W. A. Sinclair

PHYTOPLASMA IDENTITY AND DISEASE ETIOLOGY

ABSTRACT Many plant diseases believed to be caused by phytoplasmas were described before phytoplasma groups were delineated through molecular analyses. It is now possible to assess the relationships between phytoplasma identity or classification and specific plant diseases. Data were consistent with the hypothesis of a common ancestral origin of pathogenicity genes in many phytoplasmas and a limited repertoire of plant responses to certain pathogen signals. Observations also were consistent with the hypotheses that the botanical host ranges of some phytoplasmas reflect specificities in transmission by vectors and vector feeding preferences; phytoplasma-insect vector relationships are keys to understanding evolutionary divergence of phytoplasma lineages; small differences in a highly conserved phytoplasma gene may be regarded as potential indicators of separate gene pools; the reliability of a diagnosis based on symptoms must be learned empirically (i.e., through case study for each syndrome); and some discrete diseases can be ascribed to phytoplasma taxa at the 16S rRNA group level, whereas others are clearly associated with phytoplasma taxa below this level.

Phytoplasmas Associated with Elm Yellows: Molecular Variability and Differentiation from Related Organisms

Restriction fragment length polymorphism (RFLP) analyses were performed on polymerase chain reaction (PCR) amplimers of phytoplasmal DNA from eight samples obtained from Ulmus spp. (elms) affected by elm yellows (EY) in Italy and the United States, from Catharanthus roseus infected with strain EY1, and from five other plant species infected with phytoplasmas of the EY group sensu lato (group 16SrV). RFLP profiles obtained with restriction enzyme TaqI from ribosomal DNA amplified with primer pair P1/P7 differentiated elm-associated phytoplasmas from strains originally detected in Apocynum cannabinum, Prunus spp., Rubus fruticosus, Vitis vinifera, and Ziziphus jujuba. RFLP profiles obtained similarly with BfaI differentiated strains from A. cannabinum and V. vinifera from other phytoplasmas of group 16SrV. Elm-associated strains from within the United States had two RFLP patterns in ribosomal DNA based on presence or absence of an RsaI site in the 16S-23S spacer. Elm-associated phytoplasma strains from Italy were distinguished from those of American origin by RFLPs obtained with MseI in the same fragment of non-ribosomal DNA. Strain HD1, which was discovered in A. cannabinum associated with EY-diseased elms in New York State, was unique among the strains studied.

Variation in Aggressiveness of Ash Yellows Phytoplasmas

Twelve strains of phytoplasmas belonging to the ash yellows (AshY) group, from across the known range of AshY and representing six host species, were assessed for differences in ability to suppress growth and cause chlorosis in graft-inoculated Fraxinus pennsylvanica (green ash) and Catharanthus roseus (periwinkle). In each of two experiments with ash and one with periwinkle, different strains caused significantly different degrees of growth suppression and loss of foliar greenness. These growth and color impacts were positively and significantly correlated among experiments and between ash and periwinkle, indicating strain variation in aggressiveness. After two strains that differed in aggressiveness were coinoculated to periwinkle plants, polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) assays of DNA from leaves remote from the inoculation sites revealed the presence of the aggressive strain sooner and more frequently than that of the less aggressive strain. Thus, aggressiveness was associated with more rapid multiplication and/or movement than was achieved by the less aggressive strain. When either strain was inoculated 11 weeks before the other into the same plant, only the initial strain could be detected after a further 12 weeks of incubation. Thus, the initial strain or its effect on the host may have interfered with multiplication and/or long-distance movement of the second strain. A concept of preemptive dominance is proposed to account for detection by primary PCR of only single phytoplasma strains in plants that may harbor two or more strains.

Taxa of leafhoppers carrying phytoplasmas at sites of ash yellows occurrence in New York State

Leafhopper (Homoptera: Cicadellidae) populations were sampled and leafhopper carriers of ash yellows (AshY) phytoplasmas were identified as first steps toward vector identification. Nearly 5,000 leafhoppers were collected in malaise traps at two sites of high AshY incidence in New York state in 1996 and 1997. These insects comprised 33 taxa, including representatives of 13 genera known to contain phytoplasma vectors. The most abundant genus was Scaphoideus, with numbers about six times greater than any other genus. A total of 1,632 insects were assayed individually for phytoplasmas by polymerase chain reaction (PCR) amplification of phytoplasmal 16S rDNA and restriction fragment length polymorphism analyses of PCR products using restriction enzymes TaqI and RsaI separately. Phytoplasmas were detected in 35 insects, all but one in the subfamily Deltocephalinae. AshY phytoplasmas were detected in 19 of 812 individuals of Scaphoideus spp. and 1 of 87 of Colladonus clitellarius. Phytoplasmas of the Prunus X-disease group were detected in 1 Scaphoideus sp., 4 individuals of C. clitellarius, and 4 of 83 Scaphytopius acutus individuals. Phytoplasmas of the aster yellows group were detected in 1 of 68 individuals of Gyponana spp. and 5 of S. acutus. AshY phytoplasma carriers merit testing for possible vector ability.

Elm Yellows in North America

Elm yellows (EY) is a debilitating or lethal disease caused by an unnamed phytoplasma. Phytoplasmas are prokaryotic obligate parasites of plant phloem, of insects that act as vectors, and probably of additional insects that feed by sucking phloem sap. Phytoplasmas can be detected by electron or fluorescence microscopy or by PCR amplification of particular DNA sequences. The phytoplasmas associated with EY are distinguished from others by analysis of ribosomal DNA. EY is widespread in the eastern half of the USA, where sporadic epidemics have killed native elms but not those of Eurasian origin. In 1998, epidemics were in progress in Illinois, New York State, and an area centered in the Potomac valley northwest of Washington, DC. One EY vector, the leafhopper Scaphoideus luteolus, is known, and others probably exist. Research in progress or needed includes assessment of EY phytoplasma resistance or tolerance in elm cultivars developed for resistance to the Dutch elm disease pathogen, identification of EY vectors in different geographic regions, elucidation of vector biology, determination of reasons why elms of Eurasian origin seem to escape damage from EY in North America, and elucidation of mechanisms of phytoplasmal pathogenicity.