Date of Award:

5-1969

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Department name when degree awarded

Plant Virology

Committee Chair(s)

G. W. Cochran

Committee

G. W. Cochran

Committee

O. S. Cannon

Committee

D. W. Davis

Committee

W. G. Dewey

Committee

B. N. Wadley

Abstract

Bean plants which were most susceptible to tobacco mosaic virus (TMV) were selected from more than 1,000 plant introductions. Thirty-eight percent of the lines tested developed local lesions, an indication of susceptibility to TMV. Six percent were better local lesion hosts than the commonly used bean variety (Pinto). Some lines were nine times more susceptible than the variety Pinto. Selections from P. I. 164353, 165455, 273673, 227379 and 223006 appeared to be the best local lesion hosts. A selection from P. I. 223006 produced lesions which were larger, lighter in color and slower developing than those of any other line tested. This line also produced two very different types of lesions which, when sub-cultured, appeared to be caused by two different virus strains.

The susceptibility of bean plants to TMV was influenced greatly when the plants were grown under different environmental conditions. The plants were six times more susceptible when grown at 300 foot-candles of fluorescent light than when grown at 1200 foot-candles. The bean plants grown at 15 C had a higher peak of susceptibility than plants grown at higher or lower temperatures. These plants remained at their peak of susceptibility for a longer period of time than those that were grown at warmer temperatures. Alterations of the concentrations of mineral nutrients had less effect on the susceptibility than light intensity or growth temperature. Generally the best results were obtained when Hoagland's normal solution was doubled in concentration. A short-term moisture stress of 4-5 days, coupled with 24 hours of darkness before inoculation, seemed to have a significant effect in increasing the susceptibility of bean plants to TMV. The incubation temperature of the inoculated leaves influenced the number, size and time of appearance of the lesions. The optimum temperature for each of these factors appeared to be 30 C. A post-inoculation wilting treatment induced a very significant increase in the number of lesions but did not influence the ultimate size of the lesions.

The vibrating cylindrical surface of an ultrasonic probe was shown to be an efficient tool for the inoculation of TMV. The pressure of the leaf against the probe and the speed of a single passage influenced the number of lesions produced. The optimum pressure was induced by compressing the leaf and a 12.7 mm thick Polyfoam pad to 1.58 mm. The most lesions were obtained when the compressed half-leaf was passed under the ultrasonic probe in one-half of a second. Under these conditions, the ultrasonic method was 88 times more efficient than the finger as an inoculating tool. The ultrasonic method was quicker, easier and completely reproducable from operator to operator. The addition of 15 percent ethyl alcohol to the inoculum gave 30 percent more lesions. Carborundum was the most effective abrasive in increasing infections by TMV with ultrasonic inoculation.

The amount of inoculum used per leaf is more important than generally thought. The difference is not with the slope of the distribution curves but with the efficiency of infections produced per unit volume of inoculum. The efficiency of 1 lambda (0.001 ml) of inoculum per half-leaf was twice that of 10 lambda and 18 times that of 100 lambda. When the above conditions and treatments were combined into an assay procedure the experimental variation from day to day for 2 months varied from 66 to 155 percent of the average.

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