THE ADVERSE INFLUENCE OF PHYTOTOXIC SUBSTANCES FROM DECOMPOSING PLANT RESIDUES ON RESISTANCE OF TOBACCO TO BLACK ROOT ROT

Abstract

Exposure of tobacco plants to phytotoxic solutions obtained from decomposing rye and timothy residues increased their susceptibility to black root rot caused by Thielaviopsis basicola (Berk. & Br.) Ferraris. In greenhouse experiments 16 tobacco varieties, ranging from susceptible to highly resistant to black root rot in the field, were exposed to rye and timothy toxins and their relative resistance and severity of disease development were determined. Exposure to the toxins resulted in an apparent breakdown of resistance. There were no significant differences among the varieties and the disease was equally severe on roots of the resistant and the susceptible varieties after the toxin treatment. These results might explain in part, at least, the apparent breakdown of resistance that has at times been noted in the field with black root rot resistant varieties. The effect of the toxic compounds appears to be largely on the host rather than on the fungus. The precise biochemical and physiological changes which are induced in the root tissues by the toxins to make them more favorable for fungus invasion and colonization are at present not known. This study suggests, however, that phytotoxins may be important host-conditioning factors in the development of root diseases.