Corn Disease Update

Trey Price and Boyd Padgett, LSU AgCenter pathologists

Article Highlights:

• Corn disease identification
• Management considerations

Holcus spot/paraquat drift

Symptoms for Paraquat drift and Holcus spot are similar and are difficult to distinguish from each other. Symptoms appear as round to oval, light tan to white spots with or without yellow halos. 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.

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 3).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.

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 4). This disease develops in warmer temperatures than for common rust 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 a fungicide.

Northern corn leaf blight

Northern corn leaf blight (NCLB) is a disease usually seen every year in susceptible hybrids (Figure 5).This disease will first appear in susceptible hybrids in fields following corn with reduced tillage. The disease will progress slowly during dry weather, and more quickly during regular rainy periods. Most of the time fungicide applications are not needed for NCLB. However, severe disease may occur in susceptible hybrids following corn in reduced tillage situations. 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 6), 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.

Growth Stage	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%
Tassel	3	7	13	21	31	42	55	68	83	100
Silked	3	7	12	20	29	39	51	65	80	97
Silks Brown	2	6	11	18	27	36	47	60	74	90
Pre-Blister	2	5	10	16	24	32	43	54	66	81
Blister	2	5	10	16	22	30	39	50	60	73
Early Milk	2	4	8	14	20	28	36	45	55	66
Milk	1	3	7	12	18	24	32	41	49	59
Late Milk	1	3	6	10	15	21	28	35	42	50
Soft Dough	1	2	4	8	12	17	23	29	35	41
Early Dent	0	1	2	5	9	13	18	23	27	32
Dent	0	0	2	4	7	10	14	17	20	23
Late Dent	0	0	1	3	5	7	9	11	13	15
Nearly Mature	0	0	0	0	1	3	5	6	7	8

Table 1. Percent yield loss (in blue) because of defoliation by crop stage. The percent of defoliation is shown on the top row of the table. For example…30% defoliation at dent stage results in a 2% yield loss.

Figure 1. Paraquat drift on corn.

Figure 2. Holcus spot of corn.

Figure 3. Common rust.

Figure 4. Southern rust.

Figure 5. Northern corn leaf blight.

Figure 6. NCLB disease severity scale.

 

Assessing Changes in Breakeven Yields from Replanting and Input Price Volatility in 2023

Michael Deliberto, LSU AgCenter Economist

Article Highlight:

• Calculate the breakeven yield required to cover the increase in replanting costs across soybean and corn price levels.

The economic impact on farm-level production costs from replanting soybeans and/or corn can result in an increase in the number of additional bushels that will be required to offset the incurred production expenses associated with replanting field operations. The severity of this will depend on the type of seed technology employed, as differences in the prices for seed, seed treatments, and seeding rate which can influence the replanting costs and, hence, the number of additional bushels required at harvest to offset those costs.

In any ‘normal’ crop year, replanting costs play a substantial role in estimating the additional production cost per unit (bushel). However, inflationary pressures and input price volatility observed have cut into expected profit margins and may affect how growers alter their marketing strategy when using the breakeven (BE) cost per unit coupled with a percentage margin when identifying selling opportunities in the futures market. A positive aspect on this situation is that soybean and corn market prices are far better than in recent years, triggered by tight domestic stocks, renewable fuel demand, and increased U.S. export demand. However, substantial increases in production costs have eroded what otherwise would have been healthy profit margins. The following economic analysis employs a general farm management approach to calculate the BE yield required to cover the increase in replanting costs across alternative soybean and corn price levels.

Under the assumption that the farm’s soybean yield is expected to be 55 bushels per acre, estimated total variable production costs for soybeans under irrigation are $495.16 per acre. The BE yield under these imposed conditions is calculated to be 39.61 bushels per acre, assuming a $12.50 per bushel price. The BE price is calculated to be $9.00 per bushel. This can be interpreted to the extent that a producer would need to receive a market price more than $9.00 per bushel to cover the total variable production costs per acre.

Planting costs (e.g., machinery, fuel, and labor) are comprised of a planter, tractor, and seed. Planter costs are estimated to be $5.30 per acre in addition to the $58.00 soybean seed cost to total $63.30 per acre. If a producer determines that a field must be replanted, the total variable costs for the growing season will increase from $495.16 to $558.46 per acre, reflecting the replanting costs. The new BE yield under these imposed conditions is calculated to be 44.68 bushels per acre, assuming a $12.50 per bushel price. Normal production conditions are assumed. This is an increase of approximately 5 bushels per acre to offset the increase in production costs while the $12.50 price is held constant. The BE price is calculated to be $10.15 per bushel, an increase of $1.15 per bushel.

When the market price is varied from $15.00 to $11.00 per bushel (viewed left-to-right on the horizontal axis) at a yield of 55 bushels per acre, it is observed from Figure 1 that as price declines, more production is required to cover the increase cost of replanting.

Figure 1. Breakeven (BE) yield comparison across multiple price levels for replanting soybeans.

Figure 2 illustrates the required increase (change) in BE yields from replanting a 55 bushel per acre potential field across multiple price levels. As the soybean market price declines, the required increase in BE yield increases from 4.22 (at a $15.00 price) to 5.75 bushels per acre (at a $11.00 price).

Figure 2. Required increase in BE yields from replanting costs in soybeans.

In a similar approach, assuming that the farm’s corn yield is expected to be 190 bushels per acre, estimated total variable production costs for corn under irrigation are $701.96 per acre. The BE yield under these imposed conditions is calculated to be 116.99 bushels per acre, assuming a $6.00 per bushel price. The BE price is calculated to be $3.69 per bushel. This can be interpreted to the extent that a producer would need to receive a market price more than $3.69 per bushel to cover the total variable production costs per acre.

Planting costs (e.g., machinery, fuel, and labor) are comprised of a planter, tractor, and seed. Planter costs are estimated to be $5.30 per acre in addition to the $118.30 corn seed cost to total $123.60 per acre. If a producer determines that a field must be replanted, the total variable costs for the growing season will increase from $701.96 to $825.56 per acre, reflective of the replanting costs. The new BE yield under these imposed conditions is calculated to be 137.59 bushels per acre, assuming a $6.00 per bushel price. Normal production conditions are assumed. This is an increase of approximately 21 (20.6) bushels per acre to offset the increase in production costs while the $6.00 price is held constant. The BE price is calculated to be $4.35 per bushel, an increase of $0.65 per bushel.

When the market price is varied from $6.70 to $4.70 per bushel (viewed left-to-right on the horizontal axis) at a yield of 190 bushels per acre, it is observed from Figure 3 that as price declines, more production is required to cover the increase cost of replanting.

Figure 3. Breakeven (BE) yield comparison across multiple price levels for replanting corn.

Figure 4 illustrates the required increase in BE yields from replanting a 190 bushel per acre potential field across multiple price levels. As the corn market price declines the required increase in BE yield increases from 18.45 (at a $6.70 price) to 26.30 bushels per acre (at a $4.70 price).

Figure 4. Required increase in BE yields from replanting costs in corn.

For information on soybean and corn enterprise budget and farm management economics, please refer to the LSU AgCenter’s website at https://lsuagcenter.com/topics/crops/soybeans/budg... and https://lsuagcenter.com/topics/crops/corn/budget .

 

2023 Louisiana Soybean Production Update: An Early Start to Planting and a Freeze

David Moseley, LSU AgCenter Soybean Specialist

Article Highlights:

• The Louisiana soybean planting season began early in 2023.
• Depending on several possible factors, some soybean fields survived a freeze on March 20^th, but others were completely lost.

The USDA-NASS survey reported that two percent of the Louisiana soybean crop was planted by March 19, 2023. The warm and dry weather during early March allowed producers to plant their corn crop early and then begin planting soybean. Unfortunately, on March 20^th, the temperature dropped to approximately 28 degrees Fahrenheit in Northeast Louisiana. Soybean tissue death can occur if the temperature falls to 28 degrees Fahrenheit for more than four hours, especially if the unifoliate leaves are exposed (Vann and Stokes, 2020).

On March 21^st and over the following few weeks, the condition of three fields was documented. Two fields were at the LSU AgCenter Northeast Research Station and the other field was near Clayton, LA. The planting dates were March 1, 15, and 6, respectively. For the March 1^st and 6^th planting dates, with only 30 miles between the two fields, almost 100 percent of the soybean plants at the Northeast research station were killed, but approximately 50 percent of the soybean plants at the field near Clayton, LA survived. One explanation could be that the plants near Clayton, LA were less developed as they were planted five days later. At least 50 percent of the unifoliate leaves were likely still insulated by the cotyledons. The soybean in the field planted on March 15^th at the Northeast Research Station was not emerged on March 20^th and survived the freeze.

The producer near Clayton, LA planted 160,000 seeds per acre. With an approximate 50 percent survival rate, the final stand was approximately 80,000 plants per acre. According to data from the LSU AgCenter Dean Lee Research Station in 2020, a final stand of approximately 61,000 plants per acre may result in similar yield as a stand of over 100,000 plants per acre. Most University soybean agronomist would agree that 70,000 – 75,000 plants per acre that are healthy and uniformly distributed will allow approximately 95 percent of the full yield potential as compared to a more full stand.

Figure 1. Soybean seedling damage on March 21, 2023. The seeds were planted on March 6^th and were in the VE growth stage (the cotyledons had emerged past the soil surface, but the unifoliate leaves had not completely unrolled and in some cases the growing point was still protected by the cotyledons) when the freeze hit Louisiana on March 20^th. The cotyledons had suffered damage from the freeze, and it was unknown if the growing point had been killed.

Figure 2. On March 27, 2023, the original main stem on some plants had ceased from growing after the apical meristem was killed during the freeze. However, after the apical meristem was killed, two axillary branches had begun to develop trifoliolate leaves from the cotyledon node.

Figure 3. By April 5, 2023, both axillary branches had developed two open trifoliolate leaves and had another trifoliolate leaf beginning to open on each branch. Both axillary branches can fully develop and replace the original main stem.

Figure 4. It was observed on April 5, 2023 that the apical meristem was not damaged on most plants. The soybean plants were progressing normally and had reached the V2 growth stage (two open trifoliolate leaves). The plants had reached the V2 growth stage approximately 30 days after planting which is within the normal average duration between planting and V2.

Figure 5. The soybean seed was planted on 7.5 inch spacing with a seeding rate of 160,000 plants per acre. After the March 20, 2023 freeze, the final stand count on April 5 was estimated to be 80,000 plants per acre which can still support close to full yield potential of a field with a full stand.

Reference:

Vann, R., D. Stokes. 2020. Soybean Cold Damage. North Carolina State Extension.

 

Stink Bugs in Wheat and Corn

James Villegas, LSU AgCenter Entomologist

Article Highlight:

• Identification and management of stink bugs in wheat and corn.

Stink bugs are typically found infesting wheat in Louisiana at this time of the year. While it’s not uncommon to find stink bugs on heading wheat, they rarely cause economic damage. The predominant stink bug species found in Louisiana wheat is the rice stink bug (Oebalus pugnax), although brown stink bugs can also be present. These pests are mostly found around the border edges of the field. It takes high numbers of stink bugs to damage wheat. Treatment is only recommended if the threshold of 1 stink bug per 10 heads during the milk stage and 3 stink bugs per 10 heads during the soft dough stage is reached. Pyrethroids such as z-cypermethrin and lambda-cyhalothrin should be effective against these pests. Once wheat reaches the hard dough stage, damage due to stink bugs is greatly reduced. It’s important to note that stink bugs in wheat are not typically treated, but they can be an indication of stink bug infestations in corn. During harvest, stink bugs can potentially be pushed to nearby corn, so it’s important to scout adjacent corn fields.

The brown stinkbug (Euschistus spp.) is the most common species to attack corn, but the green stink bug (Chinavia halaris) and southern green stink bug (Nezara viridula) can also be a pest. Treatment is recommended if 5% of plants have bugs prior to ear shoot appearance (1 stink bug per 20 plants). For early vegetative stages (V1–V6), treatment is necessary if 10% of plants are infested (1 stink bug per 10 plants). Table 1 provides recommended insecticides for managing stink bugs in corn. Managing stink bugs in corn is critical, as they can cause significant damage resulting in reduced yields and kernel quality.

Rice stink bug (Oebalus pugnax) in wheat. Photo by: J. Villegas

Table 1. Recommended Insecticides for Stink Bugs in Corn

Insecticide	Amount of Concentrate per Acre	Pounds Active Ingredient Per Acre
beta-cyfluthrin Baythroid XL (1)	1.6-2.8 ounce	0.012-0.022
z-cypermethrin Mustang Max (0.8)	2.72-4.0 ounce	0.017-0.025
bifenthrin Brigade (2)	2.1-6.4 ounce	0.033-0.1
cyfluthrin Tombstone (2)	1.6-2.8 ounce	0.025-0.044
lambda-cyhalothrin Warrior II (2.08)	1.28-1.92 ounce	0.02-0.03

 

Frost Injury and Nitrogen Management in Corn

Rasel Parvej, LSU AgCenter State Soil Fertility Specialist

Article Highlights:

1. Nitrogen (N) needs to be applied in corn at least in 2-splits: 30-45 (silt-clay) lb N/acre at planting and rest at V5-6 growth stage.
2. Injecting liquid N (urea ammonium nitrate, UAN) is the most efficient method of providing N for profitable corn production.
3. If urea is used, it needs to be broadcast on dry soil followed by immediately incorporating it using a hipper or “known” rain event. Using any urease inhibitor is highly recommended.

Most of the corn acreage in Louisiana was injured by freezing temperature from March 19-20. At that time, most corn is at VE (emergence) to V2 stage (two leaves with visible collar) and therefore the growing point was below the soil surface. Corn’s growing point remains below the soil surface until V6 growth stage for most hybrids. Since frost injury is usually limited to aboveground plant parts, unless the temperature falls below 28°F, the growing point was most likely protected from freeze injury. However, temperatures may have reached the critical temperature of 28°F in some areas, so each field should be scouted to ensure that growing points are still viable. At this point producers need to wait 5-7 days or until visible growth is observed to assess stand quality and uniformity. Many producers are inquiring about applying some nitrogen (N) fertilizer to jump start new growth. The following things should be considered prior to applying N in frost injured corn field.

1. If a corn field has already received 30-45 lb/acre N fertilizer at or near planting, extra N is not necessary at this growth stage (V2-V4), and they can wait to sidedress the bulk of their N around V5-6 stage. In silt loam to clayey soils, most of the applied N is still available in soil because temperatures have been cool, and we have not received excessive amounts of rain over the past few weeks. For sandy soils, extra N may be needed if several rain events have occurred that could have resulted in leaching N out of the current root zone.
   
   
2. If a corn field has not received any N or received a small amount of N, 5-6 lb/acre from 4-5 gallon of 10-34-0 fertilizer at or right after planting, producers need to apply 30-45 lb/acre N once they confirm their corn stand. Since urea (46-0-0) fertilizer price is going down, many producers are thinking about flying urea at this stage. Producers may broadcast small amount of urea to provide 30-45 lb/acre N, but they should not apply high rate of N by broadcasting urea without incorporation due to several potential loss mechanisms.
   
   
3. Injecting liquid N (urea ammonium nitrate, UAN 32-0-0, 30-0-0-2S, or 28-0-0-5S) near the root system is still preferred and most efficient method of providing N for profitable corn production. Corn root system is very small right now (VE to V3) and N is therefore needed close to the root system. Broadcasting urea (46-0-0) in a 36- or 38-inch row spacing system can not ensure enough N availability near the root system. Also, most of the urea granules when broadcasted fall in the furrow, which can be lost due to volatilization, leaching, or runoff before plant uptake.
   
   
4. Many producers have inquired about applying the total corn N needs by broadcasting urea. Reasons include: urea is less expensive, the time required to inject UAN on large acreage, concerns of compaction, or unsuitable soil conditions. The most efficient way would be to broadcast all the urea around V5-6 stage on dry soil followed by immediately incorporating it using a hipper. However, this may not be feasible for many producers due the time and above-mentioned soil conditions. Using any urease inhibitor will give the urea extra protection from volatilization loss as well as the producers extra time for incorporation.
   
   
5. The next best way for those who cannot incorporate urea by hipper would be to broadcast urea in 3-splits and each time shortly before a “known” rain event (at 0.25 to 0.5-inch) to help minimize loss mechanisms. Producers may also need to increase the total N rate up to 25% to compensate some losses depending on soil type and weather conditions. For example, when N is incorporated either by injecting or rehipping, LSU AgCenter recommends 200 lb/acre N for silt loam and 250 lb/acre N for clayey soils in 2-splits (30-45 lb at planting and rest at V5-6 stage) for 200-bushel/acre corn yield. The ideal 3-way split for urea should consist of 30-45 lb/acre N at VE-V2 stage, most at V7-8 stage, and another 45-60 lb/acre N at V12-13 stage before tasseling. To better assess pre-tassel N rate considering N losses during the growing season, producers need to monitor tissue N concentration at V10-12 stage.

If you have any questions on loss mechanisms and why I suggest not applying all the N using urea at one short without incorporation please contact me at 479-387-2988 or mrparvej@agcenter.lsu.edu.

 

LSU AgCenter Specialists

Specialty	Crop Responsibilities	Name	Phone
Corn, cotton, grain sorghum	Agronomic	Matt Foster	601-334-0354
Soybeans	Agronomic	David Moseley	318-473-6520
Wheat	Agronomic	Boyd Padgett	318-614-4354
Pathology	Cotton, grain sorghum, soybeans	Boyd Padgett	318-614-4354
Pathology	Corn, cotton, grain sorghum, soybeans, wheat	Trey Price	318-235-9805
Entomology	Corn, cotton, grain sorghum, soybeans, wheat	James Villegas	225-266-3805
Weed science	Corn, cotton, grain sorghum, soybeans	Daniel Stephenson	318-308-7225
Nematodes	Agronomic	Tristan Watson	225-578-1464
Irrigation	Corn, cotton, grain sorghum, soybeans	Stacia Davis Conger	904-891-1103
Ag economics	Cotton, feed grains, soybeans	Kurt Guidry	225-578-3282
Precision ag	Agronomic	Luciano Shiratsuchi	225-578-2110
Soil fertility	Corn, cotton, grain sorghum, soybeans	Rasel Parvej