A survey of corn smut based on visible smut galls and conducted near Ames, Iowa, from 1930 to 1934 inclusive, showed 10.6, 9.9, 13.9, 5.5 and 18.3 percent of affected plants.
When the leaf sheaths were stripped from 1,985 plants, exposed to artificial and natural infection in 1934, many small smut galls aggregating 39.3 percent of the total expressed infections were found at the nodes.
The symptoms of corn smut may be classed as exposed and concealed. Exposed symptoms occur as irregular yellow or reddish stripes or blotches, brownish lesions and galls. Concealed symptoms are completely hidden by the leaf sheaths and occur as small nodal galls and minute pustules of chlamydospores in the leaves of the axillary buds.
A decrease in the surface tension of the inoculum increased the infectivity of the medium containing the sporidia. One percent rosin fish oil soap was found to be an effective surface tension reducing agent. The surface tension of carrot filtrate, used as a culture medium, was 47.0 dynes per centimeter; water, 72.3; carrot filtrate with 1 percent fish oil soap, 34.0.
Ninety-two percent of 500 sweet corn plants (Golden Bantam) became infected when inoculated with a sporidial suspension in carrot filtrate plus 1 percent fish oil soap, while 36.6 percent of 90 similar plants became infected when inoculated with a sporidial suspension in carrot filtrate.
Increased bud growth was accompanied by a larger number of nodal infections. Axillary bud growth was stimulated by injury to the corn plant, or inhibition of pollination. Injured or unpollinated inoculated plants produced approximately twice as many smut galls as the checks.
Histological examination of 262 axillary buds from 50 inoculated sweet corn plants (Golden Bantam) showed 53.6 percent or 140 of the buds to be infected with smut mycelium. Mycelium was found in apparently healthy axillary buds 67 days after inoculation.
The percentage of smut infection as indicated by nodal smut galls increased with lateness in planting. An average of data for 2 years showed 12 percent on May 15 plantings while approximately 40 percent of the June 4 plantings were smutted.
The percentage of smut infection as indicated by nodal smut galls showed a tendency to increase both ways from the 2-3 rates of planting. During the years 1931, 1932, 1933 and 934 those plants growing one in a hill showed 40.8, 20.4, 9.0 and 22.8 percent infection, respectively; those at the rate of three per hill 22.5, 13.5, 4.3 and 14.6, while those at the rate of five per hill showed 32.4, 21.6, 5.9 and 18.5 percent infection.
Davis, Glen N.
"Some of the factors influencing the infection and pathogenicity of Ustilago Zeae (Beckm.) Unger on Zea Mays L.,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 17
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol17/iss199/1