Abstract
The object of this experiment has been to determine the effect of simulated biological or enviromental damage at different timies of the growing season on the vegetative and reproductive growth of apple. One method that has been used is to inhibit photosynthesis by applying terbacil a PS II inhibitor at different times during the season to different varieties of apple (Delicous, Empire and Gala) for several years (1993-1996). The degree of inhibition was dose and time dependent. We chose a concentration (50-63 ppm) that inhibited photosynthesis 50-80% within 24 hours of appliction, that recovered fully within 7-10 days. Photosynthesis was measured with an open gas exchange system, or by measuring Fv/Fm with a chlorophyll flourescence system. No differences were found in total terminal shoot growth, cold hardiness, soluble solid concentration, fruit firmness, density of the fruit or fruit color. However, when terbacil was applied at 15 and 30 days after full bloom it induced fruit abscission which was markedly higher for trees with a higher crop load. Yield was reduced in high crop load trees but not low crop load trees. The earlier the simulated stress the greater the effect on the current seasons crop. Yield and size was only affected if the simulated stress occurred early in the season on high crop load trees, or near harvest on high crop load stress. The significance of these findings in relation to pest control will be discussed.
Published Version
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