Date of Award
Master of Science
Entomology and Plant Pathology
Bonnie H. Ownley
Ernest Bernard, Robert Miller, Mark Windham
Target spot is a leaf spot disease of tobacco initiated by basidiospores of Thanatephorus cucumeris, the teleomorph stage of Rhizoctonia solani. Symptoms of target spot begin as small, circular, water-soaked spots about 2-3 mm in diameter. Under conditions of high relative humidity and moderate temperature, lesions enlarge rapidly, becoming light green and almost transparent, with irregular margins and chlorotic halos. Sore shin is caused by R. solani or T. cucumeris. The disease usually appears as a brown discoloration of the stem near or below the soil line. The stalk of an affected plant becomes brittle and may break under pressure.
The focus of this research was biological control of target spot and sore shin of tobacco seedlings. Recent studies have shown that applications of antagonistic bacteria can be an effective control measure against target spot.
The first goal of the present study was to optimize the biocontrol activity of Bacillus sp. BA55 against T. cucumeris andR. solani on tobacco by manipulating cultural conditions for growth of the bacterium. The specific objectives were: 1) to determine the effect of culture medium on biocontrol activity of Bacillus sp. BA55 against target spot and sore shin; 2) to determine the effect of time period between foliar application of Bacillus sp. BA55 and introduction of R. solani on severity of target spot and sore shin, and; 3) to determine the effect of solid and liquid minimal 3C medium (Min 3C) and cultural incubation temperature on biocontrol activity of BA55 against target spot and sore shin.
The effect of culture medium and time of introduction of R. solani on biocontrol activity of BA55 was examined by applying foliar sprays of BA55 cultured in either Min 3C broth or nutrient broth yeast extract (NBY). An untreated control was included. The foliar treatments were applied to 8-week-old plants of TN 86 tobacco one day or six days before R. solani was added. In trial 1, disease rating was significantly lower in plants that received the foliar treatment of BA55 cultured in Min 3C compared to plants that did not receive any treatment. Plants treated with BA55 cultured in NBY were intermediate in disease rating. In trial 2, the effect of culture medium was not significant; however, plants that were inoculated with R. solani six days after foliar treatments with BA55 had a significantly lower disease rating and higher shoot fresh weight than plants inoculated one day after foliar treatment.
The effects of foliar treatment with BA55 cultured on either solid or liquid Min 3C medium at 25° or 30°C, also was examined. Foliar treatment with BA55 did not affect disease rating; however, shoot fresh weight of plants treated with BA55 cultured in solid or liquid Min 3C at 2S°C was significantly greater than the untreated control. Disease pressure was low in this experiment and probably contributed to the lack of significant differences in disease rating.
The second goal of the present study was to reduce target spot and sore shin with biological seed treatments and optimized foliar sprays of BA55. The specific objectives were: 1) to determine the effect of BA55 seed treatment and concentration of BA55 in foliar spray treatments on target spot and sore shin of tobacco seedlings in the float bed system; 2) to determine the effect of Trichoderma seed treatment and foliar sprays of Bacillus BA55 on target spot and sore shin of tobacco seedlings in the float bed system; and 3) to determine the effect of seed treatment with BA55 or Trichoderma on sore shin.
The rate of foliar application of BA55 had a significant effect on fresh weight in one of two trials. In an experiment to test the effect of BA55 seed treatment and various concentrations of BA55 in foliar sprays, TN 90 tobacco seedlings receiving 6 log or 7 log colony-forming units (cfu) per ml had significantly greater fresh weight than untreated controls. Also, TN 90 seedlings from seed treated with BA55 had greater fresh weight than seedlings from untreated seed.
In an experiment to test the effect of a fungal agent, Trichoderma 22-3, and foliar application of BA55 on severity of target spot and sore shin, KY 14 tobacco plants from seed treated with Trichoderma 22-3 had a significantly greater fresh weight than plants from untreated seed. In the second trial of this experiment, two cultivars (KY 14 and TN 90) were included. For KY 14, there was a significant interaction between seed treatment and foliar spray application on percent diseased foliage. There was no effect of seed treatment or foliar spray on TN 90 plants.
The effect of BA55 and Trichoderma seed treatments on biocontrol of sore shin on two burley cultivars was examined also. The seed treatments for KY 14 tobacco were untreated, Trichoderma 22-3 and Trichoderma 22-4. The seed treatments for TN 90 tobacco were untreated, Trichoderma 22-1, Trichoderma Test, Trichoderma 22-2. Also, TN 90 seed was treated with Bacillus sp. BA55, and a separate control for this treatment was included. The effect of seed treatment was not significant for sore shin disease rating or shoot height of KY 14. For TN 90, plants from seed treated with Trichoderma 22-2 had a significantly greater shoot height than the control, but there was no effect on disease rating. Treatment of seed with BA55 had no effect on disease rating or shoot height of TN 90 plants.
McMahan, Joseph P., "Biological control of target spot and sore shin of tobacco seedlings with biological seed treatments and foliar sprays. " Master's Thesis, University of Tennessee, 1996.