The Identification of Physiological Races of a Fungal Phytopathogen Using Strains of the Slime Mold Acrasis Rosea.

Physiological races of a fungal phytopathogen represent biotypes capable of attacking certain varieties of a susceptible host-species. Attempts to distinguish physiological races by other criteria have not yet been successful. On the other hand, strains of bacterial pathogens may often be identified using bacterial viruses (bacteriophage). Attempts to demonstrate a virus of a true fungus have not yet been successful. It seemed reasonable to assume that an obligate parasite of a fungal phytopathogen might provide the means to identify its physiological races. Olive et al.1 reported that the slime mold Acrasis rosea Olive & Stoianovitch required ingested food as its primary source of nutrients and that certain strains of the slime mold would ingest the spores of some but not other species of fungi. One "susceptible" species was Colletotrichum lagenarium (Pass.) Ell. & Halst., the incitant of anthracnose of cucurbits. An investigation of the "resistance" or "susceptibility" of four physiological races of this phytopathogen grown on four media and subsequently inoculated with each of six strains of A. rosea indicated that it was possible to identify each physiological race of C. lagenarium. Materials and Methods.-Olive et al.1 have reported on the origin of five strains of A. rosea used in this investigation; strain MW-1 was isolated by S. K. Dutta. The slime molds were "cultured" on Rhodotorula mucilaginosa (red yeast) which was grown at room temperature on corn meal-dextrose agar (Difco) supplemented with 0.2 per cent yeast extract. The red yeast was highly "susceptible" and was rapidly ingested by the slime molds. The migration of myxamoebae of A. rosea from the red yeast to the agar was detected by the appearance of an obvious haze around the red yeast culture. Small rectangles (1 X 3 mm) of agar well away from the culture were removed from the hazy area and added to the center of sporulating colonies of C. lagenarium. The myxamoebae migrated from the agar block to the colony. It must be emphasized that only sporulating colonies were inoculated. The inoculated colonies were incubated at 220C for 5 days and then placed at room temperature in the light for an additional 5 days to determine if the spore masses of the phytopathogen had been ingested and if the slime mold had fruited. The slime mold ingests spore masses but not the mycelium of a "susceptible" physiological race of C. lagenarium. The area of ingested spore masses resembled a "crater" when the surface of the colony was illuminated with incident, oblique light. If spore masses of one colony in a series of replicate plates of a "susceptible" physiological race of C. lagenarium were apparently not ingested, a portion of that colony which included the site of inoculation of the slime mold was transferred to a culture of red yeast. If the yeast was ingested, it was assumed that the slime mold had indeed ingested the spore masses of C. lagenarium but at a very slow rate. The initial investigation was performed with one isolant of each of the four known physiological races of C. lagenarium; their origin was reported by Dutta et al.2 Later studies involved 13 isolants supplied by N. N. Winstead who had assigned all but one to a specific physiological race. Only those cultures of C. lagenarium which displayed excellent growth and sporulation on the following media were used: minimal agar,3 potato-dextrose agar,3 bean agar,4 and yeast extract agar (0.5 per cent yeast extract, 2 per cent glucose, 2 per cent agar). Colonies of C. lagenarium were scored on the fifth and tenth day after inoculation with the slime mold. Spore masses of "resistant" colonies were not ingested; spore masses of "susceptible" colonies were either poorly, obviously, or rapidly ingested. If the spore masses were rapidly in-