Evidence that toxins are not the causal factors of the specific apple replant disease*

Abstract

SUMMARYExperiments were made to test the hypothesis that a toxin produced by or from apple roots is the causal factor of specific apple replant disease. In sand and vermiculite culture, apple leachates were not harmful to the growth of apples. In the field, apples exerted no effects on neighbouring apples other than competition effects. In pot and field experiments, the addition of apple root residues to fresh (unused) or fumigated soil depressed the growth of both apple and cherry test plants; similar additions to used (‘replant’) apple soil, however, improved the growth of apple, but not of cherry, test plants. It is concluded that specific apple replant disease is not caused by a toxin derived from root residues. It is suggested that the cause may rather be a rhizosphere microorganism.