Wet spring conducive to potassium deficiency in corn

By Dan Fromme

The wet spring has created soil conditions that are not conducive to good corn root growth and development. Potassium (K) deficiency in corn is one result of these conditions.

The peak period of K uptake occurs between four to eight weeks after seedling emergence. During this period, the crop takes up as much as 5 pounds of K per acre per day. If early root growth and the development of nodal roots is restricted or impeded by excessive wet conditions, the plant will simply not have an extensive enough root system to meet the nutrient demands of corn.

Symptoms can appear even if soil K is adequate for corn production. If these wet soil conditions are followed by a dry spell, this can exacerbate the situation. Anything that exerts additional stress or limits growth — such as compacted soils, root pruning and sidewall smearing — will further reduce K uptake. Also, if K levels are low in the subsoil zone — which has the most active roots — K uptake will not be adequate for the demands of the corn plant.

Most K deficiency symptoms appear while the corn plant is between 15 inches tall and the tasseling stage. Symptoms appear first on the lower leaves. Potassium is a mobile nutrient, so it can move from older to younger leaves when the K needs of the younger leaves are not met by K uptake by the roots. The older leaves turn yellow, and the leaf tissue along the margins (leaf edges) starts to dry up and die back (Figures 1 and 2). Potassium deficiency symptoms should not be confused with nitrogen deficiency, which first appears as yellowing on the lower (older) leaves and is located from the leaf tip down the midrib in a “V” shape (Figure 3).

Figure 1. In potassium-deficient corn plants, the older leaves turn yellow and the leaf tissue along the margins (leaf edges) starts to dry up and die back. Photo provided by the International Plant Nutrition Institute

Figure 2. Potassium-deficient leaf showing chlorosis on the leaf edges and necrosis starting from the leaf tip. Photo provided by the International Plant Nutrition Institute

Figure 3. Nitrogen-deficient leaf showing yellow chlorosis at the leaf tip, advancing down the leaf along the mid-rib in a V-shaped pattern. Photo provided by the International Plant Nutrition Institute

If there is a true soil K deficiency, more severe symptoms develop. Older leaves die back, and yellowing symptoms and marginal necrosis appear on the younger leaves (Figure 4). Plant height may be reduced, silking delayed and mature ear size reduced. Stalk diameter may remain unchanged, but K-deficient plants may lodge late in the season because stalk strength is lower and plants are more susceptible to stalk rots (Figure 5).

Figure 4. Potassium-deficient corn plants exhibit chlorosis along the leaf margins and tips of the older leaves. This spreads from the tip to the base, then turns to necrosis. In severe cases, the leaves appear dry and scorched along the edges and tips. Photo provided by the International Plant Nutrition Institute

Figure 5. Corn ears are lower to the ground and plants lodge when there is a potassium deficiency (left). Ears are higher from the ground when potassium is sufficient (right). Differences in height were due to differences in internode lengths. Photo provided by the International Plant Nutrition Institute

Plant analysis can be used to diagnose K problems during the growing season. Whole plant samples taken 30 to 45 days after emergence should contain 3 to 5% K. Ear leaf samples collected at early silking should contain between 2.5and 3.5% K. The ear leaf taken at early silking is the best indicator of plant K status, but if the plant is found to be K deficient, very little can be done during the growing season to correct the problem. In general, K applications made later than six weeks after emergence are not economical because the peak period of plant K demands (four to eight weeks after emergence) may have been missed entirely. For next year’s crop, soil tests from good and affected field areas can help discern true soil deficiency effects from climatic effects.

 

Early-season soybean response to flooding

By Boyd Padgett

With the recent rains, I wanted to put out some information on the impact of flooding on soybean germination as well as the impact on young plants.

Young plants

The response of young soybeans (early vegetative) to flooding depends on several factors, including temperature, cloud cover, soil conditions prior to flooding (dry or saturated), soil type, duration of the flood and variety. In addition to these factors, plant response depends on the type of flooding.

There are two types of flooding:

1. Waterlogged soils. Water only covers the roots, but plants are emerged (Figure 1).

2. Submerged plants. Water covers the entire plant (Figure 2).

Figure 1. Waterlogged soil.

Figure 2. Submerged crop at the end of a field.

Waterlogged soils are less damaging to plants than when they are submerged. In general, soybeans can tolerate these conditions for 48 to 96 hours without substantial injury. The amount of time plants can tolerate depends on the environmental conditions at the time of flooding as well as the conditions following the flood. If the soil was saturated or waterlogged prior to flooding, then plant death will occur faster compared to flooding on dry soils.

Flooding reduces the oxygen concentration in the soil. Oxygen is essential to the plant for normal development (respiration, water uptake, root development and other functions). This reduction results in the buildup of toxins and carbon dioxide, which is detrimental to the plant. Researchers have found the oxygen concentration can be near zero after 24 hours of flooded conditions, depending on the environment and water movement. More information is available on the Iowa State University Extension and Outreach website.

The likelihood of soybeans surviving flooded conditions and sustaining minimal damage is best when days are cool (not cold) and cloudy and nights are clear and cool during and after the flood event. Cold conditions could predispose plants to disease. Conversely, sunny conditions and high temperatures will increase the respiration rate of the plant and demand for oxygen in the soil (which is limited); therefore, more injury is likely. Additionally, debris (soil and other residues) left on the plant after the waters have subsided can reduce the photosynthetic capacity of the plant

Germination

Flooding can adversely impact germination. When soils are saturated for 48 hours, germination can be reduced by 30 to 70%, depending on the time of the flooding event. More information on the impact of flooding on germination can be found on the University of Nebraska-Lincoln’s CropWatch site.

Assessment and management

Any practice to facilitate water movement off the field without damaging the crop would be good (clear obstructions in water furrows, ditches, culverts, etc.). Scout fields four to seven days after the waters have receded. Look for signs of good growth such as new buds on stems and good color. Determine plant populations to decide if replanting is necessary.

Information on varieties, seed rates, recommended planting dates and late planting can be found in “2019 Soybean Variety Yields and Production Practices,” a guide available on the LSU AgCenter website.

 

Corn disease update

By Trey Price and Boyd Padgett

Holcus spot and paraquat drift

A few reports of paraquat drift and Holcus spot have surfaced statewide. These two maladies display round to oval, light tan to white spots with or without yellow halos. They 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 syringaepv. syringae. Both issues are usually of minor concern, with the exception of paraquat drift heavy enough to affect corn stand and yield.

Figure 1. Paraquat drift on corn.

Figure 2. Holcus spot of corn.

Common rust

Common rust is easily found this time of year in most corn fields. Pustules of common rust are brick red to dark orange in appearance, somewhat elongated and will appear on both leaf surfaces (Figure 3). Common rust will progress during 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 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 4).

Figure 3. Common rust.

Figure 4. Southern rust.

Northern corn leaf blight

We are starting to find northern corn leaf blight (NCLB) in central and northeastern Louisiana (Figure 5). With the current dry weather pattern, the disease will progress slowly. Most of the time, fungicide applications are not needed for NCLB, as the corn crop usually fills out before the disease is severe enough to impact yield. However, severe disease may occur in susceptible hybrids that are following corn in reduced tillage situations. These fields need to be watched closely. Most corn hybrids have some degree of resistance to NCLB. We have a copy of the official corn hybrid trial planted in Winnsboro in hopes of obtaining a better set of ratings for next year.

Figure 5. Northern corn leaf blight.

Fungicide considerations

Fungicide application decisions should be carefully considered field by field based on disease severity (Figure 6), crop stage (Table 1), hybrid susceptibility (click here for data from AgCenter hybrid trials), fungicide efficacy (refer to the corn section of the Louisiana Plant Disease Management Guide, available here), 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 water volume (5 GPA by air minimum) is recommended.

Figure 6. NCLB disease severity scale.

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.

Percent Defoliation

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

 

Entomology update

By Sebe Brown

Cotton

With difficult planting conditions forcing many cotton acres into a later planting window, thrips pressure across much of the state has been light. With later planting often comes higher temperatures, which allow cotton to accumulate more DD60s at the seedling stage. Much of the cotton across Louisiana required no foliar overtreatment for thrips, and most insecticide seed treatments performed well.

However, if a foliar treatment is needed, it should be made when immature thrips are present and/or when large numbers of adults are present and damage is occurring. Seedling cotton typically will always have a few adult thrips, but the treatment trigger is the presence of immature thrips. The presence of immatures often signifies that the insecticide seed treatment has lost is efficacy and reproduction is occurring. Avoid spraying solely based on plant injury because the damage has already occurred. Be aware that residual herbicides and sandblasting injury can mirror thrips injury. Actively growing cotton is susceptible to thrips injury until the four-true-leaf stage. If cotton is drought stressed or growth is delayed by other factors, thrips can still cause injury past this stage.

Finally, with much of the cotton acreage at the stage to receive side-dressed nitrogen, fertility management practices can influence insect control later in the season. Many of the newer varieties, particularly Delta Pine 1646, are aggressive growing and respond very well to nitrogen. Elevated nitrogen rates make vegetative growth difficult to manage, causing cotton to become “rank.” Rank cotton often has a thick, dense canopy that prevents adequate spray deposition of insecticides, which compromises control. If you have any questions regarding cotton fertility management, please contact your local AgCenter agent or specialist for more information.

Soybean

Reports of redbanded stink bugs (RBSB) exceeding threshold levels (four per 25 sweeps) in early beans are occurring in south Louisiana. This scenario occurs every year, but is often more pronounced in years that winter temperatures were not cold enough to control overwintering RBSB. RBSB are typically attracted to beginning seed (R5) soybeans; however, when alternative legume hosts begin to senesce, RBSB will often move into vegetative and early reproductive stage soybeans. Although young soybeans do not have reproductive structures for RBSB to directly feed on, these insects will feed on xylem. With no soybean seeds present to injure, stink bug feeding on young beans may contribute to green bean syndrome.

With this in mind, are insecticide applications targeting stink bugs on vegetative and early reproductive soybeans justified? With low soybean prices and rising insecticide costs, protecting yield and seed quality is fundamental. Saving insecticide applications for instances that produce direct yield and quality savings may be the best option with an uncertain soybean market.

 

Paraquat product training updates

By Kim Brown

Paraquat dichloride, also known as paraquat, is a widely used product in Louisiana in many of the state’s commodities. In 2016, the Environmental Protection Agency (EPA) published the human health mitigation decision on paraquat. As part of this decision, product manufacturers will be required to use new labels that emphasize paraquat toxicity and supplemental warning materials. Also, the new labels will restrict the use of paraquat products to certified applicators only. Companies are required to have newly labeled products on the market after Nov. 14, 2019; some may produce and sell newly labeled products before that date.

On March 8, 2019, the EPA announced the availability of a required training module for certified applicators using paraquat. This training was developed by paraquat manufacturers as part of EPA’s 2016 risk mitigation requirements and has been approved by the EPA. This training covers paraquat toxicity, new label requirements and restriction, consequences of misuse and other important information.

Paraquat is a restricted-use pesticide for use only by a certified applicator. This new restriction applies to mixing, loading and applying paraquat as well as other pesticide-handling activities.

Applicators will most likely start seeing changes to paraquat labels sometime late this year or in early 2020. Applicators that are using products with the new label will be required to be a certified pesticide applicator and must take an EPA-approved training course. In addition to this training, applicators will be required to take a 15-question exam and score 100%. This product-specific certification will be good for three years once an applicator has completed the training and passed the exam. The training is only available online and is produced by the product manufacturer.

The EPA is allowing the sale of paraquat that is already in the channels of trade, so some paraquat sold this growing season may not have the new training requirement on the label. If the new training requirement is listed on the label of the product purchased, the applicator must complete the training. Growers that currently have a supply of paraquat that does not have the new label listing the required training are not required to complete the training.

In summary, when purchasing products with new labels:

• Product may only be mixed, loaded and applied by a certified applicator who has successfully completed the paraquat-specific training before use.
  • http://usparaquattraining.com
  • Application “under the direct supervision” of a certified applicator is no longer allowed.
• Training must be repeated every three years.
• Training currently is only available online.

The LSU AgCenter Pesticide Safety Education Program will be hosting private pesticide applicator certification training opportunities throughout the state. Dates and locations can be found at www.lsuagcenter.com/pesticide.

For more information, please contact Kim Brown or Bryan Gueltig at kbrown@agcenter.lsu.eduor bgueltig@agcenter.lsu.edu.

 

AgCenter field day, industry expo set for June 27 near Alexandria

The LSU AgCenter Dean Lee Research and Extension Center will host a field day and expo on June 27.

The event will open at 2:30 p.m. A sponsored meal will be served at 6 p.m.

Event highlights include:

• Industry expo featuring equipment, technology and production and pest management products.
• Cotton, corn and soybean research.
• Beef cattle and forage research.
• Horticulture demonstrations with LSU AgCenter Super Plants.
• Youth activities and 4-H Mini Farm.
• Nutrition workshop.

Also, an interagency agent training will be held from 10 a.m. to noon.

The Dean Lee Research and Extension Center is located on U.S. Highway 71 near the LSU Alexandria campus.

Contact Tara Smith at 318-473-6520 or tsmith@agcenter.lsu.edu with questions.

 

LSU AgCenter specialists

Specialty	Crop Responsibilities	Name	Phone
Corn, cotton, grain sorghum	Agronomic	Dan Fromme	318-880-8079
Cotton	Agronomic	Dan Fromme	318-880-8079
Grain sorghum	Agronomic	Dan Fromme	318-880-8079
Soybeans	Agronomic	Boyd Padgett	318-614-4354
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	Sebe Brown	318-498-1283
Weed science	Corn, cotton, grain sorghum, soybeans	Daniel Stephenson	318-308-7225
Nematodes	Agronomic	Edward McGawley	225-342-5812
Irrigation	Corn, cotton, grain sorghum, soybeans	Stacia Davis Conger	904-891-1103
Ag economics	Cotton, feed grains, soybeans	Kurt Guidry	225-578-3282