Germination of Conidia of Peronospora Tabacina Adam I. Germination in Vitro

Abstract

SummaryWashed conidia of Perono8pora tabacina Adam germinated poorly or not at all in water alone, but germinated in the presence of riboflavin. The rate of germination of conidia in liquid suspension was enhanced by the presence of carbon and nitrogen sources, phosphate, and calcium and magnesium ions. The effects of 141 metabolites on germination and germ-tube elongation have been tested.A number of analogues of natural metabolites were inhibitory to germination and germ-tube elongation. Extracts and exudates of tobacco leaves did not affect either the amount or rate of germination.Washing by centrifugation increased the percentage of conidia that germinated subsequently and the presence of a germination inhibitor in unwashed conidia is postulated.The optimum temperature for germination of conidia on agar, or in liquid suspension, was in the range 15-20°C. The pH optimum varied with the constitution of the medium; when germinated on 2 % agar a broad optimum was shown at pH's 5·5-8·0, whereas in liquid suspension the optimum was in the pH range 6· 5-8· O.When conidia were tested within 24 hI' from the time of initiation of sporulation, germination was high (85%). After 48 and 72 hI' germination had dropped to 61 and 48% respectively.Washed spores showed no loss of germination capacity when kept for up to 6· 5 hI', but germination was negligible after 18 hr storage.Visible light did not affect germination, but 9000 fLWfcm2 of ultraviolet light reduced it to 2·7%.

Effects of Metabolites and Antlmetabolites on the Sporulation of Peronospora Tabaoina Adam. On Tobacco Leaf Disks

“…In view of the effect of riboflavin on conidial germination shown by Shepherd (1962) it is interesting to note that in the present study ribofiavin itself has a significant stimulating effect on sporulation intensity (116·4% of the untreated control), the mononucleotide a greater effect (148·4%), and FAD the largest effect (173· 9%), which suggests the possible conversion of ribofiavin to FAD in the sporulating system.…”

“…Previously Shepherd (1962) suggested that uracil synthesis is limiting during the phase of germ-tube elongation. Xanthine, its analogues, and the related compounds caffeine and theophylline all show some degree of inhibition of sporulation, but the mechanism of this inhibition is unknown.…”

“…Tetramethyl thiuram disulphide and the dithiocarbamates, at concentrations of 400 fLg/m1, caused no diminution of the sporulation intensity: the white spores are fully viable, as judged by germination on agar (Shepherd 1962) or on the leaf surface (Shepherd and Mandryk 1963), and are pathogenic. Mter infection of N. tabacum by white spores, the subsequent spore crop is the normal blue colour, which indicates a phenotypic change only.…”

“…Canavanine has been described as an efficient inhibitor of conidial germination (Shepherd 1962) and would be a useful therapeutic agent for use in disease control, except for its strongly phytotoxic properties. (c) Production of White Spores by P. tabacina…”

“…fiower pots in soil mix C supplemented with fertilizer lIe as described by Matkin and Chandler (1957). When the plants were approximately 80 em high, the foliage was inoculated with a conidial suspension of P. tabacina in water (Shepherd 1962) and placed under conditions favourable for leaf infection (Cruickshank 1958). After 5 days, the sixth leaf from the base of each plant was removed, and disks (10 mm .diam.)…”