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Physiology and Biochemistry

Mechanism to Tolerance of Pythium Species to Ethazol. Ponciano M. Halos, Former Graduate Student, Department of Plant Pathology, University of California, Berkeley 94720, Present address of senior author: Department of Plant Pathology, University of the Philippines, Los Banos, Laguna, Philippines; O. C. Huisman, Assistant Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 66:152-157. Accepted for publication 19 August 1975. DOI: 10.1094/Phyto-66-152.

Pythium sylvaticum, P. vexans, P. ultimum, and P. debaryanum became tolerant to fungistatic activity of ethazol after a time proportional to the initial concentration. Mycelial extracts from nontreated mycelium or menadione (2-methyl-1,4-naphthoquinone) partially relieved ethazol inhibition of growth and respiration. The occurrence of an antimycin A-insensitive pathway in the presence of menadione was demonstrated in P. debaryanum. The compounds present in extracts of nontreated mycelium that reversed ethazol inhibition were purified by gel filtration, partitioning in petroleum ether, and ion exchange fractionation. The two active fractions developed as ultraviolet-fluorescent spots on thin-layer chromatograms. Based on ultraviolet absorption spectra and color reactions with neotetrazolium chloride, the compounds that reversed ethazol inhibition were tentatively identified as ubiquinone derivatives. These purified ubiquinones reversed ethazol toxicity in bioassays with P. ultimum. The tolerance by Pythium spp. probably depends on increased use of an alternate pathway of electron transport mediated by ubiquinone. This circumvents electron transfer through the ethazol-sensitive site in the terminal respiratory chain.

Additional keywords: antimycin A-tolerant respiration, fungicide, polarographic measurement.