Flowers and Leaves Wilt; Bark Cankered - New shoots may wilt suddenly in late spring, turn black or brown and die. The dead leaves hang downward on the affected twigs giving the tree the appearance of having been scorched.
Fire Blight - This disease, caused by a bacterium, is spread by insects and rain. It is most prevalent in the Rocky Mountains and western states such as Colorado and Nebraska. It is a major problem in certain types of fruit trees. It is a serious disease, capable of killing a mature tree. It is wise to hire a certified arborist to treat a full-grown tree by pruning and spraying with a registered bacteriacide, an antibiotic such as Agri-Strep, rather than attempt to treat the problem yourself. This disease is not serious on flowering crabapple trees if pear and apple orchards are not nearby
In small trees, cut off affected branches at least 3 inches below the infected area between November and March. Before making each cut, disinfect the pruning saw or shears by dipping them in a solution of hot water and household bleach.
Dead Spots on Leaves; Shoots Blighted - It affects both surfaces of the newer, succulent leaves and stems of crabapple trees causing spots that swell, and turn brown. This tissue dies and leaves and stems appear blighted. Sometimes affected trees are defoliated by midsummer. Olive brown colored spores develop on the dead tissues and overwinter there.
Scab - Apple scab is a fungal disease found in most types of apple trees. It is most prevalent in the Midwest. This disease develops when the temperatures are moderate, but the humidity is high in the spring and early summer. There are very few natural controls for it. It is essential to clean up and discard in the trash fallen leaves and debris around apple trees that have scab. Choose resistant varieties of crabapple. White flowered types tend to be more resistant to this disease. Spraying or dusting a garden sulfur fungicide on new leaves as they appear throughout the season protects them from infection by the fungal spores.
Powdery Orange Spots on Leaves
Rust - A rust disease caused by a fungus may attack crabapple foliage if red cedar trees infected with cedar apple galls (fungal fruiting bodies) are nearby. These leaf spots may defoliate the tree. It is mainly a problem with native crabs of the Northeast and Midwest such as ‘Brandywine’ or ‘Charlottae’. The majority of crabs, those originating in Asia, are unaffected by this rust. So, choose those if you live near red cedar. If possible, eliminate red cedars within a mile radius. Since they grow wild all over parts of central and eastern US, this is difficult. For more information see Controlling Fungal Disease
Low Flower and Fruit Production
Biennial Cycle - Like many other woody plants, some types of crabapples bear large crops of flowers and fruit only every other year. They have bumper crops one year, then a sparse or negligible one the next. Some others bear annually. There is no way to alter this cycle; it is a natural trait. Determine what kind of crabapple you have, so you will know what to expect. Some reliably annual-bearing trees include Japanese flowering crabapple and Sargent's crabapple.
Galleries Drilled in Trunk; Girdling
Borers - The flat-headed borer and the round-headed borer, as their names suggest, are beetles that bore into the inner bark and wood of crabapple trees, discoloring the wood and frequently girdling the trees. They may bore into trunks at or below ground level, often damaging young trees so badly that they topple easily. They also open the way to diseases. Borers are notoriously difficult to combat because they are protected deep in the tree. The best control is to prune away limbs and branches that show borer holes and the resulting damage such as dead leaves. Saw a clean cut below the hole back at a healthy connecting branch. Do not leave a stub end of a branch. Do not paint or cover the pruning wound. For more information see Controlling Borers
Small Bumps on Leaves and Branches
Scale Insects - Scale insects form groups of small bumps or blister-like outgrowths on tree stems and leaves. Protected by these white, yellow or brown waxy shells, they feed on tree sap. Heavy scale infestations can kill young trees. Some scale species excrete honeydew, which coats foliage and encourages ants and sooty mold growth. For more information see Controlling Scale
Aphids - Aphids are soft-bodied, pear-shaped sucking insects about the size of the head of a pin. They often cluster on tender new growth to suck sap from leaves and stems. Also called "plant lice," they can seriously retard tree growth. Crabapple leaves may turn yellow or brown. They wilt under bright sunlight, or sometimes curl and pucker. Try spraying infested leaves and stems with a vigorous water spray once every other day for 3 days to dislodge the aphids. For more information see Controlling Aphids.
Leaves Webbed and Deformed
Spider Mites - These pests, related to spiders, are about 1/50 inch long, barely visible to the unaided eye. They have four pairs of legs, piercing-sucking mouth parts, and very compact bodies. Inspect the lower leaves on each crabapple tree. If their upper surfaces are stippled with small yellow dots or red spots and swathed in fine webbing, suspect mites.
Mites often appear on plants that are already stressed--either by disease or other insects, or by environmental problems such as heat and drought. Often they appear in yards where broad-spectrum insecticides have been overused and the resident beneficial insects that normally control mite populations are gone. Try to diagnose and correct this underlying problem so that the mites do not return. For more information see the file on Controlling Mites
Sunken Lesions on Stems and Trunk
Canker - A fungus disease sometimes causes swollen, sunken lesions to appear on trunks of crabapple trees. The fungal spores often infect the trees through open wounds in the bark made by borers, or by injury from lawn mowers and other yard maintenance equipment. Avoid wounding trees. Increase their vigor and disease resistance by proper feeding and watering. Prune out and discard in the trash affected branches and twigs. Carefully cut out large cankers with a clean, sharp knife. Sterilize tools and with household bleach to avoid spreading the disease. For more information see the file on Controlling Fungal Disease
Leaves Covered With White Powder
Powdery Mildew - Powdery mildews are caused by fungi that live on the surface cells of the crabapple leaves, not inside them. Infected leaves are covered with a white or ash-gray powdery mold. Badly infected leaves become discolored and distorted, then drop off. It makes them unsightly, but is not life threatening to mature trees. Powdery mildews thrive in either very humid or very dry weather and on foliage that does not enjoy good air circulation. For more information see the file on Controlling Fungal Disease