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Extension > Garden > Yard and Garden > Diseases > Managing apple scab on ornamental trees and shrubs

Managing apple scab on ornamental trees and shrubs

Rebecca Koetter and Michelle Grabowski

Importance

Apple scab is a very common disease, and one of the most aesthetically damaging diseases of several ornamental trees and shrubs in Minnesota. The main symptoms of the disease are leaf and fruit spots. Very susceptible trees become defoliated by mid-summer, which weakens the trees over time.

M. Grabowski, UMN Extension

Apple scab infections are initially olive green with fringed edges

Pathogen and host plants

Scab is caused by the fungus Venturia inaequalis. This fungal pathogen infects several host species including crabapples and apples (Malus spp.), mountain ash (Sorbus spp.), pear (Pyrus communis) and Cotoneaster (Cotoneaster spp.). A unique trait of this fungus is that it is composed of several host specific strains that cause scab on one plant genus but not on any other. For example, the strain of V. inaequalis that infects mountain ash will only infect nearby mountain ash trees and will not infect crabapple trees. However, the fungal strain infecting either crabapple or apple can infect both apple or crabapple trees because the trees are in the same genus.

Identification

Biology

The apple scab fungus overwinters on fallen diseased leaves. In spring, the fungus forcibly ejects spores into the air from the fallen leaves. These spores are carried by wind to newly developing leaves, flowers, fruit or green twigs. Spores need several hours of moisture on the plant surface in order to start new infections. Infections grow into spots or blotches that are capable of producing new spores within 9-17 days. These spores are spread by wind and splashing rain or irrigation throughout the canopy or to neighboring trees, initiating new infections. This cycle can repeat many times throughout the growing season whenever leaves remain wet for a sufficient number of hours. Warm rainy weather in the spring and summer results in repeated disease cycles and increases the severity of the disease.

M. Grabowski, UMN Extension

Infected leaves scattered on the lawn below a crabapple tree

Severely infected leaves turn yellow and drop prematurely, weakening the tree. Several years of early leaf loss can result in decreased growth, reduced bloom and increased susceptibility to winter injury.

Management

Resistant cultivars

Disease resistance is the most effective strategy for managing scab of flowering crabapples. Several crabapple cultivars have been developed over the years that are hardy in Minnesota and include not only scab resistance but also beautiful flower color, graceful form and bright fruit that add to the element of winter interest. The following list highlights several cultivars that have shown strong resistance and are tolerant of Minnesota's low winter temperatures (Table 1).

Table 1. List of scab resistant crabapple trees that are hardy in Minnesota

Crabapple trees resistant to scab
'Adams'
'Adirondack'
'Baskatong'
'Bob White'
'Candymint'
Coralburst®
Christmas Holly™
'David'
'Donald Wyman'
Firebird®
Harvest Gold®
'Indian Magic'
'Indian Summer'
'Jewelberry'
Lollipop®
'Louisa'
Molten Lava™
M. floribunda
M. halliana var. parkmanii
'Prairie Maid'
'Prairiefire'
'Purple Prince'
Red Jewel™
'Robinson'
'Royal Beauty'
'Tina'
'Sargent'
'Silver Moon'
'Strawberry Parfait'
Malus x zumi
'Calocarpa'

Cultural control

Fungicides

Fungicides must be applied preventively to successfully manage apple scab. Because spores are released so early in the growing season, fungicide sprays must begin when the first green leaf tips emerge in spring. Sprays should be repeated until petal drop for crabapple. If the tree is healthy and free of leaf spots at this point, further treatments are unnecessary.

Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again. (e.g. 7 to 10 days after spraying, you will need to spray again). Several factors affect what spray interval is most appropriate. In plantings where there was a severe scab infection the previous year, use the shortest interval. In plantings where scab has not been a problem, a longer interval will probably give adequate protection. In addition, if the weather is dry the longer interval is acceptable, whereas during rainy weather the shorter interval is preferable.

Once leaf spots appear in the tree, fungicides provide very little control of disease. There is no point in spraying an already heavily infected tree. Contact a certified arborist to apply fungicides to large trees. Chemical treatments are listed in the table below.

The name of the plant being treated MUST BE LISTED on the fungicide label or the product cannot be used! Some products are registered for use on ornamental crabapples but are not safe to use on crabapple or apple fruit intended for human consumption. Always completely read and follow all instructions on the fungicide label.

Active ingredient Representative Trade Names*
Tebuconazole Bayer Advanced Disease Control for Roses, Flowers and Shrubs
Myclobutanil Spectracide Immunox
Captan Bonide Captan
Chlorothalonil Bonide Fungonil, Ortho Garden Disease Control
Propiconazole Bonide Infuse
Mancozeb Bonide Mancozeb
Sulfur/lime sulfur** Safer Brand Garden Fungicide
Neem oil** Garden Safe Fungicide3
Copper# Bonide liquid Copper
*Trade names are for demonstration purposes only and do not imply endorsement by UMN Extension. Trade names may change over time. Products with the same active ingredient but different trade names should offer disease control as well.
**Burning of plant tissue may be observed especially in times of high heat.
#Russeting of fruit may occur with use of copper products.
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