Ash yellows phytoplasmas
Sinclair and Griffiths have recently published a review on ash yellows describing the distribution, host range, symptoms and epidemiology of the disease and discussed its relation with ash decline. The disease has only been reported in North America (in 16 States of US, especially in north-eastern States and 2 Canadian Provinces: Quebec and Ontario). Ash yellows is caused by unnamed phytoplasmas and induces slow growth, and decline of ash species (Fraxinus sp.). Ash yellows phytoplasmas have also been found on lilac (Syringa sp.) where they cause witches' broom symptoms. Molecular studies have shown that ash yellows and lilac witches broom are closely related; and that ash yellows could be considered as a disease caused by one pathogen (i.e. a group of closely related phytoplasma strains). Phytoplasmas have been detected in 12 ash species and numerous infraspecific taxa, but symptoms are mainly observed on white ash (F. americana), and to a lesser extent on green ash (F. pennsylvatica) and velvet ash (F. velutina). Affected trees show reduced growth, short internodes, chlorotic foliage, witches' broom and root necrosis. Seedlings and saplings can present a rapid decline. However, a minority of infected trees may not show any symptoms. The vectors of ash yellows are unknown, however single instances of phytoplasma transmission into ash seedlings have been reported by Philaenus spumarius (meadow spittle-bug) and Paraphlepsisus irroratus (leafhopper) and have not been confirmed. Control measures against this disease are not available, but forest management in order to promote species diversity or avoidance of ash species on stands where drought stress is common could reduce growth losses.
Finally, the authors discussed the relation between ash yellows phytoplasmas and the decline of white ash. This decline became prominent in parts of the north-eastern USA in the 1950s, and surveys revealed that in some States, 12 % to 37 % of ash trees were dead or showing dieback. Drought was suspected as the major cause of ash decline. Further studies have shown that phytoplasma infections and decline of white ash were correlated in many locations from Indiana to Vermont. Several observations raised the possibility that many infected ash trees may tolerate infection with little growth reduction until they come under stress from factors like drought, competition with other trees, or insect damage. For white ash, the authors felt that, although phytoplasma infection may cause decline and tree mortality, it mainly leads to a reduced vigour, which exacerbates stress caused by other factors. In green and velvet ash, ash yellows phytoplasmas may have only a contributory causal role in decline.
Sources
Sinclair, W.A.; Griffiths, H.M. (1994) Ash yellows and its relationship to dieback and decline of ash.
Annual Review of Phytopathology, 32, 49-60.