1. NSW Department of Primary Industries, Tamworth Agricultural Institute, RMB 944 Tamworth, NSW 2340, Australia.
2. Corresponding author. Phone: 61 267 631 119. Fax: 61 267 631 222. E-mail: mike.sissons@agric.nsw.gov.au
3. CSIRO Plant Industry, GPO Box 1600, Canberra ACT 2600, Australia.