Corn Disease Identification and Management

Boyd Padgett and Trey Price, LSU AgCenter plant pathologists

Common rust

Common rust may be the first disease found in corn fields and usually occurs in the lower-to-mid-canopy. Pustules of common rust are brick red to dark orange, somewhat elongated, and will appear on both leaf surfaces (Figure 1).Common rust will progress during relatively cool temperatures (60-75^oF) combined with rainy weather or heavy dews (6 hours of leaf wetness), and cloudy weather; however, very rarely are fungicide applications warranted for common rust. Warmer temperatures (> 80^oF) will greatly slow common rust development.

Southern rust

Southern rust pustules are more orange than brick red, usually not as elongated, and usually appear on the upper surface of leaves (Figure 2). This disease develops during high humidity/dews (at least 4 hours of leaf wetness) and warm temperatures (80-90^oF) and can continue to develop throughout the growing season. Like common rust, the disease usually initiates in the lower-to-mid-canopy. The disease can reach the upper-canopy during conditions favorable for development. Fungicides may be justified but should be made on a field-by-field basis. The genetic resistance of the hybrid and growth stage (post tassel) and current environmental conditions are factors to consider prior to applying fungicide.

Fungicide considerations

Fungicide application decisions should be carefully considered field by field based on disease severity, crop stage (Table 1), hybrid susceptibility, Corn Foliar Efficacy 2025, 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.

Table 1. Crop growth stage, defoliation, and associated yield loss. Percent yield loss because of defoliation by crop stage. For example…30% defoliation at dent stage results in a 2% yield loss. *% defoliation

Tarnished Plant Bug Management in Blooming Cotton

Dawson Kerns, LSU AgCenter Field Crop Entomologist

As cotton starts to bloom, its time to shift from scouting for tarnished plant bug adults migrating into the field to scouting for nymphs that have established in the field as well. The intensity of plant bug infestations could be exacerbated this year with greater corn acreage and smaller pockets of cotton. Corn can act as a host for plant bugs and large waves of plant bugs are likely to be released into the landscape when corn starts to dry down. It is not recommended to apply neonicotinoids such as Centric and imidacloprid during the blooming window because they perform poorly. The best options during bloom are Transform, Diamond, acephate, and Bidrin. The best time to spray the first application of Diamond is during late squaring to the first week of bloom. This product only targets nymphs, so it is best tank-mixed with acephate or Transform. Transform is a good fit early in the blooming window because it is soft on beneficial insects which can provide some extended residual control. The organophosphates are harder on beneficials and risk flaring secondary pests like aphids or spider mites, but they are cheaper alternatives to Transform. Pyrethroids can be tank-mixed with organophosphates to increase control of tarnished plant bugs and late season stink bugs.

Square retention can be an indicator of insecticide performance in addition to insect counts. It is important to keep in mind that the ThryvOn trait has the greatest activity against small nymphs, so 1^st instar nymphs will sometimes not need to be treated because they will die after a small amount of feeding. Older nymphs with visible wings pads and adults are more capable of surviving and will require an insecticide application.

Strategies to Build Soil Organic Matter in Louisiana

Leandro O. Vieira, Ph.D., LSU AgCenter Soil Fertility Specialist

Over the past three issues of the Louisiana Crops Newsletter, we have explored the importance of organic matter in improving the chemical, physical and biological properties of the soil. But how can we increase organic matter levels in our soils? Despite the challenging climatic conditions of Louisiana (hot and humid weather for a considerable part of the year), soil organic matter can be increased, and some strategies could be adopted. Examples of those strategies are cover cropping, no-till or reduced-tillage practices, and the application of organic amendments.

Figure 1. Illustration of strategies to increase organic matter in the soil. Source: Vieira & Castro (2025). Strategies to increase organic matter in the soil. Fact Sheet, Louisiana State University Agricultural Center, P3998.

Cover crops are crops that are planted with the primary purpose of protecting the soil during the fallow season. These crops will add a considerable amount of organic material to the soil, not only from the aboveground biomass but also from the roots. Cover crops also protect the soil from runoff, which will protect the topsoil that happens to be the most fertile layer with the highest organic matter concentration. In other words, in addition to adding organic matter to the soil, it can protect the organic matter that is already there. Beyond adding organic matter to the soil, cover crops can reduce nutrient leaching (the nutrients released from the previous crop decomposition will be taken up), their root system will improve soil aeration, and nitrogen can be fixed to the soil (especially when legumes are added to the cover crop mix).

Another strategy is the application of organic amendments, such as animal manure, biosolids (treated sewage sludge), biochar, and compost. Their addition can improve microbial activity and soil structure. Additionally, there is a considerable amount of nutrients in those organic amendments. However, it is important to remember that the nutrient concentration is highly variable and will depend on several factors. Some areas of Louisiana have access to chicken litter, which can provide organic matter in addition to many plant nutrients (nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and micronutrients).

Last but not least, no-tilling could be adopted to improve organic matter status in the soil. All the above-described practices have great potential for increasing organic matter in the soil, but no-tilling is likely the one that will provide the greatest long-lasting benefits. In case the adoption of no-tilling is too challenging, reduced tillage can also be beneficial. When a soil is tilled, it is aerated, and the organic matter is exposed to microbial decomposition. Repeated tillage can lead to organic matter depletion, especially in areas with hot and humid weather.

A fact sheet was recently published on this matter, and more details can be found at https://www.lsuagcenter.com/articles/page1748451158008.

LA Soybean June Update: Early Flowering in April Planting Dates

David Moseley, LSU AgCenter Soybean Specialist

The 2025 soybean planting season in Louisiana began rapidly, with 70%-80% of the projected acres planted by May 1, significantly exceeding the average of 40%-50%. Although most of the crop was planted within the optimal window, heavy rainfall and strong winds have led to issues such as failed stands and delayed weed control applications.

There has also been concern regarding April-planted soybeans in some fields across the state that began to flower at the V3-V4 growth stage. I analyzed data from a planting date trial conducted at the Dean Lee Research and Extension Center in 2021 (Table 1), which included four varieties for each maturity group (MG) section. For an April planting date, the average growth stage was V5-V7, which aligns with expectations. However, the observed range extended from V3 to V8. Notably, no other planting date by maturity group combination exhibited flowering at the V3-V4 stage, except for those with a June (MG 3.8-4.4 only) or July planting date.

I compared the rainfall events at the Dean Lee Research and Extension Center for April and May in both 2021 and 2025. In 2021, April had 13 days of rain totaling 13 inches, while May had 17 days of rain with 11 inches. In 2025, the Dean Lee Research and Extension Center and the Sweet Potato Research Station had between 7-9 days of rain (4-7 inches) in April and 12-13 days of rain (7-9 inches) in May. The extended number of days and amount of rain could have caused the plants to flower at an earlier vegetative growth stage.

Although some of the MG 4.5-4.9 varieties planted in April flowered earlier in 2021, their average yield was higher than those planted from mid-March and May - July, though not as much as anticipated. Additionally, there was no decrease in percent canopy coverage at the R4 growth stage for the MG 4.5-4.9 and 5.0-5.6 sections planted on April 29. Final plant height also didn't appear to be negatively affected by the earlier entry into the R1 stage.

The observations from this one year of data indicate there is not a strong correlation with flowering at an earlier vegetive stage and final plant height or yield.

Table 1. Data from a planting date x maturity group x variety trial conducted at the Dean Lee Research and Extension Center in 2021.

Sprayer Drone Outlook

Randy Price, LSU AgCenter Agricultural Engineer

Sprayer drones are new technology for farmers and applicators to apply chemicals and pesticides over fields and land areas. Still, the current sprayer drones are quite small and can only cover 300 to 400 acres per day at best, but the industry is growing quickly, and we do expect over the long term (3 to 10 years) this application method to become a dominate force in spraying. We are already hearing of farms in Brazil that completely use drones for spraying and have eliminated all over forms of spraying equipment (I am not saying we should go that far yet). Reasons for adoption are the low cost of the sprayer drones (currently $30,000 to $50,000) and the ability to spray fields in a non-contact method (which is a big plus in Louisiana during wet periods). Disadvantages are the short flight times (10 to 15 minutes) and the amount of “ground-crew” operations needed to support spraying (charging, refilling tank, etc.). Still, we do expect these limitations to be reduced as equipment becomes better and more capable, as we are already hearing of drones being tested that can carry up to 30 gallons, fly up to 30 minutes, and cover more than 15 acres per flight. We suspect that when sprayer drones reach this level, they will become much more prevalent in American agriculture. For this reason, it is always good to think about the licenses you may need to operate a sprayer drone and the regulations involved, as it can take some time to obtain these licenses. Currently in Louisiana, you need a Part 107 (general commercial drone license with the FAA), a Part 137 (general pesticide business license with the FAA), and register the drone with the FAA (and various other waivers listed (and applied) for the FAA website. On the State level you need to complete the “LSU AgCenter Drone Safety Program”, take and pass the “Commercial General Standards (Core Manual) and at least 1 sub-category exam (AgPest, Aquatic, Right of Way Vegetation Management, or Mosquito, etc.) and the “The UAV Aerial Exam” (Category 11A) (some of these can be done on-line). You will also need to have liability insurance to cover drift claims or accidents and register the drone with the LDAF (as they do not have access to the Federation registration database). If you have any questions, please feel free to call me, or the LDAF.

Irrigation and Fungicide Considerations for Corn During a Rainy Growing Season

Shelly Pate Kerns, LSU AgCenter Corn and Cotton Specialist

Louisiana is expected to increase its corn acreage by 13% this season. The reason for this increase is that prices for corn remain favorable for growers, and advancements in corn hybrid technologies make corn an easily manageable crop under ideal conditions. Irrigation costs can vary greatly from season to season depending on the weather. Consistent rainfall during this season has already netted growers roughly 50% of the water needed to produce a hearty crop, and with more rain expected this week, growers can expect to water less this year than in other years. Especially compared to the drought corn growers experienced in 2023.

Most of the corn in the state has moved into reproductive growth stages. With some fields being only a week and half away from dent stage. Early reproductive stages require a lot of water usage, and if plants experience stress between R3 (milk stage) and R4 (dough stage), plants may abort kernels or kernel weights may be reduced. As plants enter dent stage (R5), plants become more resilient and will start to use less water. However, irrigation is still an important factor in finishing strong because the plants are finishing kernel fill at this stage. When plants reach R6 (physiological maturity), it is important to check the kernel moisture content and the progression of the black layer at the kernel tips. Once the black layer has developed, kernels will continue to dry down until they are suitable for harvest (15% moisture). There are many ways to track rainfall and make irrigation decisions. The LSU AgCenter also has a Drought Irrigation Response Tool (DIRT) to aid with in season irrigation decisions: https://www.lsuagcenter.com/topics/crops/irrigation/dirt

It is important to also minimize other stresses to the crop to optimize plant health and water uptake. Although the rainfall has been good for most fields, water splashes around fungal and bacterial plant pathogens. As the weather became drier the past week or so, reports of foliar diseases have been popping up across the state (Northern Corn Leaf Blight, Holcus Spot, Curvularia, Common Rust). Overall pressure is low across the state. However, some individual fields have been hit hard (especially those that have poor soil drainage and shaded field borders). With more rainfall expected, it will be important to keep an eye on the progression of diseases in the field. If corn is close to dent, a fungicide application will most likely not be needed. However, in the case of later planted corn that is either at tassel or at an early reproductive stage, fungicides should be considered if disease is showing up and spreading quickly. You can find more information on corn diseases and management considerations at the following two links: https://www.lsuagcenter.com/portals/communications/publications/management_guides/plant_disease_guide and https://loss.cropprotectionnetwork.org/crops/corn-diseases

Overall, growers should be able to save some money on irrigation this year due to all of the rainfall experienced so far. As corn gets closer to harvest, it is important to be mindful of irrigation needs up until the plant reaches black layer. Additionally, rainfall can exacerbate plant disease. Therefore, it is important to remain vigilant on disease progression in the field until at least R5 (Dent stage).

Figure 1. Corn Growth Stages

LSU AgCenter Specialists