Field studies on the effectiveness of phosphonate suppression of phytophthora root rot in proteas

Abstract

Phosphonate suppression of root rot in proteas, caused by Phytophthora cinnamomi, was tested in field trials over a two-year period. Effectiveness of chemical control varied, dependent upon plant species, site topography, soil moisture content and soil temperature.
Phosphonate, applied as a foliar spray at 1.2 g a.i.l-1 at 6 weekly intervals, resulted in 100% plant survival (cƒ 19% in unsprayed control plants) in Leucadendron ‘Sylvan Red’ and 75% survival in Leucospermum ‘Firewheel’ (cƒ 44% in unsprayed plants). Under site conditions conducive to high disease pressure, increased spraying intensity (3 weekly, 2.4 g a.i.l-1) did not prevent infection (81% of plants) and death (50%) of Leucospermum ‘Firewheel’ by P. cinnamomi.