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Responses of Pythium ultimum and Other Fungi to a Soil Extract Containing an Inhibitor with Low Molecular Weight. O. Vaartaja, Research Scientist, Forest Ecology Research Institute, Department of The Environment, Ottawa, Ontario, K1A 0W5, Canada; Phytopathology 67:67-71. Accepted for publication 25 May 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-67.

In 1971, samples of rain waters that percolated through clay loam in a lysimeter were collected, filtered (0.2 μm pore size), and bioassayed for growth response effects on mycelia of Pythium ultimum. Growth (radial colony extension) varied from 25 to 160% of that in distilled water controls. Following and during regularly spaced heavy rains, 23 August to 29 September, there were consistent inhibitory effects (growth 25-68%). Bioassays after ultrafiltration (Amicon) and gel chromatography (Sephadex, Bio-Gel) indicated the presence of many inhibitory substances with molecular weights (MW) that varied widely (<3,000, 6,500, 7,000, 18,000, and 33,000) before and after those dates. From 23 August to 8 September very strong inhibition was caused by an unidentified substance with MW about 150. After 8 months of storage at 5 C and after being boiled for 5 minutes in open containers, the extract that contained this substance remained inhibitory, with only a slight decrease in potency. The in vitro effects of a sample collected on 31 August were studied on 42 soil fungi. The response of different groups of fungi varied significantly (P = 0.001) and their descending order of sensitivity was: (i) soil basidiomycetes; (ii) Pythium spp.; (iii) miscellaneous soil fungi; (iv) Fusarium and Cylindrocarpon spp.; and (v) Penicillium and Gliocladium spp. The inhibitory effects were reduced significantly by amending the extract with 5% sucrose and 0.5% yeast extract.

Additional keywords: fungistasis, Thanatephorus practicola.