Brown Rust

Pathogen

Fungus

Hosts

Barley, wheat, rye, grasses

Symptoms

Brown rust species are specific to their host crop. First symptoms are scattered orange-brown pustules on leaves and occasionally on the stems, leaf sheaths and ears. Pustules often develop a yellow halo on barley. Small black pustules also develop on the underside of the leaf and these give rise to the infective spores. Severe attacks cover over 50% of the leaf area and occur late in the season.

Development

The rust fungus can only grow on green plants. It survives the harvest period on late tillers and then spreads via volunteers to the subsequent crop. The spread is by airborne spores. Once inside the leaf, it produces very waxy and compact pustules deep inside the leaf which are harder for leaf systemic fungicides to target.

Favourable factors

Warm temperatures (15°C to 22°C) with high humidity and free water all aid disease development. Dry windy days help further spread. Late doses of nitrogen also favour brown rust attack.

Brown rust is killed at less than 5°C and inhibited at temperatures above 25° C, so hot summers and cold winters reduce inoculum. It has a 7 day cycle in mid to late summer.

Importance

Severe brown rust infections take out significant green leaf area and reduce the rate of photosynthesis needed for grain fill, and increase the rate of transpiration to cause drought susceptibility. Grain shrivel and specific weight reductions reduce both yield and quality.

Severe attacks occur mainly in late season and can reduce yield by up to 50%. However 15% is more common.

Control

• Correct choice of fungicide applied from flag leaf just visible (growth stage 37) onward together with choice of variety will give good control.

• Susceptible varieties (NIAB rating 4 or less) of spring barley should be avoided south of the Humber.

• Removal of volunteers which provide a `bridge` to early sown crops.

Brown rust on wheat (Photo C. Lincoln)

Severe disease pressure (Photo C. Lincoln)