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eturn to table of contents SOUTHERN RUST OF CORN by Steve Butzen, Agronomy Information Consultant DISEASE FACTS • Fungal disease caused by Puccinia polysora pathogen • Does not occur as frequently as common rust but is more destructive when it occurs • Favored by high humidity and temperatures in the 80s and 90s • More frequent in the South, but may also spread into the Midwest by wind-blown spores, usually in late summer • Spreads very rapidly when conditions favor development »» New infections may occur every seven days. »» Individual fields may be damaged very quickly. »» Epidemics may occur over large areas. • May warrant a fungicide application to help minimize crop damage IMPACT ON CROP • Disease lesions reduce functional leaf area. • Photosynthesis is reduced and less sugar is produced. • Plant uses stalk carbohydrates to help fill kernels. • Stalks are weakened and stalk rot potential increases. • Premature plant death may occur. • Yield losses may result from poorly filled kernels and stalk lodging-induced harvest losses. »» Significant damage to upper leaves early in the life of the hybrid results in higher yield losses. »» If damage is confined to lower leaves or occurs after corn is well-dented, yield losses will be lower. »» The latest planted corn in an area is at higher risk for yield loss. SOUTHERN RUST DISEASE CYCLE (Puccinea polysora) Secondary spread by wind and rain Fungus overwinters on corn in Mexico and the Caribbean. Pustule development Infected plant Wind-blown spores are the primary source of infection. Spores are blown in from the South. Wind and rain carry spores to leaves. 98

eturn to table of contents SYMPTOMS • Typical pustules are small, circular to oval. • Light-orange to cinnamon-red urediniospores • Pustules develop primarily on UPPER leaf surface (unlike common rust). • Pustules may also occur on leaf sheaths, stalks, ear shanks, and husk leaves. • Rust will turn white shoes/clothing orange after walking through an infected field. SOUTHERN VS. COMMON RUST Southern rust. Southern rust. Common rust. MANAGEMENT • Scout corn to detect southern rust early. • Monitor disease development, crop growth stage, and weather forecast. • Apply a foliar fungicide if: Southern rust. »» Rust is spreading rapidly or likely to spread and yield may be affected Southern vs. Common Rust: Ideal Environment Southern Rust Warm to hot and moist, 77+ ºF Common Rust Cool to warm and moist, 60-77 ºF »» Disease exceeds threshold established by local state extension plant pathologist • Commonly used fungicides: DuPont Aproach ® Prima, Fortix ® , Headline ® AMP, Quilt Xcel ® , and Stratego ® YLD Genetic Resistance Appearance of Pustules Small circular, pinhead appearance Large, circular to elongated »» DuPont Pioneer researchers screen hybrids and parent lines for resistance and provide ratings for customers. Color of Pustules (spores) Reddish orange Brown to cinnamon-brown »» Most hybrids are rated from “3” to “5” on a scale of 1 to 9 (9 = resistant), indicating that genetic resistance is limited for southern rust. Location of Pustules Upper leaf surface, may also infect husks Both upper and lower leaf surfaces, Infects leaves only • Disease is wind-borne and does not overwinter in U.S.; therefore, rotation and tillage are not effective. 99