Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Bonnie H. Ownley

Committee Members

Brad Reddock, Charles Hadden, Dennis Onks

Abstract

Tobacco is an important crop in Tennessee and the Southeastern United States. Farmers received $34 million in revenue in 1991 from dark tobacco alone. Increasingly, tobacco seedlings are produced in float beds in the greenhouse. Float beds have several advantages over conventional seed beds; however, conditions in the greenhouse are often conducive to the spread of plant disease. Target spot (Rhizoctonia leaf spot) is a recently-described disease of float-grown tobacco and is caused by the soilborne fungus Thanatephorus cucumeris (anamorph Rhizoctonia solani). In Tennessee, significant losses in tobacco transplants grown in float systems have resulted from target spot. There also has been an increase in the number of reports of target spot on field plants, which may be related to the increase in production of transplants by the float system. Currently there are no control measures recommended for target spot on float plants except for proper sanitation and cultural practices. In this study the effect of four chemicals and four biological control agents for control of target spot on two dark-fire tobacco cultivars (DF485 and TR Madole), planted by direct-seeding or seeding-and-transfering, was evaluated. In 1993, one biological control agent, Bacillus sp. strain BA55, and two fungicides, fluazinam, and Dithane®, applied as foliar sprays, provided substantial control of target spot on cultivars DF485 and TR Madole, across both planting methods. In 1994, several biologicals and chemicals were tested for control of target spot on direct-seeded plants of cultivars DF485 and TR Madole. Three fungicides, two formulations of fluazinam, and Dithane® significantly reduced target spot severity on both cultivars. The fungicide dimethomorph and the biological Bacillus sp. BA55 provided control only on one cultivar, DF485. Another fungicide, Rovral, also provided control of target spot; however, phytotoxicity was observed also in both years.

In 1993, direct-seeded plants of both dark-fire cultivars treated with BA55 and fluazinam, or receiving no treatment, were transplanted to the field and evaluated for further development of target spot, and the effect of disease control treatment on crop index, grade index, yield, and revenue. Environmental conditions were not conducive for further target spot development in 1993. In general, cultivar DF485 yielded more pounds per acre than TR Madole. Across both cultivars, there were no differences in crop index, grade index, yield, and revenue in plants treated with BA55 or fluazinam, or untreated.

The effects of host age, cultivar, and BA55 on target spot severity was studied also. Across five age groups, ranging from 2 to 6 wk, BA55 significantly controlled target spot. The seedlings treated at two weeks had significantly less target spot than the other age groups.

The effect of fluazinam concentration on target spot severity was studied also. Different concentrations of fluazinam, ranging from 200 to 1000 ppm, were applied weekly as foliar sprays to seed-and-transfer tobacco seedlings. Fluazinam concentration had no effect on percent diseased foliage due to low disease pressure.

Based on the results from these experiments, fluazinam and Dithane® are potential candidates for control of target spot in float beds. The biological agent, BA55, has potential as for controlling target spot if the consistency of its performance can be improved.

A leaf disk assay was designed to prescreen bacteria for biocontrol of target spot. One hundred forty-three bacterial isolates from long-term tobacco field soils were tested in this assay. Inoculum of Rhizoctonia solani was placed on sterile, moistened Terralite™ horticultural mix in multi-well tissue culture plates. Disks (12 mm) were cut with a cork borer from newly emerged tobacco leaves, surface-sterilized, dipped into bacterial suspensions, and placed in the tissue culture wells. The prescreening assay was optimized for several parameters including form and age of R. solani inoculum, and length of assay incubation time. Using Bacillus sp. BA55 as a test organism, growth medium and incubation time for the bacterial test strains were optimized also. The optimized assay consisted of oat inoculum of Rhizoctonia solani placed on the surface of the Terralite™, and incubated for 1 day. Bacillus sp. BA55 was grown for 2 days in Nutrient Broth Yeast Extract (NBY). The 143 bacteria isolated from tobacco field soils at Greeneville, TN, were evaluated for their ability to inhibit colonization of R. solani on tobacco leaf disks. Bacillus isolates BA22, BA27, BA93, BA122, and BA139 significantly reduced colonization of the leaf disks by R. solani compared to a sterile Tween-20 control. All isolates were tested also with in vitro antibiosis assays against Geotrichum candidum, a nonpathogenic soil fungus, and Rhizoctonia solani. Bacteria were spotted onto Potato Dextrose Agar (PDA) and NBY plates, and oversprayed with G. candidum. Five isolates (BA27, BA29, BA57, BA83, and BA125) produced inhibition zones against G. candidum ≥ 5 mm on PDA while 12 isolates (BA8, BA27, BA28, BA29, BA40, BA44, BA91, BA95, BA96, BA118, BA120, BA121) produced similar results on NBY. In the inhibition assay against R. solani, each bacterium was streaked also across the center of three NBY plates, and two agar plugs of 2-wk-old R. solani were placed on either side of the streak. Eight isolates (BA4, BAB, BA19, BA27, BA29, BA58, BA96, BA111) substantially inhibited R. solani (inhibition zone ≥ 5 mm).

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