Research Associate, LSU AgCenter
Southern Corn Rust Confirmed in Louisiana
Trey Price, Extension/Research Plant Pathologist, Macon Ridge Research Station
Based on a tip from an industry representative, southern rust, Puccinia polysora, was suspected along the Atchafalaya River in milk to early dough stage corn. Yesterday afternoon samples were collected from two locations (Woodside and Lettsworth) and confirmed to be southern rust this morning via microscopic examination. Since then, other similar reports have come in from Rapides and Bordelonville. Incidence in these fields is very low (<1%). Given the stage of the crop and low incidence, I would not recommend treating these fields. Current conditions (warm/humid) are favorable for disease development and producers, agents, and consultants should monitor for disease development in their corn fields. It is noteworthy that we have detected southern rust about one month earlier in 2016 than in 2015.
Scouting is key to managing southern rust. First, identify the disease correctly. Southern rust pustules will appear reddish orange and will almost always occur on the upper side of the leaf (Figure 1). In severe cases pustules may appear on leaf sheaths and husks (Figure 2). Common rust, which has been very common this year, will appear more brick red, and pustules will occur on both sides of the leaf (Figure 3). Most common rust has ceased to develop because of the warm temperatures, and pustules have turned brown. There are differences in susceptibility to southern rust among hybrids; therefore, it is important to define disease incidence/severity prior to making management decisions.
If southern rust is not present, fungicide applications are not necessary. If southern rust occurs near tasseling, a fungicide application will likely be needed for management and provide economic benefit (See Table 1 for products and efficacy) as this disease can be very aggressive under optimal conditions. As the crop matures from tasseling stage, a return on fungicide investment becomes increasingly less likely (See Table 2). Application decisions must be considered on a field by field basis taking into account disease incidence/severity, crop stage, prevailing environmental conditions, and likelihood of economic return. If a fungicide application is deemed necessary, using recommended rates and maximum water volumes will increase efficacy. Ideally, fungicides should be applied prior to disease onset, but realistically, fungicides are usually applied at or just after onset. Therefore, individuals should make efforts to detect and treat diseases as early as possible to prevent losses to yield and quality. Later planted corn is at higher risk for developing southern rust that requires management.
Table 1. Fungicide efficacy for control of corn diseases.
The Corn Disease Working Group (CDWG), which includes many members from the mid-South including several pathologists from Louisiana, has developed the following information on fungicide efficacy for control of major corn diseases in the United States. Efficacy ratings for each fungicide listed in the table were determined by field testing the materials over multiple years and locations by the members of the committee. Efficacy ratings are based upon level of disease control achieved by product, and are not necessarily reflective of yield increases obtained from product application. Efficacy depends upon proper application timing, rate, and application method to achieve optimum effectiveness of the fungicide as determined by labeled instructions and overall level of disease in the field at the time of application. Differences in efficacy among fungicide products were determined by direct comparisons among products in field tests and are based on a single application of the labeled rate as listed in the table. Table includes systemic fungicides available that have been tested over multiple years and locations. The table is not intended to be a list of all labeled products1. Efficacy categories: NR=Not Recommended; P=Poor; F=Fair; G=Good; VG=Very Good; E=Excellent; NL = Not Labeled for use against this disease; U = Unknown efficacy or insufficient data to rank product
1Additional fungicides are labeled for disease on corn, including contact fungicides such as chlorothalonil. Certain fungicides may be available for diseases not listed in the table, including Gibberella and Fusarium ear rot. Applications of Proline 480 SC for use on ear rots requires a FIFRA Section 2(ee) and is only approved for use in Illinois, Indiana, Iowa, Louisiana, Maryland, Michigan, Mississippi, North Dakota, Ohio, Pennsylvania, and Virginia.
2Harvest restrictions are listed for field corn harvested for grain. Restrictions may vary for other types of corn (sweet, seed or popcorn, etc.), and corn for other uses such as forage or fodder.
Many products have specific use restrictions about the amount of active ingredient that can be applied within a period of time or the amount of sequential applications that can occur. Please read and follow all specific use restrictions prior to fungicide use. This information is provided only as a guide. It is the responsibility of the pesticide applicator by law to read and follow all current label directions. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Persons using such products assume responsibility for their use in accordance with current directions of the manufacturer. Members or participants in the CDWG assume no liability resulting from the use of these products.
Table 2. Estimated % corn grain yield loss due to defoliation at various growth stages.
Adapted from the National Crop Insurance Service’s Corn Loss Instruction to represent the leaf collar growth staging method. Included in the Mississippi State University, Grain Crops Update June 4, 2010, Erick Larson.
If you require additional information, please do not hesitate to contact your nearest county agent, research station, or specialist.