No abstract

Differences in the size and composition of bacterial populations found in the rhizospheres of maize, wheat, and grass were compared. Significant differences in the fluorescent pseudomonad and "coryneform" groups were found among the cultivars of each plant species. Only between the two maize cultivars, reported to be resistant and susceptible to Fusarium species, and between the two grass lines, known to be resistant and susceptible to Dreschera poae, could significance be found in the total numbers of bacteria counted. In each case higher bacterial counts were recorded in the rhizospheres of cultivars susceptible to disease. No conclusion could be reached with regard to differences that occurred between the two cultivars of wheat that are known to differ in tolerance to pH. Although actinomycetes appeared to represent a high percentage of the total bacterial population in maize, insufficient data prevent further interpretation. Bacillus numbers do not differ with any significance betewen cultivars or even between cultivars and counts made in root-free soil. Both the Bacillus and fluorescent pseudomonad groups were found to be represented in lower numbers in the rhizosphere of all cultivars than either the actinomycetes or "coryneform" groups. The findings of this study correlate highly with previously published work in stressing the importance of the plant's influence on the bacteria in its own rhizosphere. As this may mean that a plant, given the right genetical configuration, can influence the development of a favourable rhizosphere flora, the mechanism involved in this relationship should be investigated.Key words: bacteria, disease resistance, pH, rhizosphere, selective stimulation.

The total bacterial populations, fluorescent pseudomonads, and actinomycetes in the rhizosphere, rhizoplane, and endorhizosphere of two wheat lines (C-R5B and C-R5D) were investigated weekly during plant growth. The total numbers of bacteria (colony-forming units) were counted on a low-nutrient agar medium, while the numbers of fluorescent pseudomonads and actinomycetes were determined by means of selective media. Fluorescent pseudomonads generally constituted less than 0.5% of the total bacterial numbers and the percentage significantly decreased with time in the rhizosphere, rhizoplane, and root-free soil. The percentages of fluorescent pseudomonads were lower in the rhizoplane than in the rhizosphere, and in the endorhizosphere they were barely detectable. The rapid decline of fluorescent pseudomonads frequently observed in survival studies may be explained by the inability of the plant to support this group of bacteria during later development of its rhizosphere. Actinomycetes generally represented no more than 7% of the total bacterial numbers. Actinomycete percentages were highest in the rhizosphere and root-free soil and lowest in the endorhizosphere. Only minor differences were found in total bacterial numbers and in the numbers of fluorescent pseudomonads between the two wheat lines. However, numbers of actinomycetes were significantly different over the whole experiment between wheat line C-R5B and C-R5D in the rhizosphere, rhizoplane, and endorhizosphere. Key words: actinomycetes, bacteria, fluorescent pseudomonads, rhizosphere, sampling time.

Different wood preparation techniques for a SEM study of the growth of Ophiostoma ulmi (Buisman) Nannf. in elm tissue were compared. Critical point drying appeared to be the most suitable method. All observations were made 10-14 cm above the site of inoculation. In the resistant elm, Ulmus hollandica cl. 390, inoculated with a non-aggressive strain of O. ulmi, no hyphae could be found until 5 days after inoculation. However, when U. hollandica cl. 390 was inoculated with the aggressive strain, the fungus could already be observed after 3 days. In the susceptible elm, U. hollandica cl. Belgica, whether inoculated with an aggressive or non-aggressive strain, hyphae were found within 2 days after inoculation. The fungus appears only to penetrate the vessel pits and has at no time been seen to penetrate the cell wall directly.

Corynebacterium fascians, which causes leafy gall, has been shown to be responsible for the unusual symptoms recently found in lilies. The symptoms are described and the different bacterial isolates compared by biochemical, serological and pathogenicity experiments. No evidence could be found for the existence of specialized strains even though considerable variation in virulence could be demonstrated. Although the role of variation in susceptibility of the different lily cultivars should not be underestimated, it would appear that high inoculum levels of C.faseians in the soil may be largely responsible for these outbreaks.Also included in this study are the results of biochemical, serological and pathogenicity experiments of C. fascians which has been isolated from Kalanchoe, Euphorbia, Brodiaea, Hebe and Verbascum.