Relative Capacities of Filamentous and Non-filamentous Bacteria from Two Forest Soils To Inhibit Phellinus weirii in Culture

Alder and conifer soil were examined for Streptomyces spp. and non-filamentous bacteria antagonistic to Phellinus weirii. Conifer soil supported higher populations Streptomyces spp. and higher percentage of Streptomyces species antagonistic to P. weirii than did alder soil. The populations of non-filamentous bacteria were low in both soils. Alder soil, however, had a higher percentage of non-filamentous bacteria antagonistic to P. weirii than did conifer soil. Identified Streptomyces show that different species are present in each soil type. Introduction Phellinus weirii (Murrill) Gilbertson Poria weirii (Murrill) Murrill] causes serious root rot of many species of native conifers in western North America. The fungus can survive for many years in stumps and roots of dead and living trees, and it may infect roots of adjacent host trees when ectotrophic mycelia pass from one to another at points of root contact. Some evidence suggests that damage to susceptible conifers is suppressed by the presence of red alder, Alnus rubra Bong. (Trappe, 1972; Nelson et al. 1978). Alder soils contain phenolic compounds shown to inhibit growth of P. weirii in vitro (Li et al., 1972) and high levels of nitrogen including NO3 (Franklin et al., 1968), a form of N not utilized by P. weirii (Li et al., 1967). Researchers have also shown significant differences in microbial populations between alder and conifer soil (Lu et al., 1968; Wicklow et al., 1974). The relative numbers or percentage of microbes antagonistic to P. weirii, however, have not been reported. Soil microbes, in addition to nitrogen and phenolic compounds, might influence disease development. In this study, we examine differences in populations of Streptomyces and non-filamentous bacteria antagonistic to P. weirii in adjacent stands of red alder and of mixed conifers, primarily Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]. This study was conducted at Cascade Head Experimental Forest of coastal Oregon, an area chosen for its history of intensive research and available scientific information. Methods and Materials Three soil samples were collected from one alder stand and from one conifer stand at two-month intervals for one year. Each sample consisted of three sub-samples taken approximately 60 cm from the base of a tree to a depth of 64 cm after the first 5 cm of top litter had been removed. The soil was immediately taken to the laboratory and screened through a 6-mm sieve. Northwest Science, Vol. 55, No. 3, 1981 219 Soil dilution plates (1:50,000 dil.) were made in triplicate from 20 gm each of the soil samples using sodium albuminate agar (pH 6.8) as the, selective medium for Streptomycef and non-filamentous bacteria. The plates were incubated at 28° C for eight days and colonies counted. A double-layer plate technique (Li et al., 1969) was used to test antagonism towards P. weirii. The fungus was first grown in malt broth, then the mycelium was washed in sterile, distilled water, and homogenized. One ml of the homogenate was pipetted into each petri plate and 10 ml of 1 percent water-agar was poured on top and allowed to solidify. Ten ml of malt yeast peptone (MYP) agar medium was layered over the water-agar and the surface inoculated with bacterial colonies, randomly selected from the dilution plates. Double-layer plates were incubated at 26° C for two weeks, and antagonism was recorded if a zone of inhibition was produced around a bacterial colony (Fig. 1). Totals of 2144 Streptomyces colonies from conifer soil and 1571 colonies from alder soil were tested for their antagonism to P. weirii. Of the non-filamentous bacteria, Figure 1. Inhibition of P. ueirii by Streptomyces isolated from conifer soil.