Sensitivity of plant pathogenic fungi to taxane extracts from ornamental yews

Taxanes were extracted from the needles of ornamental yews using methanol followed by solid phase extraction (SPE) cleanup. The concentrations of paclitaxel (Taxol) and two other taxanes, cephalomannine and baccatin III were present typically in a 10:5:1 ratio along with several other taxane compounds. A 95% ethanol preparation of the extracts was amended into potato-dextrose agar (PDA), which was then characterized by its paclitaxel content. The radial growths of 12 plant pathogens in the Ascomycetes, Deuteromycetes, and Oomycetes were recorded and used to determine their EC50 values on the taxane-amended PDA. The fungi in the Ascomycetes and Deuteromycetes were classified as taxane insensitive, and their EC50 values were not determined within the concentration range examined (paclitaxel at EC50 > 4.00 microgram/ml). The five Oomycetes examined were classified as taxane sensitive. Pythium aphanidermatum had the lowest EC50 (paclitaxel at 0.05 microgram/ml [0.058 micromolar]), and Pythium irregulare had the highest EC50 (paclitaxel at 1.3 microgram/ml) [1.52 micromolar]). Compared with a pure standard of paclitaxel or a combination of the three authentic taxane standards, the partially purified taxane extract was more toxic to P. aphanidermatum and Pythium myriotylum. but less toxic to P. irregulare. Authentic cephalomannine was less toxic than paclitaxel, and baccatin III had no significant effect on these Pythium spp. at concentrations up to 2.0 microgram/ml. The wide variation of fungal sensitivity to the taxane extract suggests that different mechanisms and, or different target sites may exist across fungal species. Taxanes may offer a new chemistry for inhibiting the Oomycetes pathogens.