The effect of soil factors on the incidence and severity of black root rot of burley tobacco, caused by Thielaviopsis basicola

Populations of Thielaviopsis basicola and occurrence of black root rot on burley tobacco were determined in a survey of 80 fields in western North Carolina. The pathogen was detected in 59% of the fields at a mean population of 40 cfu gSoil surrounding 20 plants per field in 12 survey fields was sampled for inoculum density and chemical analyses. At inoculum densities less than 50 cfu/g soil, severe root rot was observed on 52% of the plants with low resistance but on only 3% of the plants with moderate resistance. Disease developed where the soil pH was 5.6 or greater but did not develop where the pH was less than 5.2 unless the base saturation was greater than 70%. Soils apparently suppressive to black root rot were identified. Suppressiveness was confirmed with fumigated soils reinfested with T. basicola.Suppressive soils were characterized by low base saturation, low Ca, exchangeable Al levels of 1 meq/100g soil and a pH near 5. Autoclaving soil or transfer of 10% suppressive soil to conducive soil did not induce suppressiveness. Amending suppressive soils with Ca(OH)...