Molecular classification of phytoplasmas
Seemüller et al. (1998) have recently analysed the similarity of 246 phytoplasma isolates according to various molecular criteria and assigned them to 75 distinguishable taxa (not yet officially recognized as species, but virtually so) in 20 groups. It is interesting to consider the status of various phytoplasmas which are of concern to EPPO:
1) ; ; ; ;in tomato, tomato big bud phytoplasma is recognized as a 'species'. However tomato big bud disease may also be caused by Maryland aster yellows phytoplasma, by a possible 'species' (Brazilian big bud phytoplasma) in the Western X group, by sunn hemp witches' broom phytoplasma (in Australia) and by clover proliferation phytoplasma (in California, US). Stolbur phytoplasma also causes big bud-like symptoms in tomato.
2) aubergine is infected by Maryland aster yellows phytoplasma (causing dwarf), stolbur phytoplasma, sunn hemp witches' broom phytoplasma (causing little leaf) and clover proliferation phytoplasma (causing little leaf).
3) ; ; ; ;potato is infected by Maryland aster yellows phytoplasma (causing Tulelake western aster yellows) and by clover proliferation phytoplasma (causing witches' broom). Neither the potato isolate of stolbur (EPPO A2 quarantine pest), nor any of the potato purple top-wilt complex (EPPO A1 quarantine pest) was considered. Isolates may not be available.
4) strawberry is infected by tomato big bud phytoplasma (causing multiplier disease in Florida, US), clover phyllody phytoplasma (causing green petal), Mexican periwinkle virescence phytoplasma (causing green petal), stolbur phytoplasma (causing yellows). The A1 pathogen strawberry witches' broom phytoplasma was not included in the analysis (isolates may not be available).
5) ; ; ; ;Prunus spp. are infected (in North America) by Western X disease phytoplasma and Canadian peach X disease phytoplasma (different 'species'). The phytoplasmas causing peach yellows and rosette (A1 list) are probably members of the same X-disease group but their status is not currently clear. In Europe, Prunus spp. are infected by European stone fruit yellows phytoplasma (formerly A2 list), a member of the Apple Proliferation group. Other phytoplasmas found in Prunus include: Maryland aster yellows, blueberry stunt, pear decline.
6) ; ; ; ;apple and pear are infected respectively by apple proliferation phytoplasma and pear decline phytoplasma, distinct 'species' in the Apple Proliferation group. This is one of the few relatively clear-cut associations between pathogen, symptoms and host in the whole analysis. Western X disease phytoplasma and sunn hemp witches' broom phytoplasma have, however, also been found in pear.
7) grapevine is infected in particular by flavescence dorée phytoplasma (in the Elm Yellows group). German flavescence dorée phytoplasma is apparently a different 'species', distinct again from stolbur phytoplasma (which causes bois noir, Vergilbungskrankheit and other yellows). Grapevine is also infected by Phytoplasma australiense (causing Australian grapevine yellows), one of the two specifically recognized candidate species, and by Maryland aster yellows phytoplasma and clover yellow edge phytoplasma (causing yellows).
8) olive, for which phytoplasmas have been mentioned in the Alert List, is infected by Maryland aster yellows phytoplasma (causing witches' broom), clover phyllody phytoplasma and a possible olive yellows phytoplasma in the Elm Yellows group.
9) ; ; ; ;elm is infected by elm yellows phytoplasma and by Maryland aster yellows phytoplasma. The A1 pathogen elm phloem necrosis phytoplasma is not mentioned (it may be synonymous with elm yellows).
10) ; ; ; ;on citrus, Phytoplasma aurantifoliae (A1 list, causing witches' broom of lime) is the principal 'species' mentioned. Sunn hemp witches' broom phytoplasma infects grapefuit in Australia (Marsh grapefruit dieback). Both these phytoplasmas are in the Faba Bean Phyllody group.
11) ; ; ; ;on palms, four distinct 'species' are recognized: Tanzanian lethal decline phytoplasma, Cape St. Paul wilt phytoplasma, coconut lethal yellowing phytoplasma and Yucatan lethal decline phytoplasma. These form a quite distinct set, in two groups.
12) finally, it may be noted that monocotyledonous plants are hardly mentioned among the hosts of any of the phytoplasmas, so far considered (except the palms under 11). Only the extremely polyphagous Maryland aster yellows phytoplasma has been found in Allium, Alstroemeria, Gladiolus and maize. Phytoplasmas of the Sugarcane White Leaf and Bermuda Grass White Leaf groups infect various Poaceae, most importantly sugarcane and rice, but no dicots.
In conclusion, few of the A1 and A2 phytoplasmas stand up to this analysis. Some should be divided into two or more 'species', others are only forms of other 'species', often widespread in the EPPO region, others have not been considered (possibly because it is difficult to obtain authentic isolates). It is clearly too soon for EPPO to rename its phytoplasmas, but the writing is on the wall. All are likely to be reconsidered as the phytoplasmas are progressively named as species in the genus Phytoplasma.
Sources
Seemüller, E.; Marcone, C.; Lauer, U.; Ragozzino, A.; Göschl, M. (1998) Current status of molecular classification of the phytoplasmas.
Journal of Plant Pathology, 80(1), 3-26.