Thesis (Ph. D.)--University of Wisconsin--Madison, 1926. / Typescript. With this is bound: Histological studies of resistance in tobacco to Thielavia basicola / George H. Conant. Reprinted from American journal of botany, vol. 14, no. 8 (Oct. 1927), p. 457-480, plates LVII-LXII. Includes bibliographical references (leaves 41-45).

Thielaviopsis basicola (Berk. and Br.) is a widespread, soil-borne, pathogenic fungus which causes a root rot on tobacco (Nicotiana tabacum L.). Presently, control is achieved primarily through the use of resistant cultivars of tobacco or chemicals. In order to develop a disease prediction program, disease-inoculum density relationships must be determined. It was the purpose of this study to develop an improved procedure for estimating the populations of T. basicola in naturally infested soil. A second objective was to determine the horizontal inoculum pattern of T. basicola in tobacco field soil, and the third objective was to determine the relationship between black root rot of tobacco and inoculum density of T. basicola in naturally infested soil. Two fields in southwestern Virginia were randomly and systematically sampled to assay the population density of T. basicola in the soil. Populations of T. basicola were isolated using a modified carrot-disc baiting technique and calculated using a colonization correction factor. Frequencies of calculated population densities of T. basicola in soil samples were compared, using a Chi-square test, to the expected frequencies indicated by the Poisson, Neyman Type A, and negative binomial distributions. Temperature-tank studies were conducted to investigate inoculum density-disease relationships. Two cultivars of tobacco were grown in soil naturally infested with T. basicola. The soil was thoroughly mixed before use to preclude inoculum clumping. The results obtained with either systematic or random sampling indicated that propagules of T. basicola are clumped or aggregated in tobacco field soil, as there was a good fit of the data to the negative binomial distribution. Also, the variance to mean ratios were significantly (P = 0.05) greater than 1.0 which is indicative of a clumped pattern. Other indexes of aggregation indicated a low to moderate degree of inoculum clumping in soil. For soil temperature-tank tests, root rot incidence was 96-100% at all natural population densities for both slightly resistant cv. Burley 21 and susceptible cv. Judy's Pride plants, and T. basicola was consistently associated with rotted roots. Low R² values were obtained in regression analyses of T. basicola inoculum density versus mean percent root rot per Burley 21 plant and per Judy's Pride plant. / Master of Science

A new selective medium (TB-CEN) was developed for isolating Thielaviopsis basicola, cause of black root rot of tobacco, from soil. TB-CEN medium contains etridiazol and nystatin to inhibit the growth of undesired fungi, and unautoclaved extract from carrot to selectively enhance for the growth of T. basicola. Inoculum and/or population densities of T. basicola in five burley tobacco fields were 74-166 propagules per g of soil, and 0-12 propagules per g of soil in three other burley fields. Inoculum and/or population densities of T. basicola in 12 flue-cured and 2 sun-cured tobacco fields were 0-26 propagules per g of soil, and 101 and 402 propagules per g of soil in two other flue-cured fields. Environmental factors apparently had a strong effect on black root rot development, since root rot and plant stunting were severe in two burley fields that had 148 and 158 propagules per g of soil, but were not severe in the two flue-cured fields that had 101 and 402 propagules per g of soil. All of the cultivars planted in the four fields were susceptible. Black root rot was the major disease associated with the stunting of tobacco plants in the burley region of Virginia, but not in the flue- and sun-cured regions. No evidence was found to indicate that endomycorrhizae were involved in tobacco stunting in Virginia. T basicola inoculum density-disease severity studies were conducted both in soil-temperature tanks and in the field. Tobacco seedlings were grown in temperature tanks (20-23 C) for 30-31 days in naturally infested field soil (pH 6.5). For all cultivars tested (Burley 21, NC95, and Va Gold), the mean percent of roots that were rotted increased significantly (P=0.001) as inoculum density increased (R² range for regressions=0.93-0.97). Severe levels of root rot occurred at inoculum densities of 50-200 propagules per g of soil. Significant (P=0.01) reductions in plant growth occurred at inoculum densities as low as 5-10 propagules per g of soil. In a study conducted on a commercial burley tobacco (cv. B21-Ky10) field, inoculum densities of 150 and 683 propagules per g of soil were associated with moderate and severe levels of black root rot, respectively. Differences between soil-temperature tank and field studies appeared to be due to variations in environmental- and host-related factors. In another burley field study, the fungicide imazalil, which completely inhibited the growth of T. basicola when amended into agar media at a concentration of 1.0 μg a.i./ml, failed to control black root rot when it was added to transplanting water (50 ml/plant) at concentrations as high as 1,500 μg a.i./ml. / Ph. D. / incomplete_metadata