PLANT-GROWTH AND YIELD OF RED RASPBERRY FOLLOWING PRIMOCANE DEFOLIATION

Red raspberry (Rubus idaeus L. cv. Willamette) primocanes were artific ially defoliated to various degrees and at two dates in each of 2 year s to simulate defoliation caused by the twospotted spider mite (Tetran ychus urticae Koch). The effect on primocane diameter, lateral length, yield and four yield components was determined. When defoliation occu rred evenly dong the length of the primocanes, and all floricanes and excess primocanes were removed in early Aug, 1989, yield was reduced 2 6% at 25%, 50%, and 75% defoliation and 55% at 100% defoliation compar ed with nondefoliated controls. The number of canes per stool, number of fruit per lateral, and weight per fruit were reduced when defoliati on occurred earlier, in August rather than September, but the number o f laterals per cane increased with early defoliation. The effect of in creasing defoliation on plant growth and yield was generally nonlinear . When defoliation occurred in sections along the lower 2 m of all pri mocanes, and all floricanes and excess primocanes mere removed in Nov. 1992, no significant differences in yield or three yield components w ere detected. The effects of primocane defoliation are not necessarily predictable, so T. urticae should be controlled before mite-induced d efoliation occurs.