The role of root exudates in specific apple (Malus x domestica Borkh.) replant disease (SARD)

In an effort to improve our understanding of the specific apple replant disease (SARD), direct and indirect effects of phytohormones and related compounds (abscisic acid, 6-benzyladenine, indole-3-acetic acid, 1-naphthaleneacetic acid, and gibberellins GA(3) or GA(4)) on root exudates of apple seedlings were evaluated as a potential mediating factor in the infection process. In the first type of experiments, radioactively labeled hormonal substances were applied to the stumps of decapitated apple-seedlings and the occurrence of radioactivity in root exudates and their qualitative characterization were examined (direct influence). In another set of experiments, the effects of leaf-applied plant growth regulators on the amount and composition of the predominant organic acids, carbohydrates, and amino acids/amides were studied. Cherry seedlings resistant to SARD and apple seedlings with dormant apical buds and, thus, not susceptible to infection were used for comparison. The results showed no differences in exudation of applied plant growth regulators between growing cherry and apple seedlings. Thus, a direct effect of plant hormones on the infection process is unlikely. However, leaf treatments with growth regulators, in particular with auxin-type compounds and abscisic arid, increased exudation of alditols. This may indicate that plant hormones are, to some degree, indirectly involved in the infection process.