Received for publication 6 Apr. 1987. Paper no. 2795, Massachusetts Agricultural Experiment Station, Univ. of Massachusetts, Amherst. We gratefully acknowledge the valuable assistance of Lionel Jager and Max Hammond in the collection of data and the support and assistance by the Victorian Dept. of Agriculture while D.W.G. was on sabbatical leave at the Horticultural Research Institute, Knoxfield. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.