The phylogenetic relationships among members of the family Comamonadaceae and several unclassified strains were studied by direct sequencing of their PCR-amplified 165 rRNA genes. Based on the I65 rRNA gene sequence analysis, members of the family formed a coherent group. The closest relatives are species of the Rubrivivax sub-group: Leptothrix discophora, ldeonella dechloratans and Rubrivivax gelatinosus. The genus Hydrogenophaga formed two subclusters, as did the species of Acidovorax, whereas the five species of the genus Vlquaspirillum] were polyphyletic. Comamonas acidovorans was phylogenetically distant from the type species of Comamonas, Comamonas terrigena. On the basis of this work and previous studies, Comamonas acidovorans is removed from the genus Comamonas and renamed as Delftia acidovorans gen. nov., comb. nov. Descriptions of the new genus Delftia and of the type species Delftia acidoworans, for which the type strain is ATCC 15668T, are presented.

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The new Liberibacter species, 'Candidatus Liberibacter solanacearum' (Lso) recently associated with potato/tomato psyllid-transmitted diseases in tomato and capsicum in New Zealand, was found to be consistently associated with a newly emerging potato zebra chip (ZC) disease in Texas and other southwestern states in the USA. A species-specific primer LsoF was developed for both quantitative real-time PCR (qPCR) and conventional PCR (cPCR) to detect and quantify Lso in infected samples. In multiplex qPCR, a plant cytochrome oxidase (COX)-based probe-primer set was used as a positive internal control for host plants, which could be used to reliably access the DNA extraction quality and to normalize qPCR data for accurate quantification of the bacterial populations in environment samples. Neither the qPCR nor the cPCR using the primer and/or probe sets with LsoF reacted with other Liberibacter species infecting citrus or other potato pathogens. The low detection limit of the multiplex qPCR was about 20 copies of the target 16S rDNA templates per reaction for field samples. Lso was readily detected and quantified in various tissues of ZC-affected potato plants collected from fields in Texas. A thorough but uneven colonization of Lso was revealed in various tissues of potato plants. The highest Lso populations were about 3x10(8) genomes/g tissue in the root, which were 3-order higher than those in the above-ground tissues of potato plants. The Lso bacterial populations were normally distributed across the ZC-affected potato plants collected from fields in Texas, with 60% of ZC-affected potato plants harboring an average Lso population from 10(5) to 10(6) genomes/g tissue, 4% of plants hosting above 10(7) Lso genomes/g tissue, and 8% of plants holding below 10(3) Lso genomes/g tissue. The rapid, sensitive, specific and reliable multiplex qPCR showed its potential to become a powerful tool for early detection and quantification of the new Liberibacter species associated with potato ZC, and will be very useful for the potato quarantine programs and seed potato certification programs to ensure the availability of clean seed potato stocks and also for epidemiological studies on the disease.

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The specificity and sensitivity of polymerase chain reaction (PCR) primers developed for ‘Candidatus Liberibacter solanacearum’ and ‘Candidatus Liberibacter psyllaurous’ were evaluated in conventional and real-time PCR assays. All PCR primers were specific for ‘Ca. L. psyllaurous’ and ‘Ca. L. solanacearum’ insomuch as they did not detect other prokaryotic plant pathogens that affect potato except for the putative pathogens associated with psyllid-yellows and haywire. Conventional PCR assays were capable of detecting 0.19 to 1.56 ng of total DNA per reaction, and real-time PCR was found capable of detecting 1.56 to 6.25 ng of total DNA per reaction, depending on the specific PCR primer set used. ‘Ca. Liberibacter’ species associated with zebra complex disease (ZC) was confirmed in plants affected by this disease throughout Texas from 2005 to 2008, in seed tubers produced in Wyoming in 2007, and in Colorado, Kansas, Nebraska, and Mexico in 2008. A multiplex PCR assay using ‘Ca. L. solanacearum’–specific primers and primers specific for the β-tubulin DNA regions from potato was developed, providing possible utility of the multiplex assay for ‘Ca. Liberibacter’ detection in different solanaceous plant species. Preliminary studies suggest silverleaf nightshade (Solanum elaeagnifolium), wolfberry (Lycium barbarum), black nightshade (S. ptychanthum), and jalapeno pepper (Capsicum annuum) as additional solanaceous hosts for the ZC-associated bacterium. The ‘Ca. Liberibacter’ species detected in all samples divided into two clusters sharing similarity of 99.8% in their partial 16S rRNA gene sequences and 99.3% in their partial intergenic spacer region (ISR)-23S rRNA gene sequences. Genetic variation in the 16S rDNA region consistently matched that of the ISR-23S rDNA region. In this partial 16S-ISR-23S rDNA region, there was a total of eight single nucleotide polymorphisms among ‘Ca. L. psyllaurous’ and ‘Ca. L. solanacearum’ “strains” investigated in this study. ‘Ca. L. solanacearum’ and ‘Ca. L. psyllaurous’ were shown to be very closely related bacteria, if not the same, by successful amplification using a combination of forward primer of ‘Ca. L. solanacearum’ and reverse primer of ‘Ca. L. psyllaurous’ in ZC-affected potato samples. This finding clarifies the current taxonomic status of ‘Ca. L. solanacearum’ and ‘Ca. L. psyllaurous’. The detection of ‘Ca. L. solanacearum’ from haywire-symptomatic potato samples demonstrates that this bacterium might also be associated with this disease.

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Acibenzolar-S-methyl (ASM), a plant activator known to induce systemic acquired resistance, has demonstrated an ability to manage a number of plant diseases, including bacterial spot on tomato caused by four distinct Xanthomonas spp. The aim of this study was to evaluate application rate and frequency of ASM in order to optimize field efficacy against bacterial spot in Florida, while minimizing its impact on marketable yields. ASM was applied biweekly (once every 2 weeks) as a foliar spray at a constant concentration of 12.9, 64.5, and 129 μM throughout four field experiments during 2007–08. A standard copper program and an untreated control were also included. Overall, biweekly applications of ASM did not significantly reduce disease development or the final disease severity of bacterial spot compared with the copper-mancozeb standard or the untreated control. Only one experiment showed a significant reduction in the final disease severity on plants treated with ASM at 129 μM compared with the untreated control. Three additional field trials conducted during 2009–10 to evaluate the effects of weekly and biweekly applications of ASM at concentrations of 30.3 to 200 μM found that weekly applications provided significantly better disease control than biweekly applications. The tomato yields were not statistically improved with the use of ASM relative to the untreated control and standard copper program. Weekly ASM applications at rates as low as 75 μM (equivalent to 1.58 g a.i./ha in 100 liters of water or 0.21 oz. a.i./acre in 100 gallons of water) to 200 μM (equivalent to 4.20 g a.i./ha in 100 liters of water or 0.56 oz. a.i./acre in 100 gallons of water) were statistically equivalent in managing bacterial spot of tomato without significantly reducing yield compared with the untreated control.