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.