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Biovars 2 and N2 of Pseudomonas solanacearum differ in their distribution and metabolic properties, the former generally being recovered from areas of cool climate (highland strains), whereas the latter is widespread in the tropical Amazon basin (lowland strains), and is metabolically more versatile. The relationships between 53 isolates of P. solanacearum conforming to biovar 2 or N2 were assessed using analysis of total genomic DNA digested with Hae III. Separation of these digests on 5% polyacrylamide gels and subsequent silver‐staining was shown to be a rapid and sensitive method of distinguishing between biovar 2 and N2 strains. Five restriction fragment length polymorphism (RFLP) groups could be identified which were further divisible into a total of 11 clonal lines. The majority of isolates examined were biovar 2 and of one clonal type (RFLP 26A), which was found in South America and is also a world‐wide pathogen of potato. All other RFLP groups were found only in South America, where they only occurred east of the Andes, with the exception of RFLP group 27 which was only found west of the Andes. The confinement of these closely related groups to South America supports the hypothesis that this continent was the source of the common potato bacterial wilt pathogen (RFLP 26A). The genomic DNA analysis clearly separated the highland (biovar 2) strains of P. solanacearum into the two subgroups previously recognized on the basis of nutritional and DNA hybridization data (RFLP groups 26 and 27), although these subgroups were shown to be closely related, having a similarity coefficient of 85%. The metabolically more versatile lowland strains (biovar N2) exhibited considerable diversity at the DNA level, which is at odds with reports of their uniform phenotype. To demonstrate the general utility of the total genomic DNA analysis, restriction patterns of a limited number of other biovars (1, 3 and 4) are also presented.

The colonization of plant roots by Verticillium dahliae was studied with use of fluorescent antibodies to stain the fungus in soil and on plant roots. Microsclerotia were observed germinating in rhizosphere soil and in soil amended with nutrients. Germination occurred much earlier in soil amended with either a sugariamino acid solution or a solution of wheat root extract than in soil amended with distilled water. Usually only a single germ tube was observed, but some microsclerotia produced two, three or more initials. These grew into hyphal strands up to 100 μm long, but most began to lyse when the length exceeded 30 μm. Hyphae of V. dahliae were observed on the roots of both immune (wheat) and susceptible (common thorn-apple, cotton and Noogoora burr) plants growing in field soils infested with microsclerotia. These hyphae were sparse, up to 1500 μm in length and appeared to enter the cortex of the young roots directly, without the aid of specialized penetrating structures such as appressoria. Sporulating hyphae were observed on the rhizoplane of each of the four species examined. Conidial germination, however, was not observed on the rhizoplane. By cultural methods it was established that V . Dahliae colonized plant roots 2-3 cm back from the root tip, suggesting that microsclerotia are stimulated to germinate by the advancing root. Examination of the rhizosphere soil around the root cap and the zone prior to vascular differentiation failed to produce evidence of conidia or other structures that may have functioned as an intermediate stage in the colonization of plant roots by V. dahliae.

E.J. COTHER, J.K. BRADLEY, M.R. GILLINGS AND P.C. FAHY. 1992. Erwinia chrysanthemi isolated from the headwaters of the Murrumbidgee River was shown to be different from strains isolated from the source of the Murray River only 140 km away. Biovar 5 (sensu Samson) was found consistently in the upper reaches of the Murrumbidgee River but was not detected in the Murray River where the majority of strains isolated were characterized biochemically as biovar 3. Fatty acid methyl ester analysis of 56 water strains by gas‐liquid chromatography and the Hewlett‐Packard Microbial Identification System confirmed the existence of a distinct biotype at the source of each river system. All Erw. chrysanthemi strains contained cis 9 hexadecanoic (16 : cis9), hexadecanoic (16 : 0), and tetradecanoic (12 : 0) acids. The majority of strains contained dodecanoic acid (12 : 0) in small amounts. Twenty‐six isolates (18 from water, six from potato, and two type cultures) selected for DNA digestion with Hae III or Hinf I were placed in 10 clonal lines based on restriction patterns. No clone was common to both river systems. Restriction patterns obtained by analysis of strains from soft‐rotting potatoes were identical to those obtained from cultures from the headwaters or upper reaches of both rivers. Biochemical, fatty acid and DNA characterization methods all supported the hypothesis that Erw. chrysanthemi is a natural component of the alpine aquatic microflora which could infect potatoes via irrigation water.