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Common Rust of Corn • • • Common corn rust is a fungal disease that generally affects corn after tasseling. Weather conditions such as moderate temperatures, frequent rain events, and high relative humidity significantly impact the development and spread of the disease. Fields should be scouted for common rust to identify early infection and potentially help manage the disease. Life Cycle Common rust is caused by the fungus Puccinia sorghi and occurs in temperate to sub-tropic areas. The pathogen overwinters in the southern United States and Mexico. In the summer, storms and winds blow the fungal spores north, into the Corn Belt. Favorable Conditions Young leaves are more susceptible to infection with common rust than older leaves. Once spores land on leaf tissue, three to six hours of moisture is required for spores to germinate and infect. Disease development is favored by moist conditions caused by rainfall, dew, or high relative humidity (95% or greater), and moderate temperatures between 60° and 77° F1. for economic yield loss is fairly low. However, if infection occurs early in the season when plants are at early vegetative growth stages (V3 to V7) and weather conditions are • Disease is favored by temperafavorable for disease ture between 60o-77o F development, the and RH ≥ 95%. potential for economic • Small, < 1/4 inch long, pustules. yield loss increases. Yields can be • Found on both leaf surfaces. decreased about 0.5% for each 1% of the leaf area infected at reproductive, grain filling growth stages Because corn products differ in their level of resistance to common rust, the rate of disease development may be Symptoms Symptoms on leaves begin as light yellow spots about the size of a pinhead. In about 7 days these chlorotic flecks develop into reddish brown pustules. Pustules are found on the upper and lower surface on the leaf, in contrast to pustules of southern corn rust, which are mostly found on the upper leaf surface. These pustules then rupture to reveal the presence of small, cinnamon-brown, powdery spores (Figure 1 and 2). The pustules become darker brown to black later in the season. Pustules are often found in bands or patches, indicating that infection occurred while the leaf was in the whorl. Because pustules rupture the leaf surface, severely infected plants may show symptoms of moisture stress during hot, dry weather, even when soil moisture is adequate2. Effect on Yield Potential Common rust usually appears to some degree in the southern United States and Corn Belt every year. If infection occurs late enough in the season, the potential www.stewartseeds.com Figure 1. Common rust on a corn leaf. Figure 2. Common rust on a corn leaf with cinnamon-brown, powdery, circular to elongated pustules. Common Rust of Corn different for each product that is grown. Late-planted corn is more likely to have higher levels of infection since it will likely have young, more susceptible leaves during the time when spores first arrive from overwintering locations. Management Options The best management option is growing corn products that are resistant to common rust. Resistance to common rust can be due to specific genes or general plant resistance. General plant resistance includes a reduced number and size of pustules and increased latent period between infection and spore production. Fungicides can effectively control common rust. Fungicides are most effective against rust if initial applications are made while there are only a few pustules present per leaf. Use of fungicides is fairly common in sweet corn and seed corn production. Fungicide usually are not needed in field corn because plants are past the most susceptible, vegetative growth stages when rust spores arrive. However, when corn is planted later than normal and wet weather (rains, heavy dews) occurs frequently during vegetative growth stages, fungicide control of rust may be economical on field corn. Fungicide are most effective if started very early in the disease life cycle. Two applications may be required to prevent rust development under extremely conducive conditions. Scouting each corn field on a regular schedule will help determine if fungicide applications should be considered. If significant levels of common rust are present on the lower leaves prior to silking and the forecast is for cool, humid or wet conditions, a fungicide application may be Common Rust: Pustules are beneficial. The found on both upper and lower strobilurin and corn leaf surfaces. triazole families of fungicides, Southern Rust: Pustules are including Headline mostly found on upper leaf AMP® surface. fungicide, are effective at preventing the spread of common rust in corn and providing residual protection from other major fungal foliar diseases such as gray leaf spot, northern leaf blight, and anthracnose. Sources: 1 Sweets, L.E. and Wrather, S. 2008. Corn Diseases. University of Missouri Extension. IPM1001 (Verified 7/11/13) 2 Shaner, G. 2000. Corn Rust—An Epidemic? Purdue University. [Online] http:// www.agry.purdue.edu (Verified 7/11/13) Additional resources: Compendium of Corn Diseases. APS Press; Lipps, P. et al. Common corn rust. Ohio State University Extension. AC-0031-01. [Online] http://ohioline.osu.edu (Verified 7/11/13) Pataky, S. Differentiating common rust and southern rust. Plant Management Network. University of Illinois. http://www.plantmanagementnetwork.org/edcenter/seminars/corn/ CommonRust/ (verified 7/16/13). Pataky, S. Common rust of corn. Plant Management Network. University of Illinois. http:// www.plantmanagementnetwork.org/edcenter/seminars/corn/CommonRust/(verified 7/16/13). Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Leaf Design® is a registered trademark of Monsanto Company. Headline AMP® is a registered trademark of BASF Corporation. All other trademarks are the property of their respective owners. Stewart and Design™ is a trademark of American Seeds, LLC. ©2013 Monsanto Company. CRB07182013. For additional agronomic information, please contact your local representative