CSIRO Division of Tropical Crops and Pastures, Private Mail Bag, Post Office, Aitkenvale, Townsville, Qld 4814.
Rust (Uromyces appendiculatus) on Siratro was first reported in Australia in 1978. The effect of the disease on yield was measured by comparing yields of control plots with those of disease-free plots over a 2 year period in the field at Lansdown, near Townsville, Queensland. A third treatment measured the effect of weekly spraying with a suspension of urediniospores.
Severity of disease attack was highest in the cooler, dry spring and autumn months and lowest in, the wet summer months (Dec–March). Spraying with urediniospores did not change this pattern. Oxycarboxyn (PLANTVAX®) sprayed at weekly intervals controlled the disease, and yield from the rust-free plots was 30% greater than from the controls over six harvests.
Nylon-bag dry matter digestibility of leaves was higher for disease free plots — significantly so in June when the difference was 4 percentage units and the disease score was 2.7. Digestibility of stems was not affected by treatment. Heavily diseased leaves had in vitro digestibilities 8.6 units lower than disease-free leaves and a lower nitrogen concentration (3.4 v 2.7%).
Rust on Siratro could reduce the animal production potential of Siratro-based pastures over large areas of sub-tropical and tropical Australia. A search for sources of resistance to the disease has commenced.