SENSITIVITY OF PLANT-PATHOGENIC FUNGI TO TAXANE EXTRACTS FROM ORNAMENTAL YEWS

Taxanes were extracted from the needles of ornamental yews using metha nol followed by solid phase extraction (SPE) cleanup. The concentratio ns of paclitaxel (Taxol) and two other taxanes, cephalomannine and bac catin 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 i n the Ascomycetes, Deuteromycetes,and Oomycetes were recorded and used to determine their EC(50) values on the taxane-amended PDA. The fungi in the Ascomycetes and Deuteromycetes were classified as taxane insen sitive, and their EC(50) values were not determined within the concent ration range examined (paclitaxel at EC(50) > 4.00 mu g/ml). The five Oomycetes examined were classified as taxane sensitive. Pythium aphani dermatum had the lowest EC(50) (paclitaxel at 0.05 mu/ml [0.058 mu M]) , and Pythium irregulare had the highest EC(50) (paclitaxel at 1.3 mu g/ml [1.52 mu M]). Compared with a pure standard of paclitaxel or a co mbination of the three authentic taxane standards, the partially purif ied taxane extract was more toxic to P. aphanidermatum and Pythium myr iotylum, but less toxic to P. irregulare. Authentic cephalomannine was less toxic than paclitaxel, and baccatin III had no significant effec t on these Pythium spp. at concentrations up to 2.0 mu g/ml. The wide variation of fungal sensitivity to the taxane extract suggests that di fferent mechanisms and/or different target sites may exist across fung al species. Taxanes may offer a new chemistry for inhibiting the Oomyc etes pathogens.