Urediniospores of Puccinia graminis tritici, form 21, were sown on buffer solutions covering the range from pH 3 to pH 8. Maximum germination was obtained between pH 5.8 and 6.5, with indications that if the point of maximum germination could be determined with precision it would be found to lie in the neighborhood of pH 6.2.The effect of buffered and unbuffered solutions of pure phenolic compounds on the germination of urediniospores was studied. Greater inhibition was obtained with unbuffered solutions owing to the additional effect of hydrogen ion concentration. In buffered solutions, at pH 6.0, germination was prevented by 45 p.p.m. of hydroquinone, 125 of o-cresol, 180 of anisic acid, 250 of benzoic acid, 340 of catechol, 400 of guaiacol and 600 of phenol. A number of other compounds were also studied. The inhibitory effects of compounds, of which the bactericidal effects are known, are fairly closely related to their bactericidal efficiency.Investigations were made of the inhibitory effects on spore germination and the growth of germ tubes, of aqueous dilutions of 15, 10, 5 and 2.5% of the press-juice of Vernal, Khapli, Marquis and Little Club wheats. Statistical treatment of the data accumulated in ten series of determinations, in which a total of 160,000 spores was counted, showed that significant differences occurred between varieties and that variations in the total solid content and hydrogen ion concentration of the press-juice had no consistent effect upon the results. Although the results were not entirely clear-cut, it is concluded that the varieties fall in the following order with respect to increasing inhibitory effect of their extracts: Vernal, Marquis, Khapli and Little Club. Since Vernal and Khapli are resistant to form 21 whereas Marquis and Little Club are susceptible, the results fail to show any relationship between the effects of host extracts on the germination of urediniospores, or on the growth of germ tubes, and the rust resistance of the variety.
A Third Factor for Resistance to Puccinia graminis Tritici