Some inoculum and substrate factors in the cultural inhibition of Hypoxylon pruinatum (Klotzsche) Cke. by pyrocatechol

The experimental results indicated that the type of inoculum was a critical factor in the ability of Hypoxylon pruinalum (Klotzsche) Cke. to colonize 2% Difco Bacto malt agar containing 640 p.p.m. of pyrocatechol, or plates with 10% autoclaved bark meal of Populustremuloides Michx. in 2% water agar. Hypoxylon failed to become established on these media when the inoculum consisted of agar plugs from the margin of 5- to 7-day-old colonies on 2% Difco Bacto malt agar. However, the fungus did colonize the same plates when infected portions of autoclaved bark meal of P. tremuloides were used as the inoculum. Apparently, the total inhibition of Hypoxylon on these media was related to the use of the agar plug type of inoculum rather than to the concentration of fungistatic substances in the media. The most rapid growth of Hypoxylon on malt agar containing pyrocatechol occurred in the colonies that developed from infected portions of untreated bark meal of P. tremuloides. In these cultures Hypoxylon grew in association with bark saprophytes that appeared to act on the pyrocatechol and provide a more favorable medium for the growth of the fungus.It was demonstrated that the nutritional properties of the untreated bark of P. tremuloides were more favorable to the growth of Hypoxylon than those of autoclaved bark. The growth of the fungus was strongly inhibited on malt agar containing suspensions of autoclaved bark and 640 p.p.m. of pyrocatechol. Excellent growth occurred when the medium contained suspensions of untreated bark and the additional pyrocatechol. Apparently, the development of the bark saprophytes inactivated the pyrocatechol in the medium and the untreated bark tissues and prevented the chemical inhibition of Hypoxylon. Therefore, the nutritional and microbiological factors of untreated bark were such that the fungus could grow and survive in these tissues despite the presence of pyrocatechol. However, the saprophytic microfloras of most samples of untreated bark included Trichoderma sp. and (or) Aspergillus sp., which overcame the competition from the other saprophytes and were mutually antagonistic with Hypoxylon. The growth of the fungus was inhibited when the untreated bark tissues were colonized by these saprophytes