The importance of ethylene production for virulence of Pseudomonas syringae pvs. glycinea and phaseolicola was assayed by comparing bacterial multiplication and symptom development in bean and soybean plants inoculated with ethylene-negative (efe) mutants and wild-type strains. The efe mutants of Pseudomonas syringae pv. glycinea were significantly reduced in their ability to grow in planta. However, the degree of reduction was strain-dependent. Population sizes of efe mutant 16/83-E1 that did not produce the phototoxin coronatine were 10- and 15-fold lower than those of the wild-type strain on soybean and on bean, and 16/83-E1 produced very weak symptoms compared with the wild-type strain. The coronatine-producing efe mutant 7a/90-E1 reached fourfold and twofold lower population sizes compared with the wild-type strain on soybean and bean, respectively, and caused disease symptoms typical of the wild-type strain. Experiments with ethylene-insensitive soybeans confirmed these results. The virulence of the wild-type strains was reduced to the same extent in ethylene-insensitive soybean plants as the virulence of the efe mutants in ethylene-susceptible soybeans. In contrast, the virulence of Pseudomonas syringae pv. phaseolicola was not affected by disruption of the efe gene.

Bacteria from necrotic branches of Asian pear trees (Pyrus pyrifolia) in Korea were consistently isolated as white colonies on nutrient agar and formed mucoid, slightly yellow colonies on a minimal medium with copper sulphate. Isolates with this colony morphology were studied in a series of microbiological, molecular and pathological tests. Most isolates allowed the verification of Koch's postulate on P. pyrifolia seedlings and on slices from immature pear (Pyrus communis) fruits and were also positive in hypersensitivity tests on tobacco leaves. They showed characteristics common to species in the genus Erwinia, but were different from Erwinia amylovora, the agent of fire blight. A relationship between the novel pathogen and E. amylovora was found in microbiological and serological tests. Both organisms had similar but not identical protein patterns in 2‐D gel electrophoresis, and in growth morphology the new pathogen produced colonies on MM2 Cu medium that were mucoid and slightly yellow, compared with the clearly yellow colonies of E. amylovora. No similarity was found in the plasmid profiles, and consequently no PCR signal was obtained with primers from the E. amylovora plasmid pEA29. REP‐PCR also produced bands differing for the two organisms.

Production of the chlorosis-inducing phytotoxin coronatine in the Pseudomonas syringae pathovars atropurpurea, glycinea, maculicola, morsprunorum, and tomato has been previously reported. DNA hybridization studies previously indicated that the coronatine biosynthetic gene cluster is highly conserved among P. syringae strains which produce the toxin. In the present study, two 17-bp oligonucleotide primers derived from the coronatine biosynthetic gene cluster of P. syringae pv. glycinea PG4180 were investigated for their ability to detect coronatine-producing P. syringae strains by PCR analysis. The primer set amplified diagnostic 0.65-kb PCR products from genomic DNAs of five different coronatine-producing pathovars of P. syringae. The 0.65-kb products were not detected when PCR experiments utilized nucleic acids of nonproducers of coronatine or those of bacteria not previously investigated for coronatine production. When the 0.65-kb PCR products were digested with ClaI, PstI, and SmaI, fragments of identical size were obtained for the five different pathovars of P. syringae. A restriction fragment length polymorphism was detected in the amplified region of P. syringae pv. atropurpurea, since this pathovar lacked a conserved PvuI site which was detected in the PCR products of the other four pathovars. The 0.65-kb PCR products from six strains comprising five different pathovars of P. syringae were cloned and sequenced. The PCR products from two different P. syringae pv. glycinea strains contained identical DNA sequences, and these showed relatedness to the sequence obtained for the pathovar morsprunorum. The PCR products obtained from the pathovars maculicola and tomato were the most similar to each other, which supports the hypothesis that these two pathovars are closely related. In conclusion, the region amplified by PCR was highly effective for the detection of coronatine-producing P. syringae strains, and the sequence analysis of PCR products proved valuable in showing relatedness between strains and pathovars.

The use of naturally occurring microbial antagonists to suppress plant diseases offers a favorable alternative to classical methods of plant protection. The soybean epiphyte Pseudomonas syringae pv. syringae strain 22d/93 shows great potential for controlling P. syringae pv. glycinea, the causal agent of bacterial blight of soybean. Its activity against P. syringae pv. glycinea is highly reproducible even in field trials, and the suppression mechanisms involved are of special interest. In this work we demonstrated that P. syringae pv. syringae 22d/93 produced a significantly larger amount of siderophores than the pathogen P. syringae pv. glycinea produced. While P. syringae pv. syringae 22d/93 and P. syringae pv. glycinea produce the same siderophores, achromobactin and pyoverdin, the regulation of siderophore biosynthesis in the former organism is very different from that in the latter organism. The epiphytic fitness of P. syringae pv. syringae 22d/93 mutants defective in siderophore biosynthesis was determined following spray inoculation of soybean leaves. The population size of the siderophore-negative mutant P. syringae pv. syringae strain 22d/93⌬Sid was 2 orders of magnitude lower than that of the wild type 10 days after inoculation. The growth deficiency was compensated for when wound inoculation was used, indicating the availability of iron in the presence of small lesions on the leaves. Our results suggest that siderophore production has an indirect effect on the biocontrol activity of P. syringae pv. syringae 22d/93. Although siderophore-defective mutants of P. syringae pv. syringae 22d/93 still suppressed development of bacterial blight caused by P. syringae pv. glycinea, siderophore production enhanced the epiphytic fitness and thus the competitiveness of the antagonist.Application of epiphytic bacteria as control agents is considered a nonpolluting approach for alternative plant protection, and a number of potential antagonistic isolates have been described. However, only a few of these isolates have proven to be as effective under field conditions as they are in laboratory setups (1, 39). It has been proposed that several attributes contribute to biocontrol, including competition for nutrients, antibiosis, niche exclusion, and interference with cell signaling systems (13, 36).Many potential antagonists have been selected from the fluorescent pseudomonad group, as this group includes various nonpathogenic species that are adapted to plant colonization and well known for their competitiveness (11,19,20). Pseudomonas fluorescens CHA0 has been proposed as biocontrol organism that can be used against several soilborne plant diseases (13). It has been suggested that secondary metabolites, such as 2,4-diacetylphloroglucinol, hydrogen cyanide, pyoverdin, and salicylate, are active principles in this isolate (11,13).P. fluorescens Pf-5 is a rhizosphere bacterium that suppresses seedling emergence diseases and produces a spectrum of antibiotics toxic to plant-pathogenic fungi (34). Pseudomonas putida WCS358, ...

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