Foliar Corn Disease Management

Trey Price and Boyd Padgett, LSU AgCenter plant pathologist

Holcus spot/paraquat drift

A few reports of paraquat drift or Holcus spot have surfaced statewide.The two maladies display round to oval, light tan to white spots with or without yellow halos and are difficult to distinguish from each other based on symptoms alone.Generally, if a drift pattern (gradient) is observed, if affected areas are large and more jagged than round, or if secondary fungi are within lesions, it is likely paraquat drift (Figure 1).If the distribution is random, the spots appear within 48 hours of a thunderstorm, and water-soaking is observed, it is likely Holcus spot (Figure 2).Microscopic observation of Holcus spot may reveal bacterial streaming, as the disease is caused by Pseudomonas syringae pv. syringae.Both issues are usually of minor concern, with the exception of paraquat drift heavy enough to affect corn stand and yield.

Common rust and Southern rust

Common rust may be found early to mid-season when temperatures are cool (60-77^oF).Pustules of common rust are brick red to dark orange in appearance, somewhat elongated, and will appear on both leaf surfaces (Figure 2).Common rust will progress during relatively cool, rainy, and cloudy weather; however, very rarely are fungicide applications warranted for common rust.Warmer temperatures will greatly slow common rust development.We do not have any confirmed common or southern rust observations in Louisiana to date.Southern rust pustules are more orange than brick red, usually not as elongated, and almost always appear on the upper surface of leaves (Figure 3).

Northern corn leaf blight

We will start to find northern corn leaf blight (NCLB) during the month of May in CENLA and NELA (Figure 4).This disease will first appear in susceptible hybrids in fields following corn with reduced tillage.NCLB will progress slowly during dry weather, and more quickly during regular rainy periods.Most of the time, fungicide applications are not needed for NCLB as most hybrids have some degree of resistance, and the corn crop usually fills out before the disease is severe enough to impact yield.However, severe disease may occur in susceptible hybrids.These are the fields that need to be watched closely.

Fungicide considerations

Fungicide application decisions should be carefully considered field by field based on:disease severity (Figure 4), crop stage (Table 1), hybrid susceptibility, fungicide efficacy , tillage regime, prevailing environmental conditions, previous experience, commodity price, and the probability of a return on the investment.If applications are warranted, apply at labeled rates using maximum (5 GPA by air, minimum) water volume is recommended.Ground applications using at least 15 GPA are significantly more effective.

Table 1.Percent yield loss (in blue) as a result of defoliation by crop stage.For example…30% defoliation at dent stage results in a 2% yield loss.

Figure 1. Paraquat drift on corn.

Figure 2. Holcus spot and common rust of corn.

Figure 3. Southern rust of corn.

Figure 4. Northern corn leaf blight disease severity scale.

Wet fields and weeds

Daniel Stephenson, LSU AgCenter Extension Weed Specialist

Louisiana is wet.The chart below shows the rainfall amounts at four LSU AgCenter research stations around the state and it tells the story, especially in Central Louisiana.Crops planted are struggling to grow properly.Weeds are affected as well.

One of the main phone calls my peers and I have received over the past couple of weeks is what do I do with the weeds that are emerging following burndown.The presence of these weeds is primarily due to delayed planted of summer crop.The phone calls usually go like this….”I need to burndown weeds again, but it has to be applied by air.I have corn on two sides, rice on one side, and young soybean on the other.What herbicide can be applied?”Well, there really is not anything that can be applied by air if a glyphosate-sensitive crop borders the targeted field.If glyphosate-tolerant crops surround the targeted field, then glyphosate can be applied.We are currently not suggesting the use of a residual herbicide if water has not drained or if any tillage, including knocking off the beds, will be performed when it dries.

If glyphosate-resistant pigweeds that are greater than 6-inches are present, no herbicide options are available that can be applied by air and no good options are available when ground applied.My suggestion in this situation is tillage, which is not popular.Simply re-hipping the field is not enough tillage to effectively control the pigweed.A disk is required at a minimum, then the field could be re-hipped.I agree that this option is not good.I feel your pain because I must disk and re-hip all my research area at the Dean Lee Research Station this year because I have pigweed that are too big to kill with herbicides.

This weather is unprecedented.I have had many individuals in the agriculture community tell me they have not seen a year like this one before, especially producers in central Louisiana.Please contact your local parish agent or me with questions.My number is 318-308-7225.Good luck.

Using a Farm Management Tool to Estimate Herbicide Program Costs in Corn, Cotton, Rice, and Soybean Production Systems

Michael Deliberto, Donnie Miller, Daniel Stephenson, and Eric Webster, LSU AgCenter Scientist

Corn, cotton, rice, and soybean producers have flexibility when it comes to specific tillage practices and weed control programs in crop production. Varying combinations of tillage practices are currently being used by producers for land preparation via seed bed preparation. And with a broad spectrum of herbicides available for curbing weed pressure in the production of corn, cotton, rice, and soybeans, the choice of specific tillage operations and herbicide type along with the number of applications chosen by a producer can directly impact variable cost per acre.

The Field Preparation and Herbicide Control Program Cost Estimator is a Microsoft Excel® decision aid that has been developed by the LSU AgCenter to assist row crop producers in planning their field preparation and herbicide program selection. The cost of current and alternative preparation/herbicide programs can be estimated within the program (for comparative purposes).

The primary purpose of this program is that it presents information so that producers can use it as a farm planning aid. Tillage/herbicide combinations that are planned for the upcoming year can be entered into the model. Total variable cost (TVC) per acre for a program will then be estimated based on the data that a producer has entered. TVC includes charges for fuel, labor and those herbicides utilized as inputs for operations performed within a weed control program. Producers can use this model to determine a specific combination of tillage operations and herbicide applications tailored to assist them in meeting their desired level of weed control at the lowest cost per acre.

Figure 1. User interface of the farm management decision tool.

The above example program (Figure 1) illustrates how various tillage operations and herbicide applications would be entered to reflect a complete program for corn. Fuel price and labor cost are entered in their appropriate cells, as increases in these two parameters (fuel and labor cost) would have a potentially significant impact on program cost. Within the field operation section, data for each pass over the field, whether for tillage or herbicide application, would be entered on a single line. Herbicides applied as part of a weed control regimen are then selected from drop down menus. Actual herbicide rates that are to be applied can be entered by the producer. Total herbicide material cost per acre, based upon the actual herbicide application rate entered and the corresponding herbicide cost per unit is shown. For each field operation entered, the TVC of that operation is calculated. This per-acre TVC includes costs for both fuel and labor for each field pass as well as the material cost of any herbicide applied. These TVC values for each operation are then summed across all tillage/herbicide application operations so that a per-acre total variable program cost for a specific program can be estimated.

The Field Preparation and Herbicide Control Program Cost Estimator contains blank worksheets so that producers can evaluate the costs of alternative tillage and herbicide program for multiple crops produced across their particular farm acreage. Herbicide product prices were obtained in December of 2020 for publication in enterprise budgets released in January 2021 and are intended to serve as planning estimates for the material cost.

The tool and accompanying user’s guide can be downloaded from the LSU AgCenter's Department of Agricultural Economics and Agribusiness website.

Appreciation is extended to The Louisiana Soybean and Grain Research and Promotion Board, the State Support Committee of Cotton Incorporated, and The Louisiana Rice Research Board for their funding of this multi-disciplinary research.

Do Soybean Plants Require Supplemental Nitrogen?

David Moseley and Rasel Parvej, LSU AgCenter Scientist

Soybean plants require approximately 4-5 lbs of nitrogen (N) per bushel of grain. For each bushel produced, more N is required by the plants than any other nutrient. However, soybean plants can fix up to 70-75% of their N requirement through a symbiotic relationship with Rhizobium (Bradyrhizobium japonicum) bacteria. The bacteria form colonies that appear as nodules on the soybean root hairs. Through the symbiotic relationship, the soybean plant provides carbohydrates and minerals to the bacteria and the bacteria convert atmospheric N gas (N₂) into a plant available form of N (ammonium (NH₄⁺)). The remaining 25-30% of the required N usually come from residual N from the soils and breakdown of previous crop residues. However, soybean plants in a good year can produce >80 bushels yield in high yielding irrigated fields without any addition of N. Therefore, supplemental N is not required as N is not generally a yield limiting factor for soybean plants. In many cases, if the soils contain high residual N or if supplemental N is added, there can be a negative effect on nitrogen fixation from the Rhizobium bacteria.

The N fixation process can fail for several reasons including cropping history and soil conditions. If soybean seed are sown in a field not planted to soybean for the previous 3-5 years, the seed should be inoculated with Bradyrhizobium japonicum bacteria prior to planting. In addition, soil conditions including coarse-textured, compacted, and saturated (for three days or more) can cause a decrease in N fixation by the bacteria. For more information on N fixation please read the Nitrogen Fixation in Soybeans article in the LA Crops Newsletter - Volume 10, Issue 4.

At the V3-V5 growth stage (see Identifying Soybean Growth Stages from the LA Crops Newsletter Volume 11, Issue 2 – March 2021), soybean roots should have at least approximately seven nodules (2 mm or greater in size) (Figure 1) with a pink or red colored cross-section (Figure 2). If the nodules are insufficient in number or appear unhealthy, the nodules and the leaves should be regularly scouted to help determine if supplemental N is required. Plants deficient in N may contain symptoms of light green to yellow leaves, that begin in the older growth. However, other problems can mimic N deficiency, such as sulfur deficiency (Figure 3). A N deficiency can be properly identified by taking a tissue sample. If supplemental N is required in soybean, the application should be applied with a granular/dry N fertilizer in the early reproductive growth stages. The rate of supplemental N can vary depending on the growth stage, yield potential, and plant available soil N content. Read Identifying and Responding to Poor Nodulation in Soybeans from Michigan State University for more information on applying supplemental N for poor nodulation.

Figure 1: Bradyrhizobium japonicum bacteria nodules attached to the roots of a soybean plant at the V3 growth stage. A penny gives perspective to the size of the nodules.

Figure 2: A cross-section view of a Bradyrhizobium japonicum bacteria nodule from a soybean root. The cross-section view of the nodule has a pink or red color, suggesting the nodule is healthy and actively fixing nitrogen. A penny gives perspective to the size of the nodules.

Figure 3: Soybean plants with light green to yellow foliage suggesting a nitrogen or sulfur deficiency.

Wind Damage to Corn

Matt Foster, LSU AgCenter Corn Specialist

Recent storms have damaged corn in some areas of Louisiana. High wind events can cause minor leaning/bending of plants, root lodging, and “greensnap”. Corn is most susceptible to strong winds from V7 to pollination since the stalk is rapidly developing and can be somewhat brittle due to slow lignin production.

Fields should be scouted 4 to 5 days after the wind event to assess recovery potential. Corn that is leaning/bending will most likely recover to an upright position; however, if the damage occurs at the beginning of pollen shed/silking, leaning plants could shade the exposed silks and hinder pollination.

Root lodging occurs when the roots are unable to anchor the corn plant against the force of the wind. Corn hybrid, soil and environmental conditions, insect damage, and herbicide injury are factors that can contribute to root lodging. If growing conditions are favorable after the wind event, corn plants can typically recover by “goosenecking” back upright. Similar to plants that are simply leaning from strong winds, if root lodging occurs during pollen shed/silking, exposed silks may be shaded by leaning plants, therefore, affecting pollination.

Greensnap refers to corn stalks completely broken off by strong winds (Figure 1). It is much more common in the Great Plains states, but it does occur occasionally in Louisiana corn. Stalk breakage normally occurs below where the ear normally develops. When greensnap occurs late in the growing season, there is very little opportunity for compensation from the neighboring plants, therefore, yield reduction is closely correlated with the percentage of snapped stalks. Research has shown that factors such as corn hybrid characteristics, high plant population, row orientation, excess soil moisture, and in-season nitrogen application can influence greensnap occurrence.

Figure 1. Greensnap in corn. LSU AgCenter photo.

Two Good Drones for Crop Scouting and General Farm Use

Randy R. Price, LSU AgCenter Agricultural Engineer

Although drones have been touted for many high-end agricultural operations such as mapping and spraying, farmers may find that a regular retail store type drone (with a good “live view” camera) maybe just as good (useful) as a higher-end system. These drones can be used for crop scouting (high altitude oblique shots (see Figure 2) and low altitude close-ups, counting cattle, checking fence lines (especially around rivers and bayous), observing machinery operations, and creating videos of show and sale cattle. Two drones I highly recommend for this purpose are the DJI Mavic Air 2 and the DJI Mini 2. These drones are easy-to-fly, locally available (Best Buy), low-cost ($449 to $988), and have the longest flight times of most other drones (25 to 35 minutes).

Figure 1: Photos of the DJI Mini 2 (A) and the DJI Mavic Air 2 (B).

The Mini 2 fits in the palm of your hand (Figure 1-A) and is composed mainly of plastic. It weighs under 250 grams and because this light weight and size, it is considered safe enough to not cause significant damage to humans (in crowds) and does not require FAA registration for recreational flying. Still, despite its small size, the drone flies very well in wind and outdoors, and is capable of scouting large farm areas up to 100 acres. Also, the flight batteries charge from a standard 5-volt vehicle charger allowing you to easily operate and service it from your vehicle.

The DJI Mavic Air 2 (Figure 1-B) is a slightly larger drone, but has much improved flight speeds, flight duration (30 to 35 minutes), and distance, allowing you to get to the farthest extents of your farm (note via FAA rules you must always keep the drone in sight). During mapping operations, we could endlessly hover over fields waiting for cloud covers to pass, and then resume mapping when full sunlight was available. The camera is also capable of 48 megapixels for close shots of plants and leaves (both of these drones are capable of hovering at very low altitudes [< 10 ft.] over fields) and in one case we were able to see the individual lesions on sugarcane plant leaves (note that the camera can also operate at 20 megapixels for normal pictures).

Both drones also have the new ADS-B receiver system that receives location data from manned aircraft and allows your transmitter to alert you (and show their location). This system works very well and indicates aircraft well in-time for you to evaluate, land, or move out of the way. This feature is especially useful here in Louisiana where many oil field and medical helicopters exist (note that this system may not work with agricultural aircraft as they typically do not have the ADS-B system).

Disadvantages of these drones are that third-party software (Drone Deploy, Pix 4-D Capture, etc.) do not currently support these drones and automated flights for mapping must be performed manually. This procedure can easily be mastered (and performed with these drones) by flying up to altitude (200 to 400 ft.) at one corner of the field, pointing the camera down, and then either manually capturing images (or using the timed capture function (2 sec., etc., 20 megapixels only) and then slowly flying passes back and forth across the field. Make sure to leave plenty of overlap (> 50%) and you can use the live camera view to help judge, turns, distance, and overlap (a flat, forward speed works turn works well at ends during turns). The recorded images can then be put into most standard mapping program - Agisoft, Pix4-D, Drone Deploy, Microsoft Image Composite Editor, etc. - to create a composite single field images (note that the Green minus Red and the VARI indices work well at indicating high and low growth areas).

Both these drones are low cost, easy to fly, and can enhance your crop scouting abilities. If you have any questions, please contact me at: rprice@agcenter.lsu.edu. Note that if you use drones for commercial use, you must get a Part 107 license from the FAA.

Figure 2: Image taken with the Mavic Air 2 showing areas in fields affected by yellow leaf mosaic virus.

Evaluation of In-Season Nitrogen Loss and Aftermath Management in Corn

Rasel Parvej, Brenda Tubana, Syam Dodla, and Matt Foster, LSU AgCenter Scientist

Phone calls are coming in regarding the nitrogen (N) losses from more than 10-inches of rainfall within 2-wks after sidedress N application in corn. Like this year, excessive rainfall often occurs in most years in the lower Mississippi Delta during the early growing season, resulting in saturated soils for several days, which accelerates N losses via denitrification, leaching, and/or runoff and reduces corn yield potential. Therefore, LSU AgCenter always recommends applying N at least in 2 splits for silt loam and clayey soils and 3 splits for sandy soils i.e., a small rate at planting (either by 2 by 2 or dribbling on the top of the bed), the rest around V6 stage (6 leaves with visible collars and plant is about 12 to 18-inch tall) as sidedress for silt loam and clayey soils, and/or another small amount before tasseling especially for sandy soils. For example, for a 220-bushel corn crop in silt loam soils, 220 lb N per acre should be split as about 30 to 50 lb at planting and 170 to 190 lb at V6 stage. Applying a small rate of N at planting provides the corn plant enough N for setting maximum yield components at or after V6 stage. It also provides a wide window of opportunity to sidedress N application during the growing season from V6 to V8 stage. For instance, having at-planting N application would allow the producers to delay their sidedress application if missed at V6 stage due to rainfall and wet soil conditions. Although researchers from the mid-South states have showed that maximizing corn yield by a single N application during the growing season in both silt loam and clay soils is possible, for this to occur, the growing season must be ideal with moderate temperature and adequate and evenly distributed rainfall, which seldom occurs in Louisiana. Like this year, a single application is, therefore, a risky N management plan for corn production in most years in Louisiana.

Unfortunately, this year several corn producers applied their total N fertilizer in a single application as sidedress at or 1-3 weeks after corn emergence around V1-3 stages. After getting more than 10-inches of rainfall within 2-wks of sidedress, a significant amount of applied N may be lost during the early growing season before plant uptake since the root systems were very small at the time of application and were not even close to sidedress N fertilizer for active uptake. Most interestingly, all these rainfall events occurred during V1 to V4 stages, as typical in most years. If the producers had a small amount (30-50 lb) of N at planting as recommended, they could easily wait until V6-8 stages for sidedress application and be better off. At this stage, it is hard to measure how much N has been lost from our corn fields because N losses associated with excessive rainfall depend on several factors such as soil type, drainage, and cation exchange capacity (CEC). Although N can be lost via different loss mechanisms, N loss via denitrification is the main concern due to excessive rainfall especially in poorly drained soils but can occur in any soil with excessive rainfall that creates water-logged anaerobic conditions (Figure 1). However, denitrification loss of N should not be the major concern for well-drained corn field without water-logged conditions though leaching loss of N can still be a concern with high rainfall in well-drained soils. Leaching loss is also common in sandy soils with low CEC (<10). Good news is, N leaching mainly occurs for nitrate-N (NO₃-N) fertilizer. Since urea ammonium nitrate (UAN; 32-0-0, 30-0-0-2S, or 28-0-0-5S) is the most common N fertilizer for corn production in Louisiana, which contains only 25% nitrate-N, leaching loss of N from UAN can be as maximum as 25% of the total N applied.

Right now, corn is at V7 to V9 stage in most of the fields if planted in early to mid-March. To evaluate N losses due to excessive rainfall after sidedress, producers need to wait until V10 stage and take tissue samples at or after V10 stage. Tissue sampling can be done anywhere from V10 to R1 (silking stage) stages but earlier (V10) would be better if the fields experienced water-logged conditions for several days. For tissue testing, the uppermost fully developed leaf with visible collar below the whorl from 10-15 plants should be collected and sent immediately to the lab for total N concentration. The critical (normal) corn leaf N concentration from V10 to R1 stage is 3.0% (Figure 2). Leaf N concentration below 3.0% would be considered deficient (additional N is needed for maximizing yield) and above 3.0% would be sufficient (no additional N is needed). Care should be taken in collecting leaf tissue sample and interpreting N concentration because leaf N concentration can be high due to insufficient plant growth (low dilution) associated with drought, diseases, and pest infestation. Nitrogen losses during the growing season can also be evaluated by NDVI reading from N reference strips if the producers establish it in their corn fields during sidedress N application. For producers who have N reference strips, NDVI readings need to be taken from both reference strip and standard production areas and input this info along with max yield goal (double than expected i.e., 400 bu/ac for 200 bu/ac yield goal), planting date, sensing date, and N use efficiency (NUE, usually 0.5) to the LSU AgCenter sensor-based N rate calculator app developed by Dr. Brenda Tubana (Figure 3). The app would be able to determine whether the corn field requires an additional N application, along with rate, before tasseling.

Once the producers decide to apply pre-tassel N, the rate should be 15 to 25% of the total N applied i.e., roughly 40 to 60 lb N per acre. Producers can choose either dry (Urea) or liquid (UAN) N source. Urea can easily be flown by airplane. UAN should be applied as surface application because high rates of UAN (without water dilution) as foliar application will result severe foliage burn. However, producers can apply UAN through their pivot irrigation system, if available, as fertigation. Regardless of N sources, it would be better to place N fertilizer close to plant base, if possible, with high clearance applicator using “Y-drop” to facilitate rapid uptake, minimize N losses, and avoid foliage damage. Application before an expected rain (about 0.25-inch) or pivot irrigation is recommended to incorporate applied N that will minimize volatilization loss. Further, use N-stabilizers (urease inhibitor) to reduce volatilization loss. Experiments conducted at LSU AgCenter showed that when urea was surface applied at late growth stages, use of N-stabilizer decreased ammonia volatilization losses by 74% and increased corn grain yield by 12 to 25% compared to uncoated urea. Overall, a producer should consider rainfall amount following sidedress N application, field conditions, crop growth, yield potential, and tissue-testing to evaluate N losses and pre-tassel N application.

Figure 1. Nitrogen deficient corn in saturated soils due to excessive rainfall. (Source: Pioneer - Nitrogen Application Timing in Corn Production.)

Figure 2. Critical leaf nitrogen concentration from V10 to R1 stages of corn. (Source: Dr. Trent Roberts, University of Arkansas)

Figure 3. Sensor based nitrogen rate calculator for corn (Source: Dr. Brenda Tubana, LSU AgCenter).

Insect Situation for May 2021

Sebe Brown and Tyler Towles, LSU AgCenter Entomologists

Corn:

Insect pressure around much of Louisiana’s corn has been very light thus far in the growing season. Isolated reports of sugarcane beetles and stink bug injury occurred primarily in April. Insecticide seed treatments performed well, and instances of control failures were rare.

Soybean:

With the severe cold Louisiana experienced in February, redbanded stink bug (RBSB) numbers are low across the state. LSU AgCenter entomologists found their first RBSB adult in crimson clover a few miles south of Alexandria mid-April. In early-May, several RBSB arising from senescing white clover were collected in the southeastern portion of the state. Both adult and nymphal RBSB were collected meaning that oviposition is occurring, and populations are building. Soybean producers in the southern portion of the state should be aware that they are present in the landscape, however, timely planted soybeans in the northern region of the state should escape the risk of RBSB to some degree. The map below shows the parishes that have been sampled in which redbanded stink bugs were found in roadside clover species.

Wheat:

Rice stink bug numbers are increasing in wheat across Louisiana. Numbers required to cause economic damage are extremely high and wheat in the milk stage is most susceptible. LSU AgCenter thresholds are 10% infested heads in the milk stage and 25% infested heads in the soft dough stage.Applications for stinkbugs in wheat at hard dough is not recommended.If an application is warranted, pyrethroid insecticides are economical and effective.