minor necrotic spotting of leaves, to blighting of leaves and shoots. Severe infections can cause leaves to become distorted, and drop prematurely. They may also result in dieback of twigs and branches. These injuries become more extensive as the infection progresses. If the infection persists over a number of years, the affected tree may decline, and eventually expire.
- Prune and remove any dead wood from trees. Dead tissue is susceptible to infection from anthracnose fungi.
- Remove any symptomatic tissues from the tree, including infected leaves, branches, shoots, and twigs attached to or surrounding the tree. This will help reduce the number of fungal spores available to infect emerging shoots and leaves in spring.
- Maintain overall tree vigor by employing sound cultural practices.
- Ensure that trees are sufficiently watered, especially during dry periods.
- Fertilizing, mulching, and pruning are beneficial to trees. Applying a layer of organic mulch will improve soil quality, and promote healthy tree growth.
- For stressed trees, chemical control practices can be beneficial. Apply a fungicide spray just prior to bud break, before new leaves and shoots expand. Two or three additional sprays should be subsequently applied at ten to fourteen day intervals. Further applications may be necessary during wet or prolonged spring conditions.
Apple Scab (Venturia inaequalis)
Apple scab is a fungal disease that infects various apple and crabapple trees. It is caused by the pathogen Venturia inaequalis. Apple scab is one of the most serious diseases of apples and crabapples. Apple scab infections are capable of disfiguring trees. Severe infections may also reduce or eliminate fruit yield for a single growing season.
The apple scab pathogen primarily infects apples and crabapples. Infections may also occur on hawthorns, mountain ash, firethorn, and loquat.
Symptoms of Infection
Infections are generally most conspicuous on leaves and fruit. Lesions initially become apparent in spring. They may be observed on the underside of expanding leaves. As the leaves develop, the upper surface becomes vulnerable to infection. Lesions first appear as light green sections on infected leaves. Lesions are generally circular in shape. As the growing season progresses, the lesions enlarge, and deepen in color. Lesions that form on young leaves may exceed 1 cm in diameter. Older leaves are more resistant to infection. As such, lesions that are observed on older leaves tend to be smaller. Infected leaves will often become distorted, with the lesions appearing cracked and torn. Many infected leaves will be prematurely shed, resulting in significant defoliation of the host. Successive defoliations can reduce tree vigor, rendering trees more prone to insect infestations, and infiltration from additional disease pathogens.
Symptoms on plant blossoms are usually characterized as small, dark green lesions at the base of the flower, on the sepals, and on the stem pedicel. Extensive infection of flowers pedicels may cause developing fruit to drop, resulting in a lower fruit yield. The first noticeable symptom of infection on fruit is the appearance of water-soaked areas, which coalesce to form green to brown lesions. Infections of young fruit will often result in fruit distortion. Severely infected fruit may be malformed.
- Many varieties of apple and crabapple are partially resistant or immune to apple scab. Resistant varieties of apple include ‘Dayton’, ‘Freedom’, ‘Honeycrisp’, ‘Liberty’, ‘McShay’, ‘Pristine’, ‘Pixie Crunch’, ‘Redfree’, and ‘William’s Pride’. Varieties of crabapple that exhibit resistance to apple scab include ‘Adams’, ‘Adirondack’, ‘Baskatong’, ‘Bob White’, ‘Calocarpa’, ‘Candywhite’, ‘Christmas Holly’, ‘Coralburst’, ‘David’, ‘Donald Wyman’, ‘Firebird’, ‘Harvest Gold’, ‘Indian Magic’, ‘Indian Summer’, ‘Jewelberry’, ‘Lollipop’, ‘Louisa’, ‘Malus x zumi’, ‘Mary Potter’, ‘Molten Lava’, ‘M. Floribunda’, ‘M. halliana var. parkmanii’, ‘Prarie Maid’, ‘Prariefire’, ‘Purple Prince’, ‘Red Jewel’, ‘Robinson’, ‘Royal Beauty’, ‘Tina’, ‘Sargent’, ‘Silver Moon’, and ‘Strawberry Parfait’.
- Susceptible trees require fungicide sprays every year to control the apple scab fungus. Chemical fungicides are generally utilized for two purposes: as a preventative practice, or as a curative tool. Preventative fungicides are sprayed on the leaves and fruit prior to infection. Fungicidal applications prevent the fungal spores from effectively germinating, or penetrating the host tissue. Curative fungicides express limited systemic activity. If applied shortly after infection, curative fungicides may hinder development of the fungus, limiting disease progression. Initial applications should be performed in spring, as leaf expansion begins. Applications should continue until mid-June, when petal drop occurs. During this period, susceptible foliage and fruit should be inspected for scab lesions. If no leaf spots are present, applications may cease. If scab lesions are detected, applications should proceed until the end of summer. Refer to fungicide labels to determine the recommended spray interval.
- Rake and dispose of fallen leaves in autumn. Infected leaves can be burned or buried.
- Prune trees periodically to improve air circulation throughout the crown. Remove vigorous water sprouts that have formed along the main trunk, or within the canopy to increase sun penetration, and promote rapid drying of the leaves following rainstorms. This will help to create an environment that is less favorable to infection.
Photo courtesy of International Institute of Tropical Agriculture CC-by-2.0