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Growth of Ceratocystis ulmi on Low Concentrations of Hydrochloride and Phosphate Salts of Methyl 2-Benzimidazolecarbamate and on Thiabendazole Hypophosphite. L. R. Schreiber, Supervisory plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Nursery Crops Research Laboratory, Delaware, OH 43015; G. F. Gregory, principal plant pathologist, Forest Service, U.S. Department of Agriculture, Northeastern Forest Experiment Station, Delaware, OH 43015. Phytopathology 71:1261-1263. Accepted for publication 22 April 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981. DOI: 10.1094/Phyto-71-1261.

We compared the growth of 27 isolates of the Dutch elm disease (DED) fungus, Ceratocystis ulmi, on potato-dextrose agar amended with MBC-HCl (methyl 2-benzimidazolecarbamate hydrochloride), Lignasan BLP (methyl 2-benzimidazolecarbamate phosphate) (MBC-H3PO4), and Arbotect 20-S (2-(4-thiazolyl) benzimidazole hypophosphite) (thiabendazole hypophosphite) at 0.1, 0.3, 0.9, and 2.7 μg/ml of the acid salts and at 0.3 and 0.9 μg/ml of the organic moieties. Increase in colony diameter of the isolates varied significantly within each treatment as well as between fungitoxicants. MBC-HCl was the most fungitoxic chemical followed by MBC-H3PO4 and thiabendazole hypophosphite based upon concentrations of acid salts. In comparisons of the organic moieties, MBC-HCl and MBC-H3PO4 were equally fungitoxic and both were more fungitoxic than thiabendazole hypophosphite. All isolates (except one) that were tolerant to one fungicide also were tolerant to the others; the exceptional one was more sensitive to thiabendazole hypophosphite than to fungicides containing MBC. Thiabendazole hypophosphite was not only generally less fungitoxic at low concentrations than were the MBC compounds, but the colony diameters of some isolates even increased above that of the controls.

Additional keywords: chemotherapy, tolerance, benzimidazole, carbendazim.