STIMULATORY AND TOXIC EFFECTS OF COPPER SPRAYS ON POWDERY MILDEWS

Abstract

WHILE COPPER iS toxic to many fungi at concentrations above 0.01 per cent and is the most widely used of all chemicals for the control of plant diseases, different fungi vary greatly in their sensitivity to it. At concentrations below 0.001 per cent copper is frequently stimulatory, and Steinberg (1939) regards copper as an essential element for several fungi. The literature on the stimulatory effects of copper at low concentrations for fungi which are grown in culture solutions and on the toxic effect of copper on plant parasitic fungi in general is very extensive and cannot be reviewed here. The writer is not aware of detailed studies of the copper tolerance of powdery mildews, but copper fungicides are successfully used for the control of certain powdery mildews, though sulphur fungicides are generally considered superior (Martin, 1940). The writer (unpublished data) has secured control of several powdery mildews with several copper as well as several sulphur fungicides used as eradicant or protective treatments in field and greenhouse tests. Copper fungicides are especially useful for the control of powdery mildews on such sulphur-sensitive crops as cucurbits. The stimulation of powdery mildews by dosages of copper which are highly toxic to certain other plant disease fungi and even to powdery mildews under certain conditions is, therefore, of interest. MATERIALS AND METHODS.-A strain of Erysiphe polygoni DC. from clover, Trifolium pratense L., and another from bean, Phaseolus vulgaris L. var. Pinto, were maintained on greenhouse plants. Bean plants were inoculated with the fungus or treated with the test fungicide, or both, when the primary, true, unifoliate leaves had reached about two-thirds full size. Inoculation was accomplished by dusting dry spores or spraying a water suspension of spores on the upper and lower surfaces of the leaves. Infection on the primary leaves was recorded within ten days after inoculation. Because of the heavy infection in the controls in these tests, counting of individual colonies was impractical and fungus development was rated on a relative scale of 0 to 10 in which 0 indicated no mildew mycelium apparent to the unaided eye, and 10 indicated that the leaf surface was entirely covered with mycelium and conidia. In germination tests 100 spores were counted for each treatment for each test. Bordeaux mixture containing equal parts of CuSO4-5H2O and CaO in water was prepared by adding 13 per cent hydrated lime to the specified amount of copper sulphate which had first been diluted with water to approximately spray strength, and the concentration of bordeaux is expressed as the percentage by weight of CuSO45H20 in the final spray. The cottonseed oil used was a brand of self-