FACTORS AFFECTING THE VERTICAL DISTRIBUTION OF RHIZOCTONIA SOLANI WITH SPECIAL REFERENCE TO CO2 CONCENTRATION

DURBIN, RICHARD D. (U. Minnesota, St. Paul.) Factors affecting the vertical distribution of Rhizoctonia solani, with special reference to C02 concentration. Amer. Jour. Bot. 46(1): 22-25. Illus. 1959.-Thirty-three heterogeneous clones of 3 ecological types of Rhizoctonia solani (aerial, surface, and subterranean, based on their vertical distribution on the host plant) were grown in flowing atmospheres containing various levels of 'CO2. The linear growth rates were compared with those obtained when the clones were grown in normal air. Significant differences were found among the clones with respect to their tolerance of CO2. Those found normally in the soil were more tolerant of CO2 than were clones occurring in either surface or aerial environments. Additional experiments with other plant pathogenic soil fungi, plus data obtained from the literature, indicate that tolerance of C02 is a common attribute of fungi normally found in the soil and in rotting organic matter, environments where high C02 concentrations are to be expected. Significant differences were also found in the linear growth rates of 86 clones from the 3 types. The ratio of the mean growth rates for aerial, surface, and subterranean clones was about 4:3:2, respectively. The 2 characteristics, linear growth rate and CO2 tolerance, were negatively correlated (r = +.773). The percentage of the aerial, surface, and subterranean types producing sclerotia was 82, 48, and 21, respectively. In 9 clones tested linear growth rate was not found to be correlated with endogenous 02 uptake rate. Carbon dioxide tolerance, linear growth rate, and sclerotial formation are characteristics which seem to play a part in determining the habitat of clones of this fungus. A NUMBER of hosts are attacked at different vertical levels by the fungus Rhizoctonia solani Kuhn. For example, on alfalfa a foliar blight (Allison and Wells, 1951), crown rot (Cherewick, 1948), and root rot (Smith, 1943) have been reported as being caused by different clones of this fungus. The reasons for this difference in vertical distribution on any one host are not fully known. The gaseous environment, especially CO2 content, has previously been reported as an influence on the vertical distribution of soil fungi (Burges and Fenton, 1953; Durbin, 1955). Past studies have also shown that CO2 affects the growth rate o,f fungi (Foster, 1949; Thacker and Good, 1952; Tomkins, 1932). Furthermore, its concentration may increase several hundred-fold when going from an aerial to subterranean environment. In light of these facts, a study wTas carried out using a large number of diverse clones of R. solani to determine whether a correlation existed between tolerance of CO2 and habitat. During the course of experimentation, several additional phenomena were noted which, it is felt, may also influence the vertical distribution of clones of this fungus. MATERIALS AND METHODS.-The clones used in this study represent isolates from a variety of hosts and a wide geographical area of the United States (table 1) .2 A total of 86 clones was arbitrarily divided into 3 types, depending upon the position 1 Received for publication June 19, 1958. A portion of a dissertation presented to the Graduate Division of the University of California, Berkeley, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The assistance of K. F. Baker, B. L. Johnson, W. C. Snyder, and R. E. Young is gratefully acknowledged. on the host plant from which they had originally been isolated. The aerial type consisted of clones which attacked the leaves and twigs 3 or more in. above the soil level. The surface type contained clones attacking plants at or near the crown, while the subterranean-type clones parasitized roots 4-24 in. beneath the soil surface. All cultures were grown on Czapek's medium with the following composition per liter: NaNO3, 2.0 g.; KH2PO4, 1.0 g.; MgSO4. 7H20M 0.5 g.; KCI, 0.5 g.; FeSO4, 0.01 g.; glucose, 20 g.; and agar, 20 g. The pH of the medium was 4.9. A constant temperature of 250 ? 1?C. was maintained in a 500 cu. ft. walkin box used in all experiments. A 12-hr. day length controlled by a time clock was provided by Sylvania "cool white standard" fluorescent lights at 200 ft.-c. The effect of different concentrations of CO2 was determined by measuring the linear growth rates in mm./24 hr. of the different clones in U-shaped glass growth tubes through which a continuous stream of gas was passed. Commercial cylinders of 02, C02, and water-pumped "breathing air" were used as sources for the artificial atmospheres. The individual gases were first metered by passing them through calibrated manometers. An exact flow rate for each of the 3 gases was obtained by adjusting a needle valve in each line just preceding the manometer. Small C02 flow rates from 1.0 to 80.0 ml./min. were maintained by using a float-type flowmeter in place of the manometer. The gases were mixed and again passed through a manometer 2 Soine of the clones used in this study were provided by L. H. Davis, B. B. Exner, F. I. Frosheiser, R. G. Grogan, E. S. Luttrell, G. F. Weber, and J. C. Wells.