1. CSIRO Division of Plant Industry, GPO Box 1600, Canberra, ACT 2601 Australia.
2. CSIRO Division of Plant Industry, North Ryde, NSW 2113 Australia.
3. Corresponding author. E-mail: mail: J.Skerritt@pi.csiro.au Fax + 61 2 6246 5351/5000. Phone: + 61 2 6246 5350.