With the amount of rice flowering for the past couple of weeks I was expecting to get more calls about the disease shown at right. There are several things to look at in the photograph that help us identify the disease. The base of the some of the pedicels (where that part of the plant joins the spikelet) are discolored. Note its discoloration compared to others that are green. Also take a look at the band of slightly darker area of some of the spikelets in comparison to those that are completely straw colored. Third diagnostic aids are the green panicle branches. What cannot be seen in the photograph is the absence of grain in the affected spikelets.
The disease is bacterial panicle blight. It is associated with high night time temperatures during flowering. Just as a reminder, no matter how much fungicide is used it will not control this disease. Fungicides control fungi NOT bacteria. We have nothing to control bacterial panicle blight yet. The only thing we can recommend is to plant a variety that is resistant or moderately susceptible or plant early to avoid flowering when night time temperatures have risen.
I have received numerous calls from producers, consultants, and agents reporting diseases in corn. These reports are earlier compared to previous years, and disease management strategies will change based on this fact. The diseases most prevalent are southern rust, common rust, and northern corn leaf blight. The LSU AgCenter DOES NOT recommend an automatic fungicide application to corn. HOWEVER, when disease epidemics are progressing in young corn (tassel or earlier) a fungicide will be needed to slow epidemics and protect yield and quality.
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. When it is determined an application is needed, a premix fungicide will offer wide spectrum activity (examples are: Headline AMP, Stratego YLD, Quilt, and Quilt Xcel). Follow label instructions for application timings, rates, etc. In most cases, a single application at tassel is justified when disease is present and active. The decision to apply a fungicide should be made on a field by field basis. The remainder of this newsletter will address disease identification and management considerations.
Diseases can be found in Louisiana corn fields every year, but their impact on grain quality and yield is dependent on several variables. Therefore, the decision to apply a fungicide should be based on a solid understanding of disease initiation and development. Diseases commonly found in Louisiana corn include common and southern rusts, smut, and northern corn leaf blight. Other diseases that occur less frequently are ear and stalk rots, gray leaf spot, and southern corn leaf blight. While these diseases rarely develop to statewide damaging levels in most years, disease incidence and severity in individual fields may warrant a fungicide application. However, before applying a fungicide, several factors need to be considered. These include: disease identification, environmental conditions favoring disease development, and the relationship between disease severity and yield loss.
Identification and Development
Common rust usually can be found every year in Louisiana (figure 1). This disease is caused by the fungus Puccinia sorghi. In some fields signs of this disease can be present early in the growing season. Common rust is usually the first disease present in Louisiana corn, but subsides when temperatures exceed 77oF. Initial infections occur from wind-blown spores from corn-producing areas in tropical and sub-tropical regions of the world. This disease usually does not cause yield loss in Louisiana corn.
Conditions favoring development:
Moisture period: 6 hours of leaf wetness or high relative humidity
This disease can be found in fields prior to tasseling. Sporulation on the leaf surface can occur within 7 days after infection. Pustules are elongated, ragged looking and occur on the upper and lower leaf surfaces. Spores with pustules are cinnamon-brown in color. In some cases, pustules occur in bands because of infections that occurred while the leaf was in the whorl.
Southern rust, caused by the fungus Puccinia polysora, is also present in Louisiana corn (figure 2). Similar to common rust, initial infections are caused by wind-blown spores. This is a warmer-season rust compared to common rust and usually occurs late season and does not have adequate time to impact yield. However, this rust is very aggressive and if disease epidemics initiate early (prior to or at tasseling), yields could be reduced.
Conditions favoring development
Moisture: High relative humidity or abundant rainfall
Southern rust produces small circular to oval pustules and contain orange to light brown spores. The spores are lighter in color when compared to spores associated with common rust. Pustule size is usually smaller and less ragged looking than those produced by the common rust pathogen. Pustules are more abundant on the upper leaf surface and can also be found on the leaf sheath when disease is severe.
Northern Corn Leaf Blight
Northern corn leaf blight is caused by the fungus Exserohilum turcicum (figures 3&4). This disease was present at damaging levels in some fields during the 2010 growing season. The disease can be found in Louisiana corn fields during mid-season (tasseling/flowering) and, in some cases, can cause yield loss. The fungus can survive on infected corn debris left on the soil surface from the previous growing season. Therefore, the risk to this disease increases in fields where reduced-tillage practices are used and corn is planted continuously. Corn debris from the previous season provides inoculum for disease initiation and establishment. Spores produced on this debris are disseminated by wind and rain splash infecting the current crop. Subsequent infection results from spores produced within lesions on the current crop. There are several races of this pathogen. Therefore, the effectiveness of genetic resistance may vary depending on the races present in a particular field.
Conditions favoring development
Moisture: 6-18 hours
Lesions of Northern corn leaf blight usually begin in the lower canopy and progress upward. Lesions begin as small elliptical or spindle shaped lesions. Mature lesions can be six inches in length and about ½ to 1 inch wide. The lesions are grayish green in color.
Less Common Disease and Abnormalities
Gray Leaf Spot
Gray leaf spot is caused by the fungus Cercospora zeae-maydis. The fungus can overwinter on infected corn debris from the previous season. Therefore, risk to disease is increased when corn is continuously cropped and reduced tillage allows debris to overwinter.
Conditions favoring development
Moisture: Repeated moisture over 11 or more hours OR high relative humidity
(95% or more)
Initial lesions are rectangular, small, necrotic, with yellow halos. As lesions mature, they expand and turn gray where the fungus may sporulate on the underside of the leaf.
This disease is caused by the fungus Ustilago madis and is generally not thought to impact yield. This disease is usually present at very low levels in every corn field, and is most severe when actively growing tissue of young corn is wounded. The fungus overwinters on infected corn debris from the previous growing season or in the soil (for many years). The fungus is NOT seedborne, as is the case with some smuts in other crops.
Symptoms can occur on foliage and ears and are very evident. Individual kernels enlarge and are silvery gray in color. Diseased kernels can be cut in half to reveal black sooty spores.
Purple leaf sheath
Each year this abnormality can be found in some corn fields within the state. THIS IS NOT A DISEASE. While fungi and bacteria are associated with this condition, this is not harmful to the plant. The purple discoloration on the stalk and leaf sheath results from colonized (fungi and bacteria) pollen that is lodged between the sheath and stalk.
Risk and Management
Risk to disease is influenced by several factors including: genetic resistance, tillage practices, planting date, and environmental conditions. Plant debris left on the soil surface (no-till or reduced-tillage production systems) harbors disease pathogens which increase the risk to disease. Later planted corn can also heighten risk to southern rust and possibly common rust. A favorable environment will always increase risk; therefore, it is important to know what conditions favor disease development.
The first line of defense for managing corn diseases should be selecting a disease-resistant variety. When resistance is not available, a fungicide may be needed. However, when disease is not present a fungicide is not necessary. Another factor to consider is when disease epidemics initiate relative to crop growth stage. The potential for yield loss is high when disease develops prior to tasseling and conditions remain favorable for development during the growing season. When disease initiates after tasseling, the potential for disease loss decreases. Therefore, a fungicide may not be needed even in the presence of disease.
The relationship of yield and defoliation can be found in table 1 adapted from the National Crop Insurance Service ‘Corn Loss Instruction’.
Fig. 1. Common rust. Spores brick red.
Table 1. 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.
Louisiana corn producers were blessed for the most part with good soil moisture during the optimal planting window. While the drought of the previous two years is still fresh in everyone’s mind, the late winter and early spring rainfall that moistened the soil profile and refilled many bayous and irrigation reservoirs is most appreciated. However, as temperatures rise and the winds blow steadily, many producers are finding their soil moisture is moving from abundant to scarce rapidly. Early planted corn in particular, is rapidly reaching a critical phase of development and should not be left without adequate water. It may be hard to fathom, but corn planted in late February is approaching tassel. This phase of development is critical to the crop’s success and adequate water is vital. Note in Table 1 that as corn matures beyond 12 leaves, water consumption increases to over 2 inches of water per week. Two inches of rain/irrigation water is equivalent to 54,305 gallons of water per acre. Be sure to know the output capacity of your irrigation wells and how many acres it must cover, then schedule accordingly.
When to get started:
If your fields still have adequate moisture due to timely rainfall, you do not have to water based on crop stage alone. In fact, watering young corn in particular, that already has adequate soil moisture, may promote an unnecessarily shallow root system. Growers and consultants may wish to consider implementing a watering budget to help guide irrigation decisions. The University of Arkansas has a program called the Irrigation Scheduler that is based on soil texture and average pan evaporation rates, and may prove useful . You can find it at:
Dr. Dewey Lee, Feedgrain Specialist for the University of Georgia, published a corn water use table that is applicable to Louisiana (Table 1), and growers and consultants can use it to predict water use for their crop. The only requirement is to know either the planting date (so you can use the “Days After Planting” Column) or the current growth stage of your corn crop (so you can use the “Growth Stage” Column). This table will provide a good basis of understanding on how much water corn consumes as it develops. Key items to note when thinking about irrigation are how quickly water use increases as the crop matures, how critical adequate water is during tasseling, and how much an adequate water supply is still important right up to “black layer” or physiological maturity.
Table 1. Estimated Water Use of Corn in Georgia (115-119 day maturity) CREDIT: Dr. Dewey Lee, University of Georgia
Inches Per Day
Inches Per Week Equivalent
Emergence and primary root developing
Two leaves expanded and nodal roots forming.
Four to six leaves expanding. Growing point near surface.Other leaves and roots developing.
Six to eight leaves.Tassel developing. Growing point above ground.
Ten to twelve leaves expanded. Bottom 2-3 leaves lost. Stalks growing rapidly. Ear shoots developing. Potential kernel row number determined.
Twelve to sixteen leaves. Kernels per row and size of ear determined. Tassel not visible but about full size. Top two ear shoots developing rapidly.
Tassel emerging, ear shoots elongating.
Pollination and silks emerging.
Milk stage, rapid starch accumulation.
Early dough stage, kernels rapidly increasing in weight.
We have recently received reports of corn plants snapped off or crimped and laid over. The injury is most often associated with violent storms producing winds, but not always. “Brittle snap” or “Green snap” is much more widespread in the Midwest and High Plains where high winds can cause substantial damage, but occurs in Louisiana from time to time, particularly during periods of active weather. The damage usually occurs below the point on the stalk where the ear sets, is very visible, and generally results in total yield loss for that plant once the stalk has broken or severely crimped and fallen over. There are several factors that can contribute to brittle snap, including wind speed and direction, growing conditions, and hybrid type.
High wind speed, particularly when it hits perpendicular to the corn rows, is the main factor causing brittle snap. Injury may be more likely to occur during wind downbursts from a storm cloud, creating areas or pockets of damage in a field. If the winds hit during cooler periods of the day when transpiration is reduced and the plant is more turgid, the injury may be more widespread.
Growing conditions can be a factor as well. Warm weather, adequate soil moisture, and high soil nitrogen levels allow for rapid vertical stalk growth from roughly V5 to V18. Rapid cell elongation is occurring – thinning the cell walls – leaving the stalk vulnerable to snap. Inadequate potassium in relation to the amount of nitrogen available may exacerbate thin cell wall issues.
Corn seed producers recognize there are differences between hybrids as to their susceptibility to brittle snap. Some companies take the time to rate and report brittle stalk ratings, and it is worth taking these ratings into account when making seed buying decisions. Thin, rapidly growing stalks, high ear placement, and inherently thin cell wall structure in some hybrids make them more vulnerable to brittle snap than others. It may be that a few hybrids are susceptible enough that equipment (such as a boom) running through the field could knock over the plants, so producers should be aware if this is occurring and consider a different hybrid in the future.
by J Stevens, Associate Professor and Extension Soils Specialist
As of recent, I have become aware that there are some fertilizer dealers who might be unaware of some of the differences in the fertilizers they are handling and selling to producers. Let’s take Sulfur and look at it first.
The use of sulfur in soil fertility programs has become more routine. The most common chemical forms of sulfur used in fertilizers are sulfate-sulfur and elemental sulfur. However, these two forms of sulfur react quite differently in the soil. It’s very important to understand the differences between sulfate-sulfur and elemental sulfur in order to use these two forms in the most effective manner possible.
Plants can only absorb sulfur through their root system in the sulfate form. Thus all soil sulfur must be converted to a sulfate in order to be utilized by plants. For the most part, sulfates move freely with soil moisture, especially in the upper part of the soil profile. This is very much like nitrate-nitrogen in soils. As a result, sulfate levels frequently increase with increasing depth in the soil profile. Like nitrates, sulfates can leach in sandy-textured and silt loam soils.
Elemental sulfur is totally unavailable to plants. Plant roots cannot absorb elemental sulfur. Elemental sulfur is inert and is water insoluble. When elemental sulfur is added to a soil, it has to be converted to the plant-available sulfate form through the activity of soil bacteria. The rate at which this conversion takes place is the determining factor regarding the effectiveness of elemental sulfur as a fertilizer source of sulfur. This transformation of elemental sulfur to the plant-available sulfate form is a slow process often taking months to be accomplished. Thus, for most crops in the initial sulfur fertilization, a sulfate fertilizer like Ammonium sulfate is recommended and elemental sulfur is not.
Now, let’s look at Zinc, specifically zinc sulfate and zinc oxysulfate. Most of the formulations of these two products contain 35.5 – 36% zinc. Among the inorganic zinc sources on the market, the most common sources are sulfates, oxides, and oxysulfates. Zinc sulfate is essentially 100% water soluble, while the Zinc oxides are essentially insoluble in a single crop year, thus unavailable to the crop to be planted. Many agronomists consider the oxides to be ineffective as a fertilizer source. Oxysulfates are a mixture of sulfates and oxides, with varying proportions of sulfates and oxides. The solubilities of the oxysulfates vary considerably, from 0.7 to 98.3%. The effectiveness of these can be highly variable. Low solubility materials may have some value in a long-term build up program, however, when immediate results are the goal, highly soluble fertilizers are the best choice. It is suggested that in order to be effective, a Zinc fertilizer should be at least 50% water soluble.
I’ll leave you with a few questions to ponder; Are you using sulfur and/or zinc in your soil fertility program? Are you soil testing to determine if your crops could benefit from adding one or both of these nutrients? If you are applying sulfur and/or zinc, have you ever considered which form is being field-applied? The answers to these questions could lead you toward a better soil fertility program and enable your crops to improve their yields as they come closer to reaching their genetic potential.
If you have any questions on this article or would like to discuss your soil fertility program, please feel free to contact me by email JStevens@agcenter.lsu.edu , telephone, 318-308-0754 cell, or text.
Louisiana is projected to plant a little over one million acres of soybeans in 2012. Strong exports and demand for soybeans continue and soybean acres may increase throughout the planting season. The acres planted will probably be limited by weather. If the weather is favorable, we could see up to 1.1 million acres of soybeans planted. Each year seems to present different problems for production, hopefully this will be the year without one.
In Louisiana approximately 60% of the acres are planted to maturity group IV’s and 35% of the acres to maturity group V’s. The remaining 5% is planted to maturity group III’s and VI’s. Then trend has been toward maturity group IV’s.
Soybean planting got started early this year with a few acres being planted in late March. As planting continues to progress, the question always arises – What is the optimum plant population?
Too dense a plant population reduces yields, encourages diseases and lodging and increases seed cost. When calibrating planters, use seed per foot as your guide rather than pounds of seed per acre. In the following table, the estimated pounds per acre should be used only to calculate how much seed to buy. Because of varietal difference in seed size, as well as seasonal variation within lots of the same variety, planting rates can be misleading if expressed in pounds per acre. The following rates are recommended:
When planting is delayed until June 15 or later, the amount of vegetative growth that the plant produces becomes more critical. It is important to choose varieties that grow rapidly in a short time. When blooming starts, most vegetative growth ceases in determinate varieties. When planting late, seeding rates should be increased to compensate for reduced vegetative growth.
Recently I was talking to a producer who wanted to learn about making variable rate applications of nitrogen. The first question he asked was: “How much was am I going to save by making the application variable rate?” My response was not what he expected. I said “Don’t look at it as saving money, but as making money.” By matching the optimum nitrogen rate to the corresponding soil/production zone, crop use efficiency is highest and the potential profit from the nitrogen application is maximized.
To make a variable rate nitrogen application, a producer has to define the application zones. This can be based on soil types, Veris Ec soil zones, yield maps, producer knowledge or a combination. The producer’s knowledge of the field along with a yield goal helps determine the nitrogen rate assigned to each zone. The total amount applied to a field with a variable rate application may not be much different than if a producer had gone with a single rate, but by putting the correct rate in the right area the field doesn’t have areas with over or under applications of nutrients.
Soil Sampling is an essential part of variable rate applications, whether it is grid sampling or zone sampling. Each method would benefit from the addition of yield map data to the analysis. Yield maps over several crops and several years can help define the potential yield and profitability of a field. It can also assist with the definition of productivity zones for a field. This is especially obvious when a cropping history is developed over several years.
Variable rate applications of lime, P, K, and other essential nutrients need to be applied in areas defined by the sampling pattern (grid or zone). Variable rate applications of other nutrients is the most cost effective and efficient method for supplying crop needs. Variable rate also allows a producer to match fertility needs to the current crop’s needs. Supplying/maintaining fertility levels enhances the nitrogen efficiency and use by the crop.
The most useful piece of precision ag equipment a producer can own is a yield monitor. A yield monitor gathers the information from the field with which a producer can evaluate how well fertilizers, varieties, etc. performed. Verification strips of a nutrient, nitrogen rate, or another input can be used as a comparison for the rest of the field. Analyzing the results as whole strips and soil/production zones allows a producer to determine the most productive/economical practices to use on their farm.
Precision agriculture, its use, the results, and the incorporation of the practices into a farming operation is a long term process which can enhance the productivity of a farm. For more information or assistance with precision ag applications or yield data on your farm, contact Dennis Burns at 318-267-6709 or R.L. Frazier at 318-267-6714.
I have had some reports of armyworms and leaffooted bugs in commercial wheat fields. True armyworms are primarily an early season (spring) pest with a strong preference for grass crops. Usually greenish in color with orange strips running down the lateral edges of the body, true armyworms typically feed at night and during overcast days. During the day, true armyworms can be found under debris and thatch on the soil surface. In Louisiana infestations normally occur in April, but with the unseasonably warm weather, early infestations from a multitude of pests can be expected. Scout for this pest during the early morning, late evening or look for larvae on or under the soil surface. Larvae feed on the foliage of wheat plants from the base and gradually work their way up towards the flag leaf. Once the wheat has reached milk stage, the plant can tolerate greater levels of defoliation and see little to no yield loss. However, if armyworms begin to feed on or clip the wheat heads substantial yield losses can occur. Thresholds for Louisiana are 5 or more larvae per square foot with foliage loss occurring. True armyworms can be controlled with pyrethroids. If an application for armyworms is justified, use enough carrier to adequately penetrate the wheat canopy. Applications made during the late morning or afternoon may miss some armyworms in thatch or near the soil surface when direct sunlight and warm temperatures are abundant.
Leaffooted bugs are similar to stink bugs with regards to their piercing sucking mouth parts and foul odor excreted when they are disturbed. These insects are characterized by flattened leaf like expansions arising from the hind legs and a white strip running across the central part of the back. Leaffooted bugs are very flighty and can easily migrate in and out of wheat fields from adjacent weed hosts such as thistle. Flights of this pest can come from adjacent fields where burndown applications have been recently applied removing their primary host. Louisiana currently does not have a threshold for these pests and control can be quite difficult with pyrethroids. This insect is a minor pest of wheat. However, if your wheat is lodged with them and they have not migrated out of your field within a few days or been blown out by the torrential down pours this spring, a pyrethroid application can be made. If an application is deemed necessary, a high label rate of a strong pyrethroid should be used.
Aphids seem to be less of a problem this season than in previous years. The threshold for green bugs in wheat is 300-800 aphids per linear foot in wheat 6-16 inches in height. Pyrethroid applications made for other pests such as true armyworms can effectively suppress populations of green bugs. Many of the fields I have scouted have high numbers of natural enemies. These beneficial insects provide a free service in reducing aphid populations; however, aphids have the ability to outnumber their natural enemies in a short time frame.
With fungicide applications going out, tank mixing a pyrethroid in while covering ground is an option if insect pests have begun to be a problem. However, a jar test to assess fungicide/insecticide compatibility may be necessary prior to application.
For more information concerning insect pest management, contact your local LSU AgCenter parish agent, LSU AgCenter specialist, or your agricultural consultant.
by Boyd Padgett, Ph.D., Plant Pathologist, LSU AgCenter
I have received a few reports of leaf and stripe rust in commercial fields; however, not at high levels. I have not observed any rust in producer fields in Northeast and Central Louisiana. In my tests around the state (Dean Lee, Red River, Ben Hur, and Macon Ridge), I have observed leaf rust at low levels in my tests located at the Macon Ridge Research Station, at moderate levels at Ben Hur, and stripe rust at low levels in tests at Dean Lee. These tests are intentionally planted to SUSCEPTIBLE varieties, and are not representative of producer fields planted to resistant varieties. I have also observed powdery mildew in tests located at Ben Hur and the Red River Research Station. This disease is not considered to negatively impact wheat produced in Louisiana. However, if the disease is active (high incidence and severity) and present on the flag leaf prior to heading a fungicide may be justified. I HAVE NOT SEEN THIS SENARIO IN THE PAST 15 YEARS.
Producer fields: If rust incidence and severity is low (no pustules on the flag and confined to the lower canopy not active), most plants are fully headed (not flowering), and the variety is rust resistant, a fungicide is probably not needed.
Fungicides are justified if the wheat is at flag leaf to early heading and rust is active (spores are easily seen on the lower canopy). The following conditions are necessary for leaf and stripe rust development.
Stripe rust development is most aggressive when nighttime temperatures are 50 to 65oF in the presence of intermittent rain or dews. However, development can occur when temperatures are near freezing up to 70oF. Initial infections on seedling wheat may not have the characteristic striping pattern that occurs on more mature plants. Seedling infections often occur in ‘thumb-sized’ clusters on the leaves, as opposed to a random distribution that occurs with leaf rust. Infections may appear as linear rows of small yellow to light orange pustules (stripes) on the lower leaves during late winter or early spring. Striped patterns are typical of infections in older pants. If conditions remain favorable for development, pustules may cover the entire upper leaf surface, as well as portions of the head. A lifecycle (infection to reproduction) can be completed in 7 to 10 days when conditions are optimum for development.
Leaf rust is usually evident later in the season than stripe rust. This is because the leaf rust pathogen requires warmer temperatures for development than stripe rust. Initial symptoms of leaf rust begin as light yellow spots, usually on the lower foliage. As the disease develops, small pin-point pustules form on the upper leaf surface. Pustules are brick or dark red and occur randomly on the leaf. Similar to stripe rust, pustules can cover the entire leaf surface if conditions remain favorable for development. The disease develops optimally when nighttime temperatures are 50 to 70oF and leavers remain wet for 6 to 8 hours. Similar conditions will favor the development of leaf and glume blotch caused by Stagonospora and Septoria, respectively.
For more information concerning wheat disease management, contact your local LSU AgCenter county agent, LSU AgCenter specialist, or your agricultural consultant.
Now is the time to get fields and crops organized for the upcoming season. When it comes to field names and numbers, consistency is the key to success. Analyzing yield data or documenting production practices for multiple years makes it virtually a requirement. Other information such as crops, variety names, chemicals, fertilizers, equipment, etc. can be entered now in preparation for use. This information can be entered using desktop programs (e.g. APEX, Farm Works, SMS) and copied to the data cards which go in the equipment on the tractors, sprayers, and harvesters. Or it can be entered directly into the equipment if one of the desktop programs is not available. Along with these card entries, make a reference notebook with maps and other information that can stay with each tractor, sprayer, harvester, and other equipment. Time spent now getting data prepared and organized will help keep field operations moving smoothly with fewer glitches to hinder data analysis after the tasks are finished. For more information or assistance with this or other precision ag applications please contact Dennis Burns (318-267-6709) or R.L. Frazier (318-267-6714) with the LSU AgCenter.