Anti-oomycete compounds from Ganoderma appalantum, a wood rot basidiomycete

Abstract

Solvent extracts of 17 different basidiomycetes were tested for their ability to inhibit Sclerospora graminicola. Among those tested, only three basidiomycete crude extracts exhibited significant inhibition on pathogen sporulation, zoospore release and zoospore motility. In vitro, the chloroform extract of Ganoderma appalantum (3 mg mL⁻¹) recorded a maximum sporangial inhibition of 52.5 and 82.0% zoospore release and 92.5% motility. Crude extracts of three different basidiomycete fungi were treated to pearl millet seeds and assessed for seed germination, seedling vigour and effectiveness against downy mildew disease under greenhouse conditions. None of the solvent extracts were found to be phytotoxic. Chloroform and petroleum ether extracts of G. appalantum (3 mg mL⁻¹) proved to be the best by offering disease protection of 55.6 and 43.7%, respectively, followed by chloroform and petroleum ether extracts of Polyphorus spp., with 42.5 and 39.1% disease protection. The thin layer chromatography (TLC) spots that developed were eluted and tested for inhibition of S. graminicola zoosporangia. The partially purified compound from TLC chloroform extract of G. appalantum consistently showed good inhibitory effect against S. graminicola, which exhibited inhibition of sporangia (42.3%), zoospore release (76.7%) and zoospore motility (86.7%), compared to the chloroform control which offered only 1.9, 2.9 and 1.7% inhibition of sporangia, zoospore release and zoospore motility, respectively. The partially purified compound from petroleum ether extract of G. appalantum resulted in 38.0, 63.0 and 81.6% inhibition of sporangia, zoospore release and motility, respectively, compared to petroleum ether control. However, azoxystrobin 250 SC (2 μg mL⁻¹) and apron 35 SD (0.015 mg mL⁻¹) treated on sporangial suspension showed the highest inhibition of S. graminicola pathogen compared to chloroform and petroleum ether TLC fractions.