Impact of foliar herbicides on pod and seed behaviour of rust-infected rubber vine (Cryptostegia grandiflora) plants
F.F. Bebawi, P.L. Jeffrey, J.R. McKenzie, J.S. Vitelli and A.M. Lindsay, Tropical Weeds Research Centre, Department of Natural Resources, PO Box 187, Charters Towers, Queensland 4820, Australia.
Field trials evaluated the impact of foliar herbicides on pod characteristics and germination behaviour of seed within pods of rust-infected rubber vine plants (Cryptostegia grandiflora). There were three herbicide treatments: triclopyr/picloram (Grazon DS®) at 1.05/0.35 g L-1, metsulfuron (Brush Off®) at 0.09 g L-1 and control; three rust (Maravalia cryptostegiae) infection levels: low, medium and heavy; and three pod maturity stages: juvenile, immature and mature. The effects of herbicide and rust infection varied with pod maturity stages. On average, both herbicides reduced pod dry weight (DWT) and relative water content (RWC) by 22 and 91% respectively. Heavy rust infection on its own increased pod DWT and RWC by 7 and 15% respectively. The effects of herbicides, rust infection and pod maturity stage on seed germination varied with sites.
A synergistic reaction between triclopyr/picloram and heavy rust infection resulted in complete inhibition of germination of both immature and mature seeds of rubber vine. On average, triclopyr/picloram and metsulfuron reduced seed germination by 77 and 42% respectively. In contrast to its effects on seed germination, the rust had no significant effect on seed viability. Maximum seed mortality (84%) was caused by triclopyr/picloram. Results suggest that early treatment with Grazon DS and Brush Off during the reproductive growth stages of rust-infected rubber vine plants may disrupt pod development, but may not kill all seeds within pods.
Plant Protection Quarterly (1999) 14 (2) 57-62.