Growth of Some Citrus-Fruit Pathogens as Affected by 2,4-D and 2,4,5-T

Abstract

1. The plant growth-regulators 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were tested in vitro on nine species of citrus-fruit pathogens to determine the degree of inhibition of growth by concentrations of from 5 x 10-6 M to 5 x 10-3 M. 2. The species tested were Alternaria citri El. and Pierce, Botrytis cinerea Pers. ex Fr., Diaporthe citri Wolf, Diplodia natalensis P. Evans, Oospora citri-aurantii Ferr., Penicillium digitatum Sacc., P. italicum Wehmer, Sclerotinia sclerotiorum (Lib.) d By., and Trichoderma lignorum (Tode) Harz. 3. The highest concentration of both growth-regulators significantly retarded growth in all species except P. italicum. 4. In no species, however, was growth completely inhibited, nor was there any instance of a significant stimulation at lower concentrations. 5. The degree of toxicity exhibited by 2,4-D and 2,4,5-T toward the fungi appears to be entirely inadequate to explain the control of Alternaria rot of stored lemons in southern California or of Diplodia rot of oranges in South Africa.