Gardeners grow crab apple trees (Malus spp.) for their lush foliage and brightly-colored fruit and flowers. The tree's hardy nature and small size--the trees typically reach a maximum height of only 25 feet--makes crab apples ideal for the backyard landscape. However, even a healthy crab apple tree can fall victim to occasional problems, including several common crab apple diseases.
Erwinia amylovora bacterium causes fire blight, a disease that Colorado State University calls especially destructive to crab apples. It can also afflict other shrubs and trees like pears and raspberries. The disease typically occurs in the spring when the outdoor temperatures reach 65 degrees F. Symptoms include wilted, brown crab apple blossoms followed by wilted leaves and widespread branch death. A severely-infected crab apple tree will ooze a thick sap from both its branches and fruit. There is no chemical treatment for fire blight, according to Colorado State University. Pruning off infected branches or completely removing the infected crab apple tree are the only treatment options.
Several different fungi cause rust, a fungal disease that afflicts crab apple trees across North America. Symptoms include an orange or red dust that covers the crab apple tree's foliage, wilted leaves and stunted fruit growth. The disease often arises during the summer. Oregon State University recommends treating rust-infected crab apples with a mancozeb or propiconazole-based fungicide.
Colorado State University says powdery mildews is one of the most widespread diseases to attack crab apple trees, and is most common on crab apples that are grown in the shade or whose leaves are always wet. The most identifiable symptom is a white powder that covers the crab apple tree's foliage. Left untreated, powdery mildew can cause widespread stunted growth. Pruning back surrounding plants can increase ventilation among the tree's foliage, drying out the leaves and keeping the fungus growth at bay. Watering the tree at its base instead of getting its leaves wet can also minimize the risk of contracting powdery mildew. Standard fungicide sprays formulated with triforine, sulfur or neem oil will also kill the mildew, according to Colorado State University.