Abstract
Comparative tests were conducted on Phymatotrichum omnivorum (Shear) Duggar and Pythium ultimum (Trow) to determine if an alternative mode of action exists for two different triazole fungicides. The accepted mode of action of penconazole (Topas) and propiconazole (Tilt) is the inhibition of ergosterol biosynthesis in sterol-synthesizing fungi (4,5,9,72,73,93). However, P. ultimum, which does not synthesize or require sterols for vegetative growth, has shown "moderate sensitivity" to penconazole in greenhouse and in vitro tests (28,63). The ED5 0 (minimum inhibitory concentration) for P. ultimum is 25 ppm with penconazole and 98 ppm with propiconazole, while the ED[50] for P. omnivorum is 9 ppm with penconazole and 0.01 ppm with propiconazole. This difference could be due to solubility since propiconazole is more polar (110 ppm at 20 C in water) than penconazole (70 ppm). It is possible propiconazole is more effective than penconazole on P. omnivorum because its uptake is greater in the low -lipid containing (1.5-2.0% lipid content based on blot-dry w eight) vegetative hyphae. It is possible that penconazole being more non-polar than propiconazole is taken up better in the relatively high lipid containing (11-13% lipid content) vegetative hyphae of P. ultimum. The increased sensitivity shown by the sterol-synthesizing P. omnivorum over the nonsterol-synthesizing P. ultimum to both triazoles, suggests that uptake of triazoles is mediated by sterols or "sterol-like" compounds. Levels at and below the ED5 0 were chosen as the concentrations to observe biochemical and ultrastructural changes. Only penconazole at and below minimum inhibitory concentrations caused the accumulations of compounds in P. ultimum, which were eluted in the sterol range during high performance liquid chromatography. Both fungicides caused the accumulations of lanosterol as well as ergosterol in P. omnivorum. The importance of the reduction in ergosterol in triazole-treated fungi as a component of growth inhibition might be questioned in view of the results with P. omnivorum...
Anciso, Juan Reymundo (1989). Biochemical site(s) of growth inhibition in Phymatotrichum omnivorum and Pythium ultimum by triazoles [fungicides]. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1016980.