INCIDENCE OF SCAB (VENTURIA INAEQUALIS) IN APPLE AS AFFECTED BY DIFFERENT PLANT GROWTH RETARDANTS

To further investigate the efficacy of prohexadione-Ca on scab control, experiments were carried out on Golden Delicious seedlings and scions grown under greenhouse conditions. The efficacy of prohexadione-Ca against scab was compared with the one of other types of growth retardant. Furthermore, the time required to build up resistance was determined. Four growth retardants (chlormequat chloride, paclobutrazol, prohexadione-Ca, and trinexapac-ethyl) were applied at different concentrations and timings prior to inoculation with Venturia inaequalis. The results showed differential effects of the tested chemicals as related to time of application and to concentration. Chlormequat chloride was unable to control scab under all the tested experimental conditions. Paclobutrazol showed an initial, but transient effect, which is most likely due to its fungicidal properties. Prohexadione-Ca and trinexapac ethyl led to reduced scab incidence starting approximately 15 days after application. It appears likely that the latter two compounds, which are structurally related and have a similar mode of action, are inducing a physiological form of resistance, which is independent of morphological or anatomical effects. Accompanying experiments with prohexadione-Ca on older apple plants also pointed out that some three weeks are required to allow the treated trees to build up resistance against scab.