
                         Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: THURSDAY, July 20, 1995                   TAG: 9507200017
SECTION: CURRENT                    PAGE: NRV-6   EDITION: NEW RIVER VALLEY 
SOURCE: JOE HUNNINGS
DATELINE:                                 LENGTH: Medium

HOW TO KEEP SHOWY CRAB APPLES FROM TURNING INTO UGLY DUCKLINGS

Because of their spectacular flower show, crab apples are popular landscape trees in spring, but crab apples can become very unpopular, unsightly trees later in the summer if they lose all their leaves due to a fungal disease called scab. Unfortunately, this is not uncommon. In fact, scab is the most common disease of crab apples in Virginia.

Apple scab, caused by the fungus Venturia inaequalis, affects both apples and crab apples. The disease begins as diffuse, charcoal-gray to brown lesions on leaves. Infected leaves eventually turn yellow and drop off the tree. Severely affected trees can be completely defoliated by midsummer.

Some homeowners may think the tree has died after it has lost its leaves; however, while scab weakens the tree, it does not kill it outright. Fruits, as well as leaves, are infected by the fungus. The fruit lesions appear as dark, sunken scabs.

Scab can be controlled with fungicides, but sprays must be applied every 10 to 14 days throughout the growing season to achieve effective control, and this is very expensive. A better means of control is to select and plant scab-resistant cultivars. A number of cultivars with high levels of resistance to scab are available.

Scab resistance is not the only consideration in selecting crab apple cultivars, however. Other diseases, such as fire blight, cedar-apple rust, and powdery mildew (described below) can affect crab apples. Resistance to these diseases also should be a selection factor.

Fire blight is caused by the bacterium Erwinia amylovora. Although it is not as common on crab apples as scab, it can be serious. When fire blight strikes, shoots turn brown or black, as if scorched by fire, and leaves cling to the twigs. The tips of affected shoots may be curved, forming what is commonly known as a "shepherd's crook." Sunken, reddish-brown cankers may occur on larger branches or on the trunk.

Bacteria overwinter in the cankers and can spread causing new infections the following year. Fire blight symptoms appear in spring. Affected branches should be pruned out well below darkened tissue later in the summer when the bacteria are dormant. Pruning tools should be dipped in a dilute household bleach solution between cuts to prevent spread of the bacteria

Cedar-apple rust, caused by the fungus Gymnosporangium juniperi-virginianae, appears first as yellow spots on leaves. Later, the spots turn orange and then reddish with tiny black dots in the centers. This disease may cause some defoliation. The pathogen requires two hosts, red cedar and apple or crab apple, to complete its life cycle, so the disease will be more of a problem if red cedars are growing near crab apples.

Fungicides can be used to control the disease, but like scab, resistance is a better means of control.

Powdery mildew, caused by the fungus Podosphaera leucotricha, can be a serious problem in crab apples. Leaves and twigs become covered with a white, powdery substance that is actually the mycelium of the fungus. Mycelia are the vegetative parts of the fungus, consisting of branching, threadlike filaments. Early infections may cause curling and twisting of young shoots.

Fungicides can be used for control, but resistant cultivars are available.

Of course, a cultivar that has resistance to the major diseases of crab apple is useless unless it has desirable horticultural characteristics. crab apples come in various sizes and shapes. Some cultivars produce flowers on the current year's growth, others on the previous year's growth. Also, some cultivars bloom every year, while others bloom only every other year.

It is possible to select cultivars that are both aesthetically pleasing and have disease resistance. In Virginia, scab resistance is the most important type. Unfortunately, the literature on disease-resistant cultivars of crab apple is confusing. Some publications might list a cultivar as highly resistant to scab while another publication lists it as highly susceptible. This apparent contradiction may result as cultivars respond differently to the scab pathogen depending on the environment in which the resistance trial is conducted.

The cultivars with the most resistance to scab all have white flowers. There are red-flowered cultivars that have moderate resistance to scab.

by CNB