Effects of infestation with fruit tree red spider mite, Panonychus ulmi (Koch), on the growth and cropping of young fruit trees

Abstract

SUMMARYWhen Brompton plum, and M. IV and Crab C apple rootstocks grown in pots were inoculated with fruit tree red spider mites (Panonychus ulmi (Koch)), shoot extension was decreased by 7–17%, the dry weight of the new shoot by 21–32%, and the dry weight increments of old stem and roots by 12–30%, and 7–50%, respectively, compared with control plants sprayed with the acaricide tetradifon. Some of these effects were found in a field experiment with Brompton plum, but not with potted M. I apple rootstocks. Infestation of fruiting Lord Lambourne apple trees, also in pots, had little effect on vegetative growth but in one experiment modified fruit setting, and decreased the dry‐matter content, but increased the seed content of harvested fruit. Lighter inoculation caused no damage to Brompton plum. Spraying inoculated Brompton plum rootstocks with tetradifon + TEPP in mid‐June and mid‐July prevented damage, and though August sprays did not prevent damage to the new shoot, the decreases in dry weight increments of old stem and roots were only 9–13% and 11–19%, respectively, compared with 21–25% and 45–50% in infested, unsprayed plants.