Southern Rust of Maize

Southern rust produces small, circular to oval pustules that are densely clustered and predominantly on the upper leaf surface. The pustules are orange to light cinnamon-brown and release powdery spores. As the disease intensifies, leaves can become heavily covered, yellow, and die prematurely. Concentration of pustules on the upper surface helps distinguish it from common rust, whose pustules occur on both surfaces and are typically darker and more scattered.

Puccinia polysora is an obligate parasite that needs living maize tissue and produces repeating generations of wind-dispersed urediniospores. In warm regions it can persist year-round on maize, while in cooler regions epidemics typically begin when spores blow in from warmer areas. Under hot, humid conditions with dew, a new generation of pustules can form within a short interval, allowing very rapid build-up.

• Requires living maize tissue and cannot survive on residue alone
• Wind-dispersed urediniospores can travel long distances between regions
• Hot, humid weather with dew drives rapid, repeating infection cycles
• Late arrival can still be damaging if it coincides with grain fill

Maize, including sweetcorn, is the host of southern rust. Unlike many maize leaf diseases, it is favoured by high rather than moderate temperatures, combined with high humidity and free moisture on the leaves. Warm-region source populations and prevailing winds strongly influence when and where the disease appears in a given season.

• Hot temperatures combined with high humidity and dew or rain
• Proximity or wind connection to warm regions where the fungus persists
• Susceptible hybrids and late-planted crops still filling grain
• Prolonged periods of leaf wetness that favour rapid cycling

Because southern rust often arrives on the wind and can intensify rapidly, management relies on resistant hybrids and timely monitoring so that any response can be made before the disease overwhelms the upper canopy during grain fill.

• Grow hybrids with better southern-rust tolerance where available
• Follow regional rust monitoring reports that track its northward movement
• Scout the upper leaves closely during hot, humid weather at grain fill
• Consider planting date to avoid the most susceptible stages coinciding with peak spore arrival
• Prioritise later-planted or later-maturing fields most at risk