SOILS SUPPRESSIVE TO BLACK ROOT-ROT OF BURLEY TOBACCO, CAUSED BY THIELAVIOPSIS-BASICOLA

Soils suppressive to black root rot were detected in fields in which the pathogen was present but in which little or no disease had developed on susceptible cultivars of burley tobacco. Suppressive soils were characterized by low base saturation, low calcium, exchangeable aluminum levels of 1 meq/100 g of soil or higher, and soil pH less than 5. Suppressiveness was confirmed under controlled environmental conditions with fumigated field soils reinfested with Thielaviopsis basicola. Isolates of T. basicola from suppressive soils caused black root rot when placed in conducive soil, indicating that the absence of disease was not due to differences in pathogen virulence. The mechanism of suppression was not biotic; autoclaving soil had no effect on suppressiveness, and transfer of suppressive soil to conducive soil (fumigated soil; 1:9, by volume) did not induce suppressiveness. Soil calcium level was not the mechanism of suppression because amending suppressive soils with calcium hydroxide nullified suppressiveness and amending soils with calcium sulfate did not. Disease developed in acidified conducive soil only if exchangeable aluminum levels were low. The acidification treatments had no effect on the inoculum density of the fungus, and the survival of the chlamydospores of T. basicola was not affected by the soil or soil treatments. Mechanisms of soil suppression to black root rot on burley tobacco are abiotic and dependent on the interrelationships among soil pH, base saturation, and exchangeable aluminum.