Crabapple diseases ruin landscape designs. Bacteria and fungi cause the majority of crabapple diseases. Awareness and proper environments lead to healthy crabapples.

Overview

Crabapple tree diseases can cause ugly and hazardous conditions in the most beautiful of planned landscape designs. One bacterial and four fungal diseases cause most crabapple leaf and fruit discoloration and eventual fall. Branches become bare, sometimes with only a few dry leaves remaining. Tree disfiguration or death can result when these crabapple diseases are relentless for many years.

Filthy leaves and contaminated fruit usually litter areas surrounding infected trees. This debris can cause dangerous and ugly conditions for walking and leisure activities. What starts as a fine-looking and attractive landscape area can become unacceptable and impractical for its original use.

Crabapple Diseases

Crabapple trees belong to plant genus Malus, the same as conventional eating apples. The same diseases trouble cultivars (cultivated varieties) from both groups. One key to success with both groups is to identify diseases and deal with them.

Sources of diseases are fungi or bacteria. Listed and detailed below are the five most common Malus-related disease problems:

Bacteria

Fire Blight (Erwinia amylovora) thrives when weather is warm and moist during blossom time. It is common in areas such as the mid-Atlantic states, but not so in others like the New England states.

Diseased blossoms turn brown and look as if scorched by fire. If conditions are suitable, diseased blossoms are an infection source for young shoots and branches. Severe conditions instigate fire blight infection in crabapple roots. Severe fire blight infections kill large tree branches and sometimes entire crabapple trees.

Fungi

Apple scab (Venturia inaequalis) is the most widespread and troublesome of crabapple diseases. Apple scab is especially severe in the northeastern United States where tree surfaces often are continually wet and damp in April, May and June.

Sanitation is a cultural key to prevention because the infection overwinters in crabapple leaf litter and infects blossoms and new shoots in early spring. Once infected, blossoms and shoots rapidly wilt and die. They appear scorched.

Leaf infection begins with the appearance of velvety, olive-colored spots that ultimately grow larger, come together, and change to a dark brown color. Severe infection may completely defoliate a crabapple tree and cause discolored and deformed fruit.

Repeated infections do not infect the vascular system or kill crabapple trees. However, the trees do become weaker and unsightly.

Cedar-apple rust (Gymnosporangium juniperi-virginianae) is a rust disease that occurs only when Eastern red cedar (Juniperus virginiana) trees are nearby. During moist weather, orange masses of infecting spores (galls) develop on the cedar and spores spread to crabapples.

Cedar-apple rust causes mainly cosmetic problems for both juniper and crabapple trees. In junipers, galls and twig dieback are present; on crabapples, fruit infections and leaf drop can occur.

After spring infection, orange spots occur on leaves, twigs and fruit. Shortly thereafter, tiny black spots appear on the orange spots. Small cup-shaped structures form on the leaf undersides. Both upper and lower structures produce spores that blow back to the juniper.

Powdery mildew (Podosphaera leucotricha) occurs during a combination of warmth, high humidity and poor air circulation. Rainfall inhibits the disease because spores wash off and do not germinate in water.

Powdery mildew may attack all growing parts of crabapple trees with a white powdery coating. Leaves become twisted and disfigured; new shoot growth stops or is retarded; mature fruit exhibits brownish, roughened areas (russet-like). This infection disfigures both tree and fruit, but usually damage is moderate.

Frogeye leaf spot and black rot fruit damage (Physalospora obtusa / Botryosphaeria obtusa) causes raised leaf spots, fruit that rots during growth, and eroding bark cankers.

The name frogeye leaf spot results from initial circular brown leaf spots with purple margins that develop into secondary concentric layers with brown areas that surround the original, now gray, center. To many people, this leaf infestation looks like a frog's eye. These symptoms and later leaf drop look as if the disease is crabapple scab. However, rotting fruit and obvious bark cankers determine the correct disease identification.

Initial infection takes place in warm, humid weather around blossom time. Failure to cleanup mummified apples and canker-infested dead wood allows disease-spreading spores to overwinter. Continuing poor sanitation results in persistent and resistant outbreaks. These lead to weak trees from defoliation, and dieback of twigs and limbs because of cankers. The best control is rigorous sanitation and keeping plants as healthy as possible with good pruning, fertilization, mulching, and watering practices

©Text and photograph by Georgene A. Bramlage. 2007. Reproduction without permission prohibited.