An isolate of Fusarium solani (Sud 96) obtained from infected Striga plants in Sudan and six other isolates from Japan were evaluated for their effects on Striga germination. Among all the isolates, only the one from Sudan demonstrated high inhibitory activity. Aqueous and organic solvent culture extracts, as well as fungus suspension, when mixed with GR24, a synthetic analog of the natural germination stimulant strigol, inhibited germination of conditioned Striga seeds. Fusarium solani (Sud 96) filtrates, from cultures grown on autoclaved rice, sorghum grains, and potato dextrose agar (PDA), were more effective in reducing Striga germination than those from cultures grown on wheat straw. A significant difference between rice compared to sorghum and PDA cultures only occurred at high dilutions (40-fold). Complete inhibition of germination occurred when F. solani (Sud 96) culture filtrates and GR24 were applied simultaneously. Filtrate treatments made 2, 4 and 6 h subsequent to treatment with GR24 were less inhibitory. Filtrate treatments applied 8 h or more following GR24 had negligible effects on germination. Chromatographic separation on a silica gel column indicated the presence of several compounds with high inhibitory activity.

Metabolites of the fungus Fusarium solani (Sud 96) inhibited Striga hermonthica germination induced by the germination stimulant GR24. The active principles were identified as trichothecenes acuminatin, neosolaniol, 8-acetylneosolaniol, and tetraacetoxy T-2 tetraol (neosolaniol diacetate) on the basis of their chromatographic behavior and nuclear magnetic resonance and mass spectra. Inhibitory activity of the four trichothecenes against Striga germination increased with acetylation of the hydroxyl moieties. The most abundant inhibitor produced by the fungus, 8-acetylneosolaniol, completely inhibited Striga germination at 24 μM. The fungal toxin did not affect the germination of sorghum, a host crop, but retarded root and shoot elongation of the seedlings by 60 and 30%, respectively, at the same concentration.

Plant roots enter symbiotic as well as pathogenic interactions with fungi in the rhizosphere. We studied the response of bean (Phaseolus vulgaris L. cv. Saxa) roots to infection by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and the pathogen Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burkholder) Snyder & Hansen. In a time-course study of the symbiotic interaction between bean roots and G. mosseae, covering all stages of mycorrhiza development, we detected little change in the expression of the defence-related genes chitinase, β-1,3-glucanase and phenylalanine ammonia-lyase compared with non-mycorrhizal control roots. The only difference observed was a transient increase in chalcone synthase transcripts at later stages of mycorrhizal root colonization. In interactions with the pathogen, a marked induction of chitinase and phenylalanine ammonia-lyase expression was observed at the level of both the transcripts and enzyme activities. Class I β-1,3-glucanase levels strongly increased at the transcript level, but there was little change in the overall β-1,3-glucanase enzyme activity. In the non-host interaction between common bean and Fusarium solani (Mart.) Sacc. f. sp. pisi (Linford) Snyder & Hansen defence responses increased only slightly and transiently, if at all.