Pathogenicity, host response and control of Cercospora leaf-spot of sugar beets

On artificial media conidia of Cercospora beticola began to appear in 12 to 20 hours after inoculation, and the optimum production was from 48 to 96 hours after transfer. Germination of the conidium may take place at any point, but more often near a septum and from the basal end of the cell first. Germination of the conidia did not take place on the living leaf in an atmosphere containing less than 90 percent humidity. The humidity of the leaf surface was found to be an average of 3.5 percent higher than the surrounding atmosphere. An air humidity of 75 percent was necessary for any conidial formation, and increasing humidity from 75 to 96 percent produced an increase in conidial length from 100.4µ to 185.2µ. Only a small percentage of the germ tubes growing over the surface of the, leaf penetrated the stomata. Varying the humidity of the atmosphere surrounding the plant did not appear to influence the penetration of the stomata by the germination tubes. Cercospora beticola readily infected seedlings of sugar beet, producing both hypocotyl and cotyledon infection. Twenty-six species of plants, distributed throughout 12 widely separated families were infected under greenhouse conditions by spraying with suspensions of conidia of C. beticola. The finding of 26 plants which could be infected with C. beticola clearly indicates that the organism has a wide host range among the plants of the sugar beet area. Under field conditions six species of plants in six widely separated families have been found infected with a Cercospora sp. morphologically and pathogenically similar to C. beticola. Cercospora beticola was found growing in the field under conditions which indicated that it might at times be a saprophyte. The application of fungicides for the control of Cercospora leaf-spot did not produce a significant increase in the yield of beets and sugar. Statistical analysis of returns from machine blocking indicate significantly greater yields of beets from 20-inch blocking than from either 12 or 24-inch. The percentage of sugar was slightly greater in the beets grown in the 12-inch blocking, but it was not significant; the increase in tonnage of the 20-inch blocking more than compensated for the difference in the percentage of sugar. No significant differences were found in purity. Blocking the sugar beets 20 inches in the row retarded the leaf-spot spread about 10 days as compared with the 12-inch blocking. This retardation of leaf-spot development among the 20-inch blocking reduced the number of dead leaves as much as 16 percent below that of the 12-inch blocking. The use of machinery makes possible a much more rapid and uniform cultivation than is usually given by hand. This makes possible, in turn, a much more effective control of the weeds and a maintenance of a lower humidity among the beet leaves. Thus the same measures which reduce the spread of leaf-spot will also reduce the cost of production and increase the yield in 20-inch blocking