Some Effects of 2,4‐Dichlorophenoxyacetic Acid, its Salts, and Esters on Several Physiologic Strains of the Potato Scab Organism Actinomyces scabies (Thaxt.) Guss.

SYNTHETIC plant hormones have been used extensively since 1941 in the control and elimination of weeds. Since much of these chemicals used as herbicides probably reaches the ground, it is desirable to know the effect of these synthetic plant hormones on the soil flora. According to Stevenson and Mitchell (8) 0.02% 2,4-D, either as acid or as sodium salt, in potato dextrose agar had an inhibitory effect on three different species of bacteria, but 0.08% 2,4-D had no apparent effect on three fungi—Fusarium sp., Penicillium sp., and Penicillium notatum. Lewis and Hamner (4) found that Rhizobium leguminosarum Frank was not affected by any of the 2,4—D samples used even at concentrations as high as 1,000 ppm at pH 6.8. Worth and McCabe (9) demontrated that several species of Rhizobium are inhibited by 2,4-D at concentrations as low as 20 ppm at pH 7.4. Carlyle and Thorpe (2) found that different species of Rhizobium differed in their sensitivity, but none was seriously inhibited in sand cultures by concentrations equivalent to less than 200 pounds of the sodium salt per acre. These reports suggest that 2,4-dichlorophenoxyacetic acid and similar chemicals may have effects on Actinomyces scabies, an organism closely related to the bacteria. In our efforts to improve crop plants we are constantly on the lookout for new practices and new chemicals. Probably the best protection we have at present for the control of the common scab of potato is the use of resistant varieties adapted to local areas. That there are numerous physiologic races of the potato scab organism, Actinomyces scabies (Thaxt.) Guss., is well known and also that these races differ in their ability to scab different potato varieties (7). New races are apparently being formed due to changes in environment and variations in heredity. Some of these new races are nonpathogenic and others are virulent toward certain varieties of potato. Therefore cheap chemical control of potato scab applicable to all or at least to extensive and diverse potato-growing areas is highly desirable. Many attempts have been made to control common scab with chemicals. Seed and soil treatment have not been entirely satisfactory. Cairns, et al. (1) obtained satisfactory control by disinfection of the affected seed tubers before planting // a sufficient interval had elapsed between the growing of successive potato crops on the same land. Mercury salts are effective in killing the scab organism on the seed piece but not in killing the organism in the soil. Under Long Island conditions Cunningham and Wessels (3) found that 4 pounds either of yellow oxide of mercury or of calomel added to each ton of 5-8-5 fertilizer mixture on soils having a pH of 5.5 or lower reduced scab. Schaal (6) found that scab was not controlled on sandy alkaline soil of northern Colorado with mercuric chloride, yellow oxide of mercury, sulfamic acid, aluminum sulfate, potassium iodide, or sulphur.