The flowering crab apple tree provides year-round interest and requires little maintenance, but it is often plagued by bacterial and fungal diseases. Some crab apple tree varieties are disease resistant, though they don't provide foliage or flowers that are as attractive as other varieties. Although the crab apple tree proves reliable for many landscape uses and a worthy investment, gardeners should consider potential crab apple tree diseases before planting.
The fungus Venturia inaequalis causes apple scab. The fungal spores develop in spring and spread to new leaves through rainwater and wind. Leaves develop olive-green spots that enlarge and turn black. The fruit cracks and becomes deformed. In severe cases, leaves yellow and drop as early as July and the fruit falls before it is ripe. Apple scab does not kill the crab apple tree, but defoliation reduces flowering and fruit the following season.
The bacterium Erwina amylovora causes fireblight. The disease attacks new terminal shoots in late spring to early summer and gives the shoots a scorched appearance. Leaves attached to blighted shoots bend over to form a characteristic "shepherd's hook." The infection moves into the main trunk and produces cankers, or dark brown to purple depressions, in older tissue. On warms days, amber or honey-colored bacterial ooze emerges from infected parts. Left unchecked, the disease progresses to the roots and results in the death of the tree. Removal and disposal of infected branches and shoots helps deter fireblight spread.
The fungus Gymnosporangium juniperi-virginianea causes cedar-apple rust in a crab apple tree. The fungus overwinters on a juniper or red cedar tree, then develops orange gelatinous spores in the spring. The spores migrate to the crab apple tree and produce bright yellow-orange spots on the leaves and lesions on the fruit. The crab apple tree may benefit from a fungicide spray on growing buds, but repeated applications in wet weather may be required.
Frogeye Leaf Spot and Black Rot
The Botryosphaeria obtusa fungus causes two related diseases known as frogeye leaf spot and black rot. Frogeye leaf spot produces brown and tan spots on leaves that give the appearance of a frog's eye. The spots coalesce and the leaves turn yellow and fall. Black rot tends to infect fruit after the onset of frogeye leaf spot. The surface of the fruit develops alternating black and brown concentric rings at the blossom end. The fruits dry out and mummify and persist on the branch until the next season. Spring pruning of diseased branches and the previous year's fruit helps control the spread of the fungus.