Plant Health Problems
See Apple for a discussion of problems that may also occur on crabapple.
Diseases caused by Fungi:
Scab, Venturia inaequalis.
This fungus causes circular, olive-black spots on the leaves, fruit, and young fruit stems. Heavy infection will cause dropping of young fruits, distortion and cracking of growing fruit, as well as extensive defoliation. The fungus overwinters on the fallen infected leaves, producing a spore stage in the spring that infects the young leaves and fruit during periods of rain. Infection can take place, with suitable weather conditions, at any time after growth starts until the middle of June. See Apple for a more detailed discussion of this disease.
Scab is most effectively controlled by planting resistant varieties such as Adams, Baskatong, Callaway, David, Dolgo, Donald Wyman, Malus floribunda, among others. Using these in the landscape should avoid almost entirely the need to spray. In the case of susceptible varieties, control is concentrated on preventing scab from becoming started in the spring. The disease overwinters on diseased leaves on the ground. Therefore, control measures should begin with the removal of the overwintering leaves from beneath the tree and in the surrounding area before budbreak. Control can also be achieved with the use of fungicide sprays. Among the compounds registered for use in Connecticut are thiophanate-methyl, mancozeb, and chlorothalonil. Consult the label for dosage rates and safety precautions. Generally, protection must be given the trees from budbreak until well into the growing season, usually two weeks or more after petal fall. For more information, see the fact sheet on Common Diseases of Crabapple.
Cedar-apple rust, Gymnosporangium juniperi-virginianae.
This fungus has one stage of its life cycle on the crabapple and the other on red cedar or juniper species. From this dual existence, the disease derives its common name, cedar-apple rust. On the cedar are produced the well-known cedar apples. The fungus causes bright orange spots on the foliage and to a lesser degree on fruit of crabapple. The infections on apple foliage produce another spore stage, borne in delicate cup-like structures, which re-infect the cedar and thus the cycle is complete.
Rust is most effectively controlled by planting resistant varieties such as Ellwangerina, Henry Kohankie, Ormiston Roy, and Red Baron. Control can also be achieved with the use of fungicide sprays. Among the compounds registered for use in Connecticut are chlorothalonil, mancozeb, and ferbam. Consult the label for dosage rates and safety precautions. Fungicide control for the Juniperus hosts is usually not practical. For more information, see the fact sheet on Cedar-Apple Rust.
Cedar-hawthorn rust, Gymnosporangium globosum.
The rust spots appear brown or orange-yellow on the upper side of the leaves, but from the underside long slender whitish tubes break open to shed orange-brown spores. Infected leaves fall prematurely. The alternate host is red cedar or juniper species. The disease cycle and control strategies are essentially the same as for cedar-apple rust. See Hawthorn and the fact sheet on Cedar-Apple Rust for more detailed discussions of this disease.