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Disease Control and Pest Management

Mechanisms Inhibiting Damping-off Pathogens of Slash Pine Seedlings with a Formulated Soil Amendment. J. W. Huang, Former graduate student, Department of Plant Pathology, University of Georgia, Current address: Department of Plant Pathology, National Chung-Hsing University, Taichung, Taiwan, Republic of China; E. G. Kuhlman, Principal plant pathologist, U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station, Carlton Street, Athens, GA 30602. Phytopathology 81:171-177. Accepted for publication 2 August 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1991. DOI: 10.1094/Phyto-81-171.

Suppressiveness of soil amended with SF-21 mixture to Rhizoctonia solani and Pythium aphanidermatum was tested with a modified pine stem-segment colonization method and germ tube growth, respectively. Soil amended with 1% (w/w) SF-21 did not immediately inhibit hyphal growth and stem-segment colonization by R. solani, but 7 days after amendment it significantly inhibited the fungus. The population of other fungi increased significantly 4 days or more after application of SF-21. The density of this population was correlated with suppression of stem-segment colonization by R. solani in amended soil. Stem-segment colonization by R. solani increased 30–40% after amended soil was autoclaved, but the inhibitory effect returned 7 days after infestation with Trichoderma harzianum or Penicillium oxalicum or both. Infestations with T. harzianum also restored suppressiveness to autoclaved nonamended soil, whereas infestations of autoclaved nonamended soil with P. oxalicum did not. Increasing the population of T. harzianum or P. oxalicum or both in amended soil at day 0 to about the same level as that in amended soil at day 7 immediately rendered it suppressive to stem colonization by R. solani. In addition, amended soil at day 0 directly inhibited and lysed germ tubes of P. aphanidermatum, and at day 7 after amendment was even more effective. Autoclaved amended soil with or without added T. harzianum or P. oxalicum or both suppressed germ tube elongation by P. aphanidermatum. However, inhibition of germ tube elongation was partially nullified in amended soil mixed with benomyl at 50 µg/g soil after 7 days. Soil amended with SF-21 and incubated for 7 days produced inhibitory substances that diffused into a synthetic medium and inhibited hyphal growth of R. solani and P. aphanidermatum by 22 and 28%, respectively, compared with diffusates from nonamended soil. When the pH of amended soil was adjusted from 4.3 to 5.8, the suppressive effect on colonization by R. solani was completely nullified, but the soil remained partially inhibitory to germ tube elongation by P. aphanidermatum. Population density of Trichoderma spp. and Penicillium spp. in amended soils was negatively correlated (r = –0.91), with pH values from 4 to 6. The capacity of Al₂(SO₄)₃ to inhibit growth of R. solani and P. aphanidermatum in water agar was much greater at pH 4 than at pH 6. P. aphanidermatum was more sensitive to Al₂(SO₄)₃ at pH 4 than was R. solani. Sporangial formation by P. aphanidermatum was inhibited by water extracts of milled pine bark-water mixtures of 2:1–1:1, v/v, whereas extracts from 1:2–1:50, v/v, enhanced zoospore release. A 50 µg/ml alcohol extract from Virginia or shortleaf pine bark more strongly inhibited sporulation by P. aphanidermatum than did that from loblolly pine bark. We conclude that the inhibitory effects of SF-21 on specific pathogens differ. Effects on R. solani are indirect through proliferation of the microbial species, especially Trichoderma harzianum and Penicillium oxalicum. Effects on P. aphanidermatum are direct by the inorganic and organic components of SF-21 and indirect by reduced soil pH and stimulation of the microbial population.