Primary infections were obtained by growing cabbage seedlings in a modified Hoagland's solution in which resting spores of Woron. were suspended. Seeds were germinated on filter paper wet with tap water, and after 2 days the plants were transferred to small glass tubes bent in the form of a shallow U or to small vials containing solution and spores. Zoosporangia were formed after several days growth at 25° in the dark. They were stained in aceto-carmine. A roughly linear relationship was found between the logarithm of number of infections/root and the logarithm of spore concentration in the medium. Numbers of infections were usually greater at 1/5 or 1/25 dilution of the culture solution than at the standard concentration, but were very much fewer or none in more dilute 1/125 or more concentrated (x 5) solutions. The concentration which permitted maximal infection tended to vary from one experiment to another. Infection was not affected by changing from pH 5 to 6 but was greatly decreased at pH 8.


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