Cause Apple proliferation is caused by Candidatus Phytoplasma mali that can be transmitted by artificial grafting, root grafts or by psyllids and a leafhopper. The disease, which occurs throughout Europe but not in the United States, is a high risk to U.S. production if it ever is introduced here. One possible entry route is illegally smuggled budwood or trees. Infected trees may appear to recover from this disease, such that there are no symptoms, but trees still carry the phytoplasma in the roots. Depending on the pathogen strain(s) found in the tree, it may or may not be present in the above ground portions of these recovered trees. As much as 80% of the fruit from infected trees is unmarketable due to reduced weight, unsatisfactory color, and poor taste.
Symptoms The most reliable characteristics include witches' brooms, small fruit, and late growth of terminal buds in the fall. Elongate stipules and leaf rosettes develop on the terminal parts of shoots. Root systems are compact and result in stunted growth of infected trees. All symptoms may not occur on all parts of the tree at the same time.
Flowering may be delayed; flower parts are deformed and may resemble leaves. Leaves emerge earlier and are irregularly serrated and small. Summer leaves often are chlorotic, and trees may exhibit early fall color (lilac or purplish red). Fruits are few, small, incompletely colored, and poorly flavored.
Cultural control Send any trees that may have these symptoms to a diagnostic lab for confirmation of the disease, and never smuggle in budwood or trees. Always use certified planting material.
Reference Seemüller, E., Kiss, E., Sule, S., and Schneider, B. 2010. Multiple infection of apple trees by distinct strains of 'Candidatus Phytoplasma mali' and its pathological relevance. Phytopathology 100:863-870.