Join us this morning as we discuss the final “R” in our nitrogen application series! #AskTheAgronomist
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#AskTheAgronomist: What’s the Right Nitrogen Application Rate?
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Stalk Lodging
Credit: The Ohio State University Stalk lodging can lead to reduced yield and grain quality. Lodging refers to the breakage of the stalk below the ear and is often connected with stalk rots, soil-borne pathogens and other environmental stressors that occur after pollination.
Causes of Stalk Lodging
Carbohydrate demand during grain fill is the primary driver of stalk lodging. When carbohydrate reserves in corn leaves are depleted during ear development, the plant will begin sourcing the macronutrient from stalks and roots. Environmental stresses can feed into this cycle. For example, warm weather and high temperatures increase plant growth which can lead to high demand for carbohydrates.
While energy demand during grain fill is the primary cause of lodging, fungal pathogens can reduce stalk vigor. Stalk rot pathogens can also weaken stalks and are another major cause of lodging. Generally, if more than 10 to 15% of stalks exhibit rotting, stalk lodging is likely to be a significant problem.
As it relates to weather, high winds and heavy rainfall can also cause stalks to become physiologically weak and eventually lodge. Injuries from hail or frost damage, or insect feeding that creates lesions on leaves, can reduce the amount of energy created by plants. This leads to a higher use of stalk carbohydrate reserves.
Nutrient deficiencies are also a major cause of stalk lodging. Excess nitrogen and low levels of potassium have been associated with reduced stalk quality. Maintaining sufficient potassium levels in corn is especially important as it is the building block for leaf and stalk tissue.
Scouting and Treatment Options
When scouting for stalk lodging, choose ten consecutive plants across ten different areas of the field. Check for signs of lodging by pinching the stalk at the first or second internode above the ground, or by pushing the plant over 8 to 12 inches at ear level. If the plant fails to stay standing upon exerting force, it is considered lodged.
Adjust your harvest plan to harvest lodged corn first before lodging becomes more widespread. Selecting the right corn hybrid for your farm can mitigate issues with insect feeding that would otherwise reduce stalk quality. The level of resistance to stalk lodging among hybrids is highly variant and some have genetically stronger stalks than others. Be sure to consult with your local Latham representative to determine the best management strategy for stalk lodging.
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Sclerotinia Stem Rot or “White Mold” in Soybeans
Credit: University of Wisconsin Sclerotinia stem rot – also known as White Mold –can lead to significant yield loss in soybeans. Yield loss from white mold is attributed to the damage it causes to leaves, pods and stems. Research from the University of Wisconsin estimates the disease has costed growers in the U.S. and Canada 101 million bushels of soybeans – equal to $1.2 billion.
White mold is caused by the fungus Sclerotinia sclerotiorum and overwinters in the soil for a number of years. After the fungus emerges from the soil, mushroom shaped structures known as apothecia will form on the soil surface. The apothecia, ranging between ¼ and ½ an inch wide, will first infect through soybean flowers before it reaches the stem.
While the fungus primarily spreads through the air via spores, moisture is required for infection to take place. As a result, cool and wet weather along with high humidity are the main causes of white mold.
Credit: University of Wisconsin Scouting for White Mold
While Sclerotinia sclerotiorum is often confused with other fungal pathogens, the sclerotia distinguishes white mold from other look-alike diseases. Symptoms of white mold will be most prevalent between R3 and R6. Infected plants will exhibit white fuzzy growth on the lower stem.
Soybean blossoms are the first area of the plant to exhibit signs of infection and neighboring stems and pods may appear water-saturated. After infecting blossoms and pods, white mold may eventually spread through the entire stem causing it to turn tan or bleached of color. As the mold growth becomes thicker, black spots will begin to surface throughout the fuzzy white surface.
White mold also creates foliar symptoms, causing leaves to completely die while still attached to the stem. In infected soybeans, the tissue area between the leaf veins will turn gray and cause leaves to become wilted and curled.
Treatment Options
Due to the overwintering nature of the sclerotia, a two to three-year rotation away from soybeans is advised. When it comes to genetics, some soybean varieties are more resistant to white mold than others. Varieties with resistance to the fungus may recover better than others. It is also important to consider planting practices that cause white mold growth. Shade created from high plant density and growing canopies can lead to the fungal disease. Increasing row width and reducing planting populations are the best methods to improve airflow through the canopy and reduce infection from white mold.
In addition, broadleaf weeds are notorious for hosting white mold and herbicides should be used to control weeds. Fungicides are a viable method for treating white mold and can reduce the negative impacts incurred by soybeans. However, they are most effective when applied just before infection takes place. Be sure to consult with your local Latham representative to determine the best treatment plan for white mold.
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Physoderma Stalk Rot
There are a variety of stalk rots that infect corn, causing extensive damage to crops and losses in yield. Common factors make corn susceptible to stalk rot including warm and wet weather, stress after pollination, fertility issues, stalk boring insects, and the presence of other foliar diseases. There are key signs, symptoms and differences that distinguish the different types of stalk rot.
Credit: Crop Protection Network Scouting for Signs and Symptoms
Physoderma stalk rot is caused by the pathogen Physoderma maydis, the same fungus responsible for causing Physoderma brown spot. The fungal disease seems to be showing up in more and more corn fields each year, but typically shows up on random plants and and has minimal impact on yield. Like most stalk rot diseases, warm and wet weather favor the development of Physoderma stalk rot.
Physoderma stalk rot infects corn between the V4 and V9 stages. The disease is not associated with any foliar signs, so it is important to inspect plants closely at the base. Dark brown or black lesions will appear at the base of the stalk, and rotting of the pith will be observed upon splitting the stalk open. Overtime, blackening of the pith will move to higher nodes. Sporangia can also be found on the outside of nodes and within the rotted pith tissue.
Scout for symptoms of Physoderma stalk rot across five areas of the field. Stalks will make a distint “pop” and snap at one of the first 3 nodes above the soil line. If more than 10 to 15% of plants exhibit stalk rot, the field should be harvested early.
Management Strategies
With this being a newer disease we are still learning differences in hybrid tolerances to both stalk and foliar phases of this disease.
As a majority of stalk rots overwinter, one to two-year rotation away from corn and controlling corn residue are key for preventing the return of the disease. Fungicides may also be applied to prevent Physoderma leaf blight , but these studies are in early phases as well given the novel nature of this diesease
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Corn Aphids
Credit: Purdue University Corn aphids have the ability to negatively impact yield end of year. Proper identification and management can protect your crop.
Identification
Corn aphids have a blue-green to gray appearance and are 1/16 inch in length. Aphids extract sap from corn plants and excrete honeydew. The honeydew creates sooty-looking mold that can interfere with pollination.
As opposed to laying eggs, female aphids reproduce by giving birth to nymphs. The insects can build large colonies comprised of more than one species such as bird-cherry oat aphids. Aphids are most often wingless, but when the size of their colonies begin to grow, some can form wings to migrate to other corn plants. Corn leaf aphids cannot overwinter and usually migrate to southern states ahead of the cold months.
Scouting and Implications
Aphids will begin to appear in corn around mid-June and early July. Mature aphids can be found near the tassels, silks and the base of the corn stalk. The insects predominately live in the whorls where they are protected from outside elements. Infected corn plants may exhibit stunting or wilting and curling of the leaves
When scouting for aphids, select 20 non-consecutive plants from five different areas of the field. Unroll the whorl to count the number of insects present. The general treatment threshold advised by Iowa State University is when 50% of corn plants have more than 100 aphids per plant.
Overall, aphid infestations have been found to reduce grain quality and decrease kernel size. A study from Penn State University finds that 30-40% of aphid-infested corn stalks will become barren or earless. Drought stress can also worsen the impacts of aphid feeding and should be included when considering a treatment plan.
Treatment Options
The best time to begin manage aphid-infested corn is two to three weeks prior to tasseling. Corn plants with aphid colonies above the ear are more susceptible to yield loss than plants with aphids found below the ear.
Since aphids mainly colonize inside corn whorls, spraying insecticides is not the most effective solution. Insecticide application is most effective when aphids are found on the outermost areas of leaves. If corn plants that have reached the hard dent stage, applying insecticides will not pay.
Fungi and other beneficial insects including lacewings and lady beetles can reduce aphid populations. Look for signs of bloating or discoloration in aphids – this could be evidence of fungi at work. If over 20% of aphids appear to be parasitized, consider holding off on a management plan. Be sure to talk with your local Latham representative to develop an effective treatment strategy for managing aphids.
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Japanese Beetle in Corn and Soybeans
Credit: Purdue University Japanese beetles cause damage to corn by interfering with pollination and lead to defoliation in soybeans. Their distinct metallic-green heads and bronze wings make them distinguishable from other look-alike beetles. The insect undergoes one life cycle per year with adults reaching 5/16 inches in length.
Eggs are laid during July and August and overwinter until temperatures rise in the spring. For both corn and soybeans, adults migrate from grass and begin feeding in late May or early June. The peak emergence for Japanese beetles occurs 4 to 5 weeks after feeding begins.
Japanese beetles tend to feed in concentrated areas and in groups. After the female burrows into the soil, she will lay 1 to 4 eggs at a time for several weeks. Adult grubs feed on turf grass in late summer and early spring before emerging to feed on crops.
Corn In corn, Japanese beetles can feed on silks and interfere with pollination. Though leaf feeding in corn is possible, a majority of the economic impacts are attributed to silk feeding. Plants should be treated if there are three or more beetles per ear.
Research from the University of Tennessee found that repeated clipping of silks did not always lead to yield loss. More importantly, the environment has a stronger influence on the damage Japanese beetles can cause. Higher yield loss in infested corn plants that were under drought stress as opposed to plants that were not.
Soybeans
In soybeans, the beetle strictly feeds on the soft tissue bordering leaf veins, leaving them skeletonized. Soybeans growing in sandy soils are even more susceptible to defoliation from Japanese beetles. Though yield loss from Japanese beetles has not been quantified, figures from North Dakota State University (see table) point to the general yield impacts from defoliation. Research from the University of Nebraska advises treatment when damage reaches 30% defoliation before bloom or 20% defoliation during flowering. As long as beetles appear to be actively feeding, insecticide application is advised.
Other Considerations
Japanese beetles are easily confused with other species of beetles, including masked chafers and May or June beetles. Be sure to consult with your local Latham representative to confirm suspected Japanese beetle infestations and determine the right treatment plan.
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Photo Credit: Purdue University
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Frogeye Leaf Spot
Credit: University of Nebraska Frogeye leaf spot is a foliar disease caused by the fungus Cerospora sojina. The fungus primarily spreads from infected plants through air and water droplets. During particularly wet years, frogeye leaf spot can lead to yield loss as high as 30%.
Warm, humid weather and heavy rainfall are key drivers of frogeye leaf spot. Areas with standing water and high moisture are notorious for hosting the fungus and fuel the damaging disease. Soybeans grown year after year in the same field are also more susceptible to frogeye.
Scouting for Frogeye
Frogeye leaf spot most often infects plants after flowering, and signs are most evident in the upper canopy of plants. Dark spots with a gray center and a red-purple border will form on infected eaves. The smaller spots can join to create larger lesions, leading to defoliation that can reduce photosynthetic leaf area.
In addition to causing defoliation, frogeye leaf spot often leads to premature leaf drop. The disease can also infect stems and pods. Later in the growing season, reddish-brown lesions will form on stems and turn the centers of the stems gray. Gray or brown cracked seeds can also form as a result of the disease and pods will be discolored with long lesions.
Treatment Options
Selecting varieties with resistance to frogeye leaf spot is the best way to prevent the disease. As the fungus Cerospora sojina is able to overwinter, tillage practices that reduce weeds and bury residue will decrease the likelihood of future infection.
Fungicide application for treating frogeye leaf spot is most effective between the R2 and R5 growth stages. Be sure to consult with your local Latham representative to determine the best strategy for managing frogeye leaf spot.
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Anthracnose Stalk Rot
Credit: Crop Protection Network There are a variety of stalk rots that infect corn, causing extensive damage to crops and losses in yield. Common factors make corn susceptible to stalk rot including warm and wet weather, stress after pollination, fertility issues, stalk boring insects, and the presence of other foliar diseases. There are key signs, symptoms and differences that distinguish the different types of stalk rot.
Anthracnose stalk rot is the most common type of stalk rot and is caused by the fungus Colletotrichum graminicola. The fungus is favored by wet, warm weather and overwinters in corn residue. Signs of the disease will be observed four to six weeks following pollination.
Scouting for Signs and Symptoms
The disease undergoes three phases with distinct signs and symptoms:
- In the first phase, foliar lesions will appear in the early part of the growing season. The leaf blight will begin on the lowest leaves, and will reach the upper leaves by late season.
- During the second phase, top-dieback will appear in the middle part of the season after tasseling, killing the parts of the plant located above the ear.
- In the third phase, shiny black lesions will appear on outside surface of stalks. Look for setae, or bristles that cover the surface of the stalk. A hand lens can be used to look for the bristle-like texture and black dots in the center of lesions.
Stalks will exhibit fragility and appear to be brittle when handled. Different from other forms of stalk rot, anthracnose stalk rot will cause plants to lodge at the upper portion of the stalk. Pinching or bending at the nodes can be used to test for stalk lodging.
Stalk rot can lead to death just before maturity and reduce yield. In addition, plants defoliated from hail damage and those that are nitrogen deficient are at an increased risk for being infected from the stalk rot.
Management Strategies
Planting hybrids with resistance to stalk rots is a helpful defense against these diseases.
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Rust Diseases in Corn
Southern Rust. Credit: Crop Protection Network Common Rust and Southern Rust infect corn in the late summer. The diseases generate raised spores known as “pustules” on the surface of leaves, leading to reduced yield and poor grain quality.
Common rust (Puccinia sorghi) and southern rust (Puccinia polysora) fungi are unable to overwinter in the Midwest and require a host plant to remain alive. The spores created by rust diseases are transported by wind to the Midwest from Southern states.
Temperatures ranging from 61–77° fuel the growth of rust diseases. Cool and humid temperatures, especially when exhibited overnight, can further drive the development of the fungi.
As long as the weather conditions are right for rust diseases, the cycle of spore development will continue. The return of hot and dry weather can prevent further development of the fungus and kill off the spores.
Implications
Rust can reduce yield and decrease grain quality. Foliar damage from rust diseases can interfere with water transpirationand reduce photosynthetic leaf area.
Nutrients designated to support plant growth are rerouted in response to the damage incurred by leaves. Damage from rust diseases deplete carbohydrate reserves in corn leaves. As a result, the plant will begin sourcing the nutrients from stalks and roots, leading to reduced yield and stalk rot.
Common Rust. Credit: Crop Protection Network Scouting for Rust Diseases
While common rust has less of an impact on yield, southern rust has been found to reduce yield by 25 bu/acre in corn with no fungicide application. The fungi can begin to infect plants under favorable conditions in as little as six hours.
Southern rust signs are evident on the upper leaf surface and are round, as opposed to elongated in plants infected with common rust. The pustules will be orange compared to the darker color of common rust. Overtime, southern rust pustules will become brown or black.
Common rust pustules are found on the upper and lower leaf surface and are oblong. Common rust pustules will be brick red in appearance and can coalesce to kill parts of leaves. In order to determine the difference between common and southern rust pustules, use a magnifying lens to inspect the leaf surface.
Management
Planting early is one of the best ways to reduce corn’s vulnerability to rust diseases. Corn planted late in the season is most susceptible to experiencing yield loss and grain damage. Many hybrids are also equipped with resistance to rust diseases, though the extent to which they are protected from the fungus can vary.
When pustules are observed on 50% of scouted plants, it is advised to begin implementing a treatment plan. Fungicides can also be used to treat corn infected with common and southern rust. Be sure to consult with your local Latham representative to determine the best management options for rust diseases.
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Herbicide Applications Update for Xtend® Soybeans in 2020
Before you spray Xtend soybeans this spring with a dicamba-based herbicide, remember that federal law requires that all applications be made by a properly trained certified applicator. Both custom or private applicators must attend annual training, so they’re up-to-date on all new rules and regulations. Most of the live training sessions have been cancelled due to the coronavirus pandemic. However, there’s still time to complete online training. Listed below are websites, by state, where you can get information about online training courses. I’ve also added some comments from state Departments of Ag about possible 2020 changes to the herbicide labels.
Iowa, North Dakota, South Dakota, and Wisconsin: No known changes to the state label in any of these states. The individual state DOA’s direct applicators to the various company websites listed below for their annual training.
Minnesota online: Applications needing dicamba training should visit https://mcpr-cca.org/dicamba-information-trainings/. Some farmers were hoping the MDA would loosen its restriction on how late dicamba applications may be made. The MDA is holding fast to June 20 or the R1 stage of the crop, whichever comes first. No changes have been made from 2019 requirements.
Nebraska: The Nebraska DOA is directing applicators to this UNL Extension website for dicamba training https://pested.unl.edu/dicamba. No listed changes to the 2019 guidelines for application.
Illinois: The IDOA has further restricted applications of dicamba herbicide over the top of Xtend soybeans to no later than June 20 or the R1 stage of the crop, whichever comes first. NOTE: This is a change from 2019 when the last date of application was June 30. For training, the IDOA directs applicators to this website, sponsored by the Illinois Fertilizer and Chemical Association: https://ifca.com/IllinoisDicambaTraining.
There are four dicamba herbicide formulations approved for use over the top of Xtend soybeans. They are XtendiMax® with VaporGrip® Technology from Bayer CropScience, Engenia® from BASF, FeXapan® with VaporGrip® from Corteva and Tavium® with VaporGrip® from Syngenta.
All four companies provide online dicamba application training for applicators. NOTE: You’re only required to attend one online session, regardless of which herbicide you eventually apply. Here’s the list of the various company sponsored training websites:
• BASF: https://bit.ly/2xTlJV9
• Bayer CropScience: https://bit.ly/2RjHt3e
• Corteva: https://bit.ly/2UTSpH7
• Syngenta: https://bit.ly/2V7OuFv
May God keep you and your family safe as we work together to provide the world with much-needed food and supplies during this difficult time. You are a blessing to all!
