The phylogenetic relationships of 17 phytopathogenic mycoplasmalike organisms (MLOs) representing seven major taxonomic groups established on the basis of MLO 16s ribosomal DNA (rDNA) restriction patterns were examined by performing a sequence analysis of the 16s rDNA gene. The sequence data showed that the MLOs which we examined are members of a relatively homogeneous group that evolved monophyletically from a common ancestor. In agreement with results obtained previously with other MLOs, our results also revealed that the organisms are more closely related to AchoZepZasma Zaidhwii and other members of the anaeroplasma clade than to any other mollicutes. A phylogenetic tree based on 16s rDNAs showed that the MLOs which we examined can be divided into the following five primary clusters: (i) the aster yellows strain cluster; (ii) the apple proliferation strain cluster; (iii) the western-X disease strain cluster; (iv) the sugarcane white leaf strain cluster; and (v) the elm yellows strain cluster. The aster yellows, western-X disease, and elm yellows strain clusters were divided into two subgroups each. MLOs whose 16s rDNA sequences have been determined previously by other workers can be placed in one of the five groups. In addition to the overall division based on 16s rDNA sequence homology data, the primary clusters and subgroups could be further defined by a number of positions in the 16s rDNAs that exhibited characteristic compositions, especially in the variable regions of the gene.Nonhelical phytopathogenic mollicutes, which are most often referred to as mycoplasmalike organisms (MLOs), are wall-less, nonculturable prokaryotes that cause diseases in several hundred plant species (23). The first attempts to differentiate and classify these organisms were based on symptoms, host ranges, and insect vector relationships (4, 9, 21).The phylogenetic interrelatedness of the MLOs was poorly understood until recently, when molecular methods were introduced into plant mycoplasmology. Many MLO strains were differentiated and partially characterized by dot and Southern hybridization techniques, as well as by serological techniques. Most of this work has been reviewed by Kirkpatrick (10) and Lee and Davis (16). Closely related MLOs belonging to the aster yellows strain cluster could be classified by Southern blot analysis (17).Neither serological methods nor nucleic acid hybridization experiments performed with randomly cloned DNA fragments have revealed the phylogenetic or taxonomic positions of MLOs in relation to each other and to other microorganisms. In contrast, the 16s rRNA gene is a universal characteristic in prokaryotes and has both conserved and sufficiently variable regions. This gene is therefore suitable for phylogenetic and taxonomic classifications at various levels, including intrageneric differentiation (30,34 yellows agent (strain SAY), and the western-X disease agent (strain WX), as well as several Japanese MLOs, showed that these organisms exhibit levels of sequence homology of at least 89.4%. Thus...

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Spatial patterns of sharka disease, caused by plum pox virus (PPV) and vectored by several species of aphid, were determined by double antibody sandwich-enzyme-linked immunosorbent assay using polyclonal antibodies in newly infected, mature apricot and peach orchards in eastern Spain. Among yearly assessments of plots examined for within-and across-row aggregation of adjacent sharka-diseased trees, only a few transects were found to have aggregation by ordinary runs analyses. Analyses, using beta-binomial index of dispersion ( 14) to determine if spatial aggregation was present in each plot for data partitioned into quadrats of different spatial dimension, demonstrated occasional aggregation and results were generally inconclusive. Significant (1;) values, when present, were generally found associated with plots with higher disease incidence. No disease gradients were discernible for any of the plots and years. More rigorous spatial analyses were used to test for spatial relationships over longer distances. Twodimensional distance class analyses indicated a spatial dependency of PPV-infected stone-fruit trees over distance, a general scarcity of significant distance classes near the origin, and the presence of significant distance classes occasionally comprising small loose clusters at distances near the center or distal end of the proximity matrices especially during the initial stages of the epidemics. Geostatistical analysis confirmed the lack of significant associations among immediately adjacent trees and the trend for higher order spatial associations in semivariograms for distances corresponding to the center and distal ends of the proximity matrices. This trend in semivariance over distance was best described by linear or exponential increase models compared with transitional models commonly used in geostatistics. Correlation analysis indicated a significant conservation of orientation of localized systemic infections in scaffold branches over years. The spatial patterns of sharka suggest the lack of movement of PPV-viruliferous aphid vectors to immediately adjacent trees and their preferential movement to trees several tree spaces away.

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