Disease Guide
University-rated fungicide efficacy, pricing per acre, and spray timing for southern rust (Puccinia polysora). Based on Crop Protection Network 2025 data.
Southern rust is a foliar disease of corn caused by the obligate pathogen Puccinia polysora. Unlike most corn diseases, southern rust does not overwinter in the Corn Belt — spores blow northward from tropical and subtropical regions each season. This means infection timing varies year to year depending on weather patterns and hurricane activity that can carry spore loads into the central U.S.
The disease produces small, circular, densely packed orange-brown pustules primarily on the upper surface of corn leaves. Southern rust favors warm temperatures (80°F+) and high humidity. Once established, it spreads explosively — a field can go from scattered pustules to severe infection in 7-14 days under favorable conditions. This rapid progression makes early detection and fast response critical.
Yield losses from southern rust range from 20% to 40%+ in severe epidemics. The disease destroys photosynthetic leaf tissue during grain fill, reducing grain weight and test weight. Losses are most severe when rust arrives before R3 (milk stage), giving the pathogen more time to damage the upper canopy. Southern rust is considered one of the most yield-destructive corn diseases when it arrives early and goes untreated.
Efficacy ratings from the Crop Protection Network 2025, based on multi-year university field trial data. Only products with tested ratings are shown — untested or not-listed products are excluded.
| Product | Mode of Action | Southern Rust Rating |
|---|
Source: Crop Protection Network 2025. Ratings reflect multi-year, multi-location field trial performance.
Efficacy ratings cross-referenced with real purchase prices. Products sorted by efficacy rating — best performers first.
| Product | Southern Rust Rating | Best $/Acre |
|---|
Prices as of early 2025 from AgChem, FBN, and Farmerceag. Actual pricing varies by geography, volume, and timing.
Southern rust requires a reactive approach. Unlike tar spot or gray leaf spot where preventive VT-R1 applications are standard, southern rust timing depends entirely on when spores arrive. The disease does not overwinter in the Corn Belt, so you cannot predict it from field history alone. Monitor USDA southern rust tracking maps and regional Extension reports throughout the season.
Spray when pustules are confirmed nearby. Once southern rust is reported in your county or adjacent counties, apply fungicide within 7 days. The disease spreads explosively under warm, humid conditions — a week of delay can mean the difference between protecting yield and watching it disappear. If you already have a VT-R1 application on for other diseases, that application may provide protection if rust arrives within 14-21 days of application.
Late-season applications can be justified. Because southern rust can arrive at any growth stage, fungicide applications as late as R4 (dough) may be economically justified if the disease arrives late and conditions favor rapid spread. This is unusual for most corn diseases, where applications after R2 rarely pay.
Act fast — southern rust doesn't wait. This is the one corn disease where speed of response matters more than product selection. Any product rated Good or better will help. Get something on the field quickly rather than waiting for the perfect product.
Efficacy ratings for all 26 products across 7 diseases, pricing from 3 suppliers, decision frameworks, and an ROI calculator.
View full guide →Search and filter 9,196 yield records from 867 university field trials by year, state, and crop.
Browse data →Fungicide ratings, pricing, and timing for tar spot (Phyllachora maydis).
View guide →Fungicide ratings, pricing, and timing for gray leaf spot (Cercospora zeae-maydis).
View guide →Fungicide ratings, pricing, and timing for NCLB (Exserohilum turcicum).
View guide →Fungicide ratings, pricing, and timing for anthracnose (Colletotrichum graminicola).
View guide →Fungicide ratings, pricing, and timing for common rust (Puccinia sorghi).
View guide →Fungicide ratings, pricing, and timing for eyespot (Aureobasidium zeae).
View guide →Based on Crop Protection Network 2025 data, Trivapro and Topguard EQ rate Excellent or Very Good to Excellent for southern rust. Quadris, Headline, Quilt Xcel, Veltyma, Revytek, Miravis Neo, and Priaxor all rate Very Good. Most premium dual or triple-MOA products provide strong southern rust control.
Southern rust (Puccinia polysora) and common rust (Puccinia sorghi) look similar but differ in important ways. Southern rust pustules are smaller, more circular, and densely packed — mostly on the upper leaf surface. Common rust pustules are larger, more elongated, and appear on both leaf surfaces. Southern rust favors warm temperatures (80°F+) while common rust prefers cooler conditions (60-75°F). Southern rust is far more yield-damaging and requires faster fungicide response.
Southern rust timing depends on when spores arrive — it does not overwinter in the Corn Belt. Monitor southern rust tracking maps and scout frequently from R1 onward. When pustules are confirmed in your area, apply fungicide immediately. Unlike diseases where VT-R1 preventive applications are standard, southern rust may require a reactive application at R2-R4 if spores arrive late in the season.
Southern rust can reduce corn yields by 20-40% or more in severe epidemics. Because it spreads extremely rapidly under favorable conditions (warm, humid weather), southern rust can go from a few scattered pustules to a full-field epidemic in just 7-14 days. Yield losses are most severe when the disease arrives before R3 (milk) and goes untreated, as it can destroy leaf tissue faster than most other corn diseases.
Ledger Drone is invoicing software built for ag drone pilots. Track acres, chemicals, and field-by-field billing — all from your phone, offline. If you're spraying fungicide commercially, you need a tool that keeps up with you in the field.
Learn about Ledger Drone →