Masters Theses

Date of Award

3-1979

Degree Type

Thesis

Degree Name

Master of Science

Major

Agricultural Biology

Major Professor

James W. Hilty

Committee Members

Charles Hadden, L. F. Johnson

Abstract

Brown spot, caused by Septoria glycines Hemmi, is one of the most destructive diseases of soybeans in Tennessee. The objectives of this study were to (a) evaluate the effect of temperature, humidity, precipitation, and age of the host plant upon infection of soybean leaves by S. glycines and (b) determine the resistance of eight soybean cultivars to the pathogen. The optimum temperature for linear growth and sporulation of S. glycines on potato-dextrose agar and V-8 juice agar was 24 to 28 C. Czapek-Dox agar was not a satisfactory medium for growth and sporulation. Atomizing a suspension of S. glycines pycnidiospores upon the soybean leaves was the most effective method of inoculation in controlled-temperature chambers. Distributing a spore or mycelial suspension over the leaves with a glass rod did not result in brown spot symptoms. Disease incidence was low in all experiments with the effect of temperature on infection. There was no significant difference among the four temperatures. Disease incidence of soybeans inoculated at 29 C day - 25 C night was significantly higher in a growth chamber containing a cool-moisture humidifier than in a chamber with ambient humidity. Brown spot symptoms of soybeans in the field were rated weekly during the summer of 1978. The two major increases in symptoms occurred in late June and in late August and September. Each increase occurred 10 to 18 days after a period with relative humidity of 90% or higher for at least 18 hours and two consecutive days with one inch or more of precipitation. Monitoring spore release with a Kramer-Collins spore sampler was of little use in predicting an increase in symptoms, as only four S. glycines pycnidiospores were collected in 1741 hours of sampling. There were no significant differences in disease severity in eight soybean cultivars inoculated in the greenhouse. However, significant differences occurred among disease severity in the same eight cultivars when detached leaves were inoculated. Brown spot symptoms developed on detached leaves of Essex, Lee 74, and York, but did not develop on Bedford, Centennial, Dare, Forrest, or Pickett 71. Additional monitoring of brown spot symptoms in the field is necessary to confirm the effect of temperature, precipitation, and humidity on the development of brown spot. Results of detached leaf inoculations should be compared with field inoculations to assess the reliability of this method of determining resistance to brown spot.

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