Abstract Vegetative inoculum of Pisolithus tinctorius (Pt) produced by research procedures at the Institute for Mycorrhizal Research and Development (IMRD) was compared with that produced in various commercial solid substrate fermentors by Abbott Laboratories. Effectiveness of inocula was examined during 1977 through 1980 in 54 discrete experiments undertaken at the IMRD Microplot Nursery and in 33 conventional bare-root nurseries located in 25 states. Over 750,000 seedlings of 11 pine species and varieties, Douglas-fir, and northern red oak were involved. Inoculum was broadcast on fumigated nursery soil at different rates and manually mixed into the upper 10 cm of soil before sowing seed. In conventional nurseries, most Abbott inoculum batches produced in 1977 and 1978 were ineffective, but modifications in fermentation procedures significantly improved effectiveness in 1979 and 1980 to such a degree that most batches of Abbott inoculum were as effective as IMRD inoculum. The minimum standard for inoculum effectiveness was formation by Pt of at least 50 percent of all ectomycorrhizae on inoculated seedlings. In the 1980 tests, a single drench application of captan (5.6 kg a.i./ha) after sowing of seed significantly improved inoculum effectiveness. A nutrient-enriched medium of vermiculite and peat moss, the latter at 5 to 10 percent by volume, was best for growing Abbott inoculum. Peat moss maintained the pH of inoculum below 6.0 and increased efficacy. Inoculum leached with water and dried was most effective. Of seven inoculum characteristics studied, no one was consistently correlated with effectiveness of inoculum in nursery tests. Generally the most effective inoculum was one with (1) abundant hyphae of Pt inside the vermiculite particles, (2) pH between 4.5 and 6.0, (3) minimum microbial contaminants, and (4) water soluble substances such as glucose minimized by leaching before drying. An inoculum broadcast rate of 1.08 l/m² of soil surface gave best results. The product of percent of seedlings ectomycorrhizal with Pt and proportion of Pt ectomycorrhizae to total ectomycorrhizae on inoculated seedlings was termed the Pt index. Attempts were made to correlate final Pt indices formed by IMRD inoculum on 1-0 loblolly pine seedlings in 20 nursery experiments with various cultural practices and soil conditions in order to determine the effects of these individual factors on inoculum effectiveness. No single factor accounted for significant differences in inoculum effectiveness even though each factor varied from nursery to nursery.Loblolly pine seedlings from 11 conventional nursery tests in which Pt indices were >50 produced 35 percent more biomass/cm² of nursery soil than did seedlings with natural ectomycorrhizae. This suggested that seedlings with more than half of their ectomycorrhizae formed by Pt utilized water and nutrients more effectively and that, in most nurseries, successful inoculation with Pt could lead to more plantable seedlings/unit area of nursery soil than could otherwise be produced with the same cultural practices. In the 1-0 pine nurseries, Pt indices above 50 were associated with larger seedlings and fewer culls in comparison to noninoculated control seedlings in over half of the nurseries. In most of the remaining nurseries, however, control seedlings had abundant natural ectomycorrhizae, grew well, and included few culls. In the 2-0 seedling nurseries, only 7 of 12 tests had Pt indices >50 after the first growing season and only two had Pt indices >50 at the end of the second growing season. Significant seedling growth increases and a corresponding reduction in seedling culls due to Pt ectomycorrhizae were found in only 4 of these 12 nurseries. Results from one 3-0 pine seedling test in Michigan were similar to those of the 2-0 tests.There were indications that soil fumigation in the fall was not as effective as spring fumigation for development of Pt ectomycorrhizae. Only 30 percent of the nurseries in the 1978 test that fumigated soil in the fall of 1977 preceding spring inoculations had positive inoculation results, whereas 80 percent of the nurseries that fumigated soil in the spring of 1978 had positive results.The number of fruit bodies produced by Pt in the various nurseries was positively correlated with high Pt indices. Thelephora terrestris was the most frequently encountered naturally occurring ectomycorrhizal fungus in these tests. In 41 of the 45 tests in conventional nurseries, fruit bodies of T. terrestris were found on all tree species in 23 of the 25 states in which tests were undertaken. Fruit bodies of Rhizopogon nigrescens and Laccaria laccata, and the jet-black ectomycorrhizae formed by Cenococcum geophilum were rarely encountered.Results of these tests showed that viable vegetative inoculum of Pt can be effectively produced by industrial fermentation procedures and used to form ectomycorrhize in bare-root seedlings of a variety of forest tree species for practical use in forestry.
Marx et al. (Sat,) studied this question.