1 SummaryThe RAPD-PCR profiles of 13 phytopathogenic Alternaria species and two closely related outgroups were examined using six different primers. Each species produced a distinct pattern of DNA fragments which were used as a measure of the degree of relatedness between species. A. brassicae isolates of diverse origin showed high levels of similarity but little similarity was noted between other species. The closest interspecific genetic distances were recorded between A. citri, A. alternatu and A. longipes. The outgroup genera Embellisia and Stemphylium, which are recognised as distinct, could not be clearly separated using RAPD banding criteria, suggesting a high level of genetic diversity amongst these groups of fungi.

Strains of Rhizobium trifolii isolated from 21 different sites in Wales were characterized for growth rate, acid and aluminium tolerance, symbiotic efficiency and plasmid content. Plasmid number ranged from 2 to 10 per isolate but it was not correlated with soil pH or altitude of the locality of isolation. Neither plasmid number nor the pH of the site of isolation affected growth in acid medium or aluminium sensitivity. Plasmid number was positively correlated with growth rate in pure culture (P > 0.001) but isolates with high plasmid numbers were inefficient in their nitrogen‐fixing ability on white clover. The possible role of plasmids in terms of competitive ability is discussed.

An assessment was made of the relative contributions of a spontaneous mutation to rifampin resistance and a cryptic plasmid, pTA2, to competitive nodulation of Medicago saliva by a strain of Rhizobium meliloti. This was facilitated by use of rifampin-resistant derivatives of this strain in which pTA2 was originally present, cured, or reintroduced. Both curing of pTA2 and spontaneous mutation to rifampin resistance significantly influenced nodulating competitiveness, but the effect of rifampin resistance was greater and such that the contribution of pTA2 was evident only in cases in which paired competitors had the common rifampin resistance background. The data suggest that rifampin-resistant derivatives contain an altered RNA polymerase insensitive to the action of rifampin. All R. melhoti derivatives had symbiotic characteristics and phage susceptibility patterns similar to those of the wild type. Plasmid pTA2 transfer or other genetic interchange was not detected in nodules of M. saliva inoculated with paired competitors.Legume inoculation in soils containing indigenous Rhizobium populations frequently results in only small proportions of nodules produced by introduced strains (27, 33). Therefore, the improvement of nodulating competitiveness in Rhizobium strains intended for legume inoculation is of considerable practical importance.It has been reported that the symbiotic plasmid in R. leguminosarum (5), antibiotic resistance in R. trifolii (8), and genetic factors in the legume host (10, 17) influence competitive success in nodulation. However, the genetic basis of nodulating competitiveness in Rhizobium spp. is still not understood. Bromfield (6) reported that a derivative of a strain of R. meliloti obtained by spontanteous mutation to rifampin resistance and cured of a 135-megadalton cryptic plasmid (pTA2) was significantly less competitive in nodulation than the wild type. The main objective of this investigation was to assess the relative contributions of spontaneous mutation to rifampin resistance and plasmid pTA2 to competitive nodulation by this strain of R. meliloti. This was facilitated by use of rifampin-resistant derivatives with pTA2 originally present, eliminated, or reintroduced. MATERIALS AND METHODS R. meliloti and medium. The R. meliloti strains used in this investigation are described in Table 1. All R. meliloti were characterized by the procedure of Lesley (20) with the same 16 distinct phages. R. meliloti IZ450(pTA2) and derivatives were grown on yeast extract-mannitol agar (YEM) modified from Fred et al. (16) by using 1 g of yeast extract (Difco Laboratories) per liter and omitting CaCO3. Competition experiment. Competition for nodulation between R. meliloti IZ450(pTA2) and derivatives was examined with Medicago sativa cv. Apollo grown in modified Leonard jars (32) containing quartz sand and supplied with nitrogen-free nutrient solution (22). Seedlings from surfacesterilized seed (32) were planted and inoculated with 1 ml of cell suspension washed from the surface of a YEM slope in * Cor...

An assessment was made of the effect of high concentration rifampin resistance on the nodulating competitiveness of five strains of Rhizobium meliloti. The results indicate that the acquisition of rifampin resistance by R. meliloti is generally associated with a significant loss of nodulating competitiveness and an altered RNA polymerase insensitive to the action of rifampin. All mutants were similar to their parent strains with respect to growth rates, phage sensitivity patterns, and symbiotic effectiveness. The data suggest that rifampin resistance in R. meliloti is unsuitable as a marker for ecological studies.