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Fatty Acids and Naturally Occurring Plant Lipids as Stimulants of Rhizomorph Production in Armillaria mellea. A. R. Moody, Assistant Research Plant Pathologist, Department of Plant Pathology, University of California, Berkeley 94720; A. R. Weinhold, Associate Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 62:264-267. Accepted for publication 13 September 1971. DOI: 10.1094/Phyto-62-264.

Armillaria mellea was grown on a basic medium consisting of 10 g glucose, 2 g L-asparagine, 1.75 g KH2PO4, 0.75 g MgSO4 7H2O,1 mg thiamine, and 10 g Difco agar/1,000 ml distilled water. Abundant mycelia but no rhizomorphs were produced on this medium. Rhizomorphs were formed when the medium was supplemented with natural lipids such as coconut, corn, cottonseed, olive, peanut, safflower or wheat germ oils, lanolin, or vegetable lecithin. Several monocarboxylic acids were then tested to determine the active portions of these naturally occurring lipids. Two groups of free acids stimulated rhizomorph production. The first consisted of propionic, butyric, and valeric acids, whereas the second contained the unsaturated fatty acids oleic, linoleic, and linolenic. The unsaturated fatty acids are major components of the natural plant oils that were active, and activity correlated with the quantity of unsaturated fatty acids in the oils. Polyoxyethylene sorbitan monolaurate (Tween 20) was highly effective in stimulating rhizomorph production, whereas related compounds were active to a lesser degree. Among additional lipid compounds tested, several sterols and glycerol were found to be inactive. No rhizomorphs were produced by A. mellea when active fatty acids or naturally occurring lipids were the sole source of carbon in the media, indicating that lipids function as growth-promoting substances rather than as sources of carbon.