Development of black root rot on burley tobacco as influenced by inoculum density of Thielaviopsis basicola, host resistance, and soil chemistry.
Twelve fields infested with T. basicola, selected from 80 surveyed the previous year, were planted with cultivars of burley tobacco with either low (B21 × Ky 10) or moderate (Ky 14) resistance to black root rot. One week after transplantation, soil samples were taken around 20 individual plants in each field to determine the inoculum density of the pathogen and to conduct soil chemical analyses. Severity of black root rot, evaluated on each of the same plants 6 weeks later, depended on inoculum density, level of host resistance, and soil chemistry. Inoculum densities of <50 c.f.u./g of soil resulted in severe root rot on 52% of the plants with low resistance but on only 3% of the plants with moderate resistance. At high inoculum densities, however, levels of disease on the 2 cultivars were similar. Black root rot did not develop on plants of either cultivar in 3 of the 6 fields with an average pH of <5.2. These soils appeared to suppress disease development, since the pathogen was present at densities sufficient to cause disease (>50 c.f.u./g of soil). Black root rot developed, however, in soils with a pH of <5.2 if the base saturation was >70%. The suppressive effect was confirmed under controlled experimental conditions with fumigated field soils reinfested with T. basicola and was reversed by altering soil chemistry with calcium hydroxide.