To save your diseased crabapple tree, you must first diagnose the disease and understand how to treat it properly. Several diseases can affect crabapple trees, including apple scab, cankers, cedar-apple rust, leaf scorch, powdery mildew and sooty mold. When you’re diagnosing and treating your diseased crabapple trees, keep in mind that fungal diseases are most likely to attack trees when the trees are weakened. So as you’re trying to save your diseased crabapple tree, be sure to keep the tree well-watered and fertilized.
Things You'll Need
- Appropriate fungicide/insecticide
- Sharp knife (optional)
- Pruning tools
- Garden hose
- Fruit tree fertilizer
Look for olive-green spots on the leaves and fruits of your crabapple tree that turn darker and velvety over time to detect apple scab, which is caused by a fungus. Treat apple scab by removing and burning all the fallen leaves in the autumn. Spray the diseased trees with a fungicide containing copper octanoate, lime sulfur or sulfur in the spring, before the trees bud.
Treat cankers by pruning away dead or weakened limbs in early spring and cutting out the cankered areas until you reach healthy wood. Cankers are caused by a fungus and trees grow discolored callus tissues on the trunk, stems and limbs. Fertilize your crabapple trees in late fall or early spring and keep them well-watered, because cankers usually affect weakened and stressed trees most.
Inspect your crabapple trees for bright yellow-orange spots on the leaves and fruits to diagnose cedar-apple rust. Your crabapple trees can be infected by this fungal disease only if juniper and red cedar trees grow nearby. Treat cedar-apple rust by removing nearby junipers or cedars and spraying the crabapple trees with a fungicide.
Treat leaf scorch by fertilizing your crabapple trees well in the spring and watering them deeply, saturating the soil 6 inches deep, at least once every two weeks. Leaf scorch is usually caused by poor conditions, occurring in July and August with the leaves turning yellow or darkening around the edges.
Treat powdery mildew by removing and burning the fallen diseased leaves in the autumn and applying a fungicidal spray to the affected crabapple trees as soon as you see the first signs of the disease. Powdery mildew appears in late summer or early fall as dusty white or grayish spots on the leaves, expanding to eventually cover the entire leaf.
Identify sooty mold by looking for a black fungus coating the surfaces of your crabapple tree’s leaves, twigs, branches and fruits. Treat sooty mold by controlling the insects that perpetuate the fungal infection, which are usually aphids, mealybugs, whiteflies and sometimes scale. Spray the tree with an insecticide or fungicide that is made to treat the type of insect involved in the sooty mold infestation on your tree.