Leon H. Leonian

Differential growth of Phytophthoras under the action of malachite green.

Differential growth is often employed to identify strains of certain difficult species of bacteria and fungi. While not always a sure criterion, such growth reactions may often possess a pertinent cumulative value, provided that the right sort of inhibitive substances be used. If the critical concentration of such substances happens to be rather rigid and narrow, permitting no large numbers of dilutions at which members of a given group of organisms may grow or fail to grow, less satisfactory results can be expected, whereas a relatively broader margin of growth tolerance is conducive to more clear-cut differentiations. Malachite green is a substance of the latter type, as it is able to induce differential growth in Phytophthora when its concentration is varied from one part of the dye in one million parts of nutrient solution to one part in sixteen million. Such a comparatively wide margin tends to create some fairly well-defined spheres where numerous species and strains may aggregate themselves into tentative groups. The toxic effect of malachite green upon certain fungi was tested by Coons 2 who found that this dye in dilutions of I part in 40,000 was able to prevent growth in a number of species. The writer 3, working with 220 different cultures of Fusarium, found that a number of them failed to grow in presence of 0.0002 percent malachite green, and only three organisms were able to make any growth when the concentration of this dye was increased to O.OI percent. It is the purpose of this paper to present the effects of malachite green upon the species of Phytophthora, tracing their fluctuating limits on the one hand and their possible interspecific relationships on the other. A taxonomic work constructed upon the morphology or the physiology of a single organism may prove highly inadequate and misleading. With this idea in view, the writer endeavored to secure as many different strains of Phytophthora species as possible; but, with the exception of P. cactorum and