Choosing crabapple tree (Malus spp.) cultivars resistant to the diseases known to thrive in your area is the best way to prevent disease. For example, hardy in U.S. Department of Agriculture plant hardiness zones 4 through 8, “Beverly” (Malus "Beverly) resists scab and rust, and “Adams” (Malus "Adams") resists scab and fire blight. Crabapple trees draw attention in spring with their showy blossoms. They add interest to your landscape again in autumn with small, colorful fruit in hues or yellow, red, orange and pink.
You can find velvet-textured, olive green fungus in spring on the surface of foliage and flower petals when your crabapple tree has apple scab. Next, the leaves fade to yellow and prematurely drop to the ground. Additionally, the fruit's surface will feature rough, circular spots. Scab starts multiplying on the wet surfaces of crabapple trees quickly. For example, spores begin to multiply within nine hours when the temperature is between 61 and 75 degrees Fahrenheit. To prevent scab, apply a natural fungicide, such as liquid lime sulfur or fixed copper, at the beginning of the season as soon as the tips of the branches break dormancy and start to grow.
Despite needing juniper and rose bushes to complete its life cycle, cedar-apple rust is one of the most common diseases of crabapple trees. The telltale sign of infection is yellow-orange spots on the leaf’s surface. To prevent cedar-apple rust, do not plant crabapple trees next to these plants. To control cedar-apple rust, start by removing surrounding juniper trees and rose bushes and pruning affected branches. Once you remove the offending plants, cedar-apple rust typically resolves itself without treatment, if you live in an area not prone to this disease. For severe cases and disease-prone areas, spray effective fungicides, including fixed copper and liquid lime sulfur, weekly as soon as buds appear at the start of the growing season.
Botryosphaeria canker causes affected branches to wither and die. You can find visible fungus clusters growing on dead branches and under the bark of affected parts of the tree. Drought stress increases crabapple’s susceptibility to botryosphaeria, making regular irrigation a must in the absence of rain. To control botryosphaeria canker, prune affected branches as soon as possible. No fungicides effectively control this disease and it may kill your crabapple tree.
Apple Powdery Mildew
The leaves of trees affected by apple powdery mildew look as though someone sprinkled their surface with baby powder and twisted them into distorted shapes. Effective fungicides to control powdery mildew include micronized sulfur or horticultural oils; application is most effective when the flowers are in the pink bud stage. Pruning affected branches as soon as possible helps to control powdery mildew as well.
- Penn State Extension: Crabapple Diseases
- University of California, Davis, IPM Online: Apple Scab
- Cornell University Department of Plant Pathology and Plant Microbe Biology
- University of California, Davis: Home Orchard: Apple Scab
- Colorado State University Extension: Flowering Crabapple Trees
- University of California, Davis, IIPM Online: Apple Powdery Mildew
- Virginia Cooperative Extension: Botryosphaeria Canker and Dieback of Trees and Shrubs in the Landscape
- Colorado State University Extension: Fire Blight
- Dynamic Graphics Group/Dynamic Graphics Group/Getty Images