MLO diseases of elm
Elm phloem necrosis MLO (an EPPO A1 organism) is thought to be confined to the North-Eastern USA. However, an MLO disease of elm is also known from Europe; this was described many years ago from central Italy and has recently been found by Conti et al. (1987) on a collection of elm breeding material from the same region. There is some doubt as to the relationship between the American and European causal organisms.
W.A. Sinclair, in the APS Compendium of Elm Diseases, gives the name elm yellows to what EPPO knows as elm phloem necrosis, on the basis that the phloem necrosis symptom is a special case in the highly susceptible Ulmus americana. He notes that natural infection of phloem necrosis is unknown in the European and Asian species of elm, grown extensively in the North American range of the MLO. Graft transmission experiments failed to infect U. glabra and U. pumila or resulted in quite different symptoms, such as witches' brooms in U. carpinifolia, U. laevis and U. parvifolia. Sinclair does not comment on the older reports of an elm yellows disease in Italy.
In addition, Morvan (INRA, Montfavet, (FR), pers. comm.) confirms that witches' broom symptoms on elm have been known in France and some other European countries for some time. A strain is maintained in culture in Vinca sp. at Versailles.
The Italian MLO is nevertheless presumed to be a different organism, on the following grounds: 1) monoclonal antibody against elm phloem necrosis MLO from North America does not react with the Italian MLO; 2) S. luteolus does not occur in Europe, so the Italian MLO must have a different vector specificity (no vector is yet known). Comparative studies are continuing.
Sources
Sinclair W.A. (1981) Elm yellows. in Compendium of Elm Diseases (eds. R.J. Stipes & R.J. Campana), pp. 25-31. American Phytopathological Society, St. Paul (US).
Conti M., D'Agostino G. & Mittemoergher L. (1987) A recent epiphytotic of elm yellows in Italy. Proceedings of the 7th Congress of the Mediterranean Phytopathological Union. Consejerla de Agricultura y Pesca de la Junta de Andalucia, Granada (ES), 208-209.