Apple Proliferation Phytoplasma (APP) is a bacterial disease affecting mostly apples although pear trees with proliferation symptoms have been identified, but it has not been positively confirmed that they are suffering from APP. Apple cultivars seem to vary in terms of the degree of their reaction to the disease, Belle de Boskoop, Gravenstein, Starking, Golden Delicious and Winter Banana appear to be the most sensitive cultivars. Presence of APP has been confirmed in Austria, Bulgaria, Czech Republic, France, Germany, Greece, Hungary, Italy, Moldova, Poland, Romania, Slovakia, Slovenia, Spain, Switzerland, UK (eradicated), Ukraine, Yugoslavia. Found but not established in Denmark, the Netherlands and most recently in Nova Scotia.
Although natural transmission via root fusion is possible, it is reported that leafhoppers are vectoring the disease among trees. It appears that the bacteria cannot be transmitted via pollen or seeds, but grafting has yielded positive transmission. The causal agent does not seem to be systemic but can be disseminated in the scion (shoot) wood. Trees may push out a seemingly high proportion of buds that appear healthy, yet are infected. Phytoplasmas are not active during the winter and content appears to decline due to the degeneration of sieve tubes. However, in April and May reinvasion occurs from concentrations built up in the roots and infection will peak in late summer or early fall. Infected trees are highly susceptible to powdery mildew.
Shoots and bark on affected trees appears thin and trees lack vigor. Necrotic areas will develop on bark and branches. Mildly infected trees may recover after the initial shock during the first 3 growing seasons, but severely infected trees could die. Infected trees produce rosettes of terminal leaves late in the season in place of a normal bud set, these subsequently get powdery mildew, this is sometimes the first noticeable symptom. Premature development of axillary buds which give rise to secondary shoots that develop into witches brooms during the second or third year following infection is a more reliable symptom to diagnose APP. Fruit can be up to 25% smaller than those produced by unaffected trees, sugar and acidity will be reduced making the flavor unpalatable.
This disease has the potential for causing major economic impacts, it affects many important cultivars reducing size by up to 50%, weight up to 65%, while quality and vigor are drastically decreased. Current control methods being researched and developed are focusing on breeding cultivars for increased resistance to this and other phytoplasmas. If you have concerns about fruit trees on your property contact your arborist as soon as possible and visit https://www.savatree.com/tree-disease-treatment.html for more information.
Data Sheets on Quarantine Pests Prepared by CABI and EPPO for the EU; Apple Proliferation Phytoplasma