Symptoms

Initial symptoms include a yellow freckle consisting of small elongated spots. These lesions are visible on both leaf surfaces. In susceptible varieties, these lesions increase in size, turn red-brown to brown and are accompanied by the development of a yellow halo.

Spore production leads to pustule development on the lower surface of the leaf with sporulating structures rupturing the leaf surface. Pustules may be 2-10mm long by 1-3mm wide. Rubbing one's hand over the lower surface of leaves bearing fresh pustules leaves a brown powdery deposit (spores) on the skin.

Heavily diseased crops take on a rusty brown appearance and may show little green leaf tissue. Good growing conditions usually lead to rapid re-greening of the crop through fresh leaf production.

In resistant varieties, symptom development beyond a yellow leaf freckle remain limited.

Cause

Rust is caused by the fungus Puccinia melanocephala. It is spread by wind-blown spores. Transmission may occur over long distances, leading to rapid spread of the disease. The disease is favoured by cool conditions where early morning dews predominate. Heavy rain does not favour transmission.

Distribution

Common rust occurs throughout Queensland and New South Wales and may be seen in susceptible varieties in every district.

Assessment of Severity

Timing and inspections

Although able to cause disease throughout the cropping period, common rust is usually more severe early in the growth period. Inspections normally should be made between October - December.

Assessing severity

As for the other foliar (leaf) disease, only moderate to heavily diseased crops are recorded in disease surveys by CPPB Boards.

History of Incidence

P.melanocephala is thought to have entered Australia via wind-blown spores in 1978. The first record was in the Cairns region but within 12 months it had spread throughout Queensland and New South Wales. Since then, the disease has affected between 15,000 - 40,000 hectares annually.

Gradient in rust disease severity going from 5% diseased leaf area to approximately 40% disease. Moderate-severe disease has been set at 15% leaf area affected (B).