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David Moseley, Kerns, Shelly, Stephenson, Daniel O., Hendrix, James, Price, III, Paul P, Padgett, Guy B., Miller, Donnie K., Villegas, James M., Parvej, Md Rasel
Shelly Pate Kerns, LSU AgCenter Cotton Specialist; Trey Price and Boyd Padgett, LSU AgCenter Plant Pathologists
As the cotton season progresses in Louisiana, there have been several reports of potassium deficiency. Potassium deficiency can weaken the plant, making it easier for plant pathogens to attack. It is very common to see fungal leaf spots on potassium stressed plants. Although pre-mature defoliation due to disease is possible, it is most likely the nutrient deficiency that is the leaf-dropping culprit. If you are seeing potassium deficiency, you may also see some of the diseases listed below. Here is how you can differentiate between these ailments:
-Potassium (K) Deficiency: K deficiency will show up as interveinal chlorosis (yellowing) starting at the leaf tips, then moving to the margins of leaves (Fig. 1). Yellowing leaves will change to a reddish-bronze color, often curling down-ward, and develop necrotic (dead) tissue near the leaf tips. In severe cases, plants will defoliate pre-maturely and boll retention can be negatively impacted.
Figure 1. Interveinal chlorosis (yellowing) caused by potassium deficiency in cotton.
-Alternaria Leaf Spot: Alternaria leaf spot (causal agent: Alternaria spp.) is a fungal disease that causes small circular leaf spots (lesions). Lesions will be brown in the center with purple borders. As lesions expand, concentric rings can develop that look very similar to target spot (Fig. 2). Alternaria lesions are usually found late in the season and lesions can be found throughout the canopy. This disease is commonly associated with potassium stressed cotton.
Figure 2. Alternaria leaf spot lesion with a brown center, purple border, and concentric rings.
-Target Spot: Target spot (causal agent: Corynespora cassiicola) is a fungal disease that thrives in areas with frequent rainfall and high humidity. Target spot lesions will have tan centers with concentric rings (Fig. 3). Although these concentric rings are similar to Alternaria leaf spot, target spot lesions will typically only be observed within the lower canopy. Lesions can also be observed on other parts of the plant such as the petiole, bracts, and bolls. Target spot can frequently be found on non-nutrient stressed cotton. Therefore, leaves may remain green (apart from lesions), whereas leaf spots associated with other foliar pathogens may be found more often on leaves exhibiting nutrient deficiency symptoms. Although incidence of this pathogen has been low so far, it is important to remain diligent in scouting efforts after the high rainfall Louisiana has received in the last 2 weeks.
Figure 3. Target spot lesions with concentric rings on a leaf and bracts.
-Cercospora Leaf Spot: Cercospora leaf spot is caused by a weak pathogen (Cercospora gossypina, syn. Mycosphaerella gossypina), and can commonly be observed on nutrient stressed plants. Cercospora typically causes small reddish lesions, with light brown centers (Fig. 4). Like other foliar cotton pathogens, these lesions may form concentric rings.
Figure 4. Small reddish lesions with light brown centers associated with Cercospora.
-Stemphylium Leaf Spot: Stemphylium leaf spot (causal agent: Stemphylium solani) shows up as small circular brown fungal lesions with reddish margins. The center of the lesion may develop concentric rings (Fig. 5). Although these concentric rings are similar to lesions formed by target spot and Alternaria, Stemphylium leaf spot lesions will often be found in the upper part of the canopy, with lesions congregated near the leaf margins. As lesions expand and the plant tissue dies, the center of the lesions may turn tan to white and fall out of the leaf, leaving leaves with a “shot hole” appearance.
Figure 5. Stemphylium and other pathogenic lesions (leaf spot complex) on a cotton leaf.
K deficiency can be confirmed through tissue and soil testing. The best management of K deficiency is being proactive. Soil testing before planting and applying K to the soil based on nutrient needs of the field is highly recommended to avoid another case of K deficiency. More information about K deficiency in cotton can be found here: https://www.lsuagcenter.com/articles/page1628783805876#title2
Often, there will be leaf spots caused by multiple fungal pathogens simultaneously present on a leaf. The way to accurately differentiate between these is through microscopic confirmation of fungal pathogenic spores. Foliar fungicide applications for management of Alternaria, Cercospora, and Stemphylium leaf spots rarely reap a return on investment. Again, the best management of these diseases is through being proactive. To avoid severe infections, reduce plant stress and maintain plant health through proper nutrient, water, and pest management.
Target spot can be managed through the planting of tolerant varieties, controlling plant growth, rotating crops to non-hosts between seasons, and spraying foliar fungicides. Fungicides are most likely to yield a return on investment when applied before the plant reaches >30% defoliation between the first and sixth week of bloom.
Rasel Parvej, State Soil Fertility Specialist, LSU AgCenter
Potassium (K) is the second most critical nutrient for cotton production after nitrogen (N). A deficiency in K can lead to significant reductions in lint yield and fiber quality. Cotton plants often experience K deficiency due to several factors. One primary reason is growing cotton in soils with low soil-test K levels without applying fertilizer-K. Additionally, various biotic and abiotic stresses during the growing season—such as insect damage, diseases, nematode infestations, soil compaction, and drought—can suppress cotton root growth, leading to reduced K uptake and increased deficiency. Although we had ample rainfall this year, the past couple of weeks required us to use irrigation to minimize water stress, particularly during the blooming period, which is the most critical stage for setting maximum yield potential. Any stress during this period can cause significant yield reduction. Figure 1 illustrates the cotton response to K deficiency.
A bale of cotton requires approximately 52 pounds of potash (K2O) per acre, with peak accumulation occurring a couple of weeks after the first bloom. During this peak period, cotton plants uptake around 2.2 to 3.5 pounds of K2O per acre per day. Any shortage in K supply during this critical period will result in deficiency and yield reduction. Not all parts of the cotton plant are equally sensitive to K deficiency. The sensitivity order is stems > roots > bolls > leaves, with stems being the most sensitive and leaves the least. Therefore, when K deficiency symptoms appear in the leaves, it indicates that other parts of the plant are already affected.
Potassium deficiency symptoms in cotton first appear as yellowish-white mottling between the veins. As the deficiency progresses, the center of these yellow spots dies, followed by numerous brown specks appearing between veins, around the margins, and at the tips of leaves. Eventually, the entire leaf turns reddish-brown and rusty-colored. These symptoms typically first manifest on the lower, older leaves early in the growing season, but are most frequently observed on the upper, younger leaves of heavily fruiting cotton plants later in the season (Figure 2).
Cotton plants can experience K deficiency without showing any visible symptoms, a condition known as hidden K deficiency or hidden hunger. The best way to diagnose hidden hunger K deficiency is through tissue sampling during the early blooming period. For tissue sampling, collect at least 20 of the uppermost recently mature leaves (without petioles) from the 3rd to 5th node from the terminal (the quarter-sized main-stem leaf at the top of the plant is counted as the 1st leaf). These samples should be taken during the early blooming period and sent promptly to a diagnostic lab (e.g., LSU AgCenter Soil Testing and Plant Analysis Laboratory, Baton Rouge, LA) for K concentration analysis.
The critical leaf-K concentration ranges from 1.5% to 3.0% at the early bloom stage and 0.75% to 2.5% at the late bloom stage. Cotton fields with a leaf K concentration lower than 1.5% during the early blooming period should be fertilized with K to mitigate yield losses. Cotton is particularly responsive to K fertilization during the blooming stage. A top-dress application of 100 pounds of dry potash (Muriate of Potash; 0-0-60) per acre should suffice for in-season management of K deficiency. Follow up with irrigation or at least 0.25 inches of rain to ensure rapid K uptake and an improved response.
Figure 1. Cotton response to K deficiency. Potassium-sufficient plants are on the left and deficient plants are on the right. The photo was taken from a research trial conducted at the LSU AgCenter – Macon Ridge Research Station, Winnsboro, LA.
Figure 2. Potassium deficiency symptoms during the mid-blooming period of cotton. The photo was taken from a research trial conducted at the LSU AgCenter – Macon Ridge Research Station, Winnsboro, LA.
James Hendrix, LSU AgCenter Conservation Agronomist, Northeast Region, and Dr. Donnie Miller, LSU AgCenter Weed Scientist
Crop harvest and subsequent soil preparation for next year’s crop will soon be at a record pace due to current harvest conditions throughout most of Louisiana. Producers planting covers this year may see seed costs decrease up to 10% for cereals other than black oats and some clovers. Now is the time to begin planning your seed purchases based on your objectives for planting as waiting too late may result in seasonal increases in prices due to shorter supplies if buyer demand is high.
Producers should consider several factors before planting covers. Even though early planting dates for winter cover crops are nearing, weather and soil conditions may not be favorable to get an early start to planting. Drought conditions have been experienced in some areas in northeastern Louisiana, and rainfall received after crop harvests may not be adequate to ensure cover crop germination and growth. In addition, the latter part of summer has been abnormally dry and hot, the ideal environment for armyworm infestations. As a result, if you do plant early, remember to scout your fields.
Once weather and soil moisture are favorable, producers should begin planting fields that will be planted to corn or grain sorghum next year by the end of September, due to the short growing season to produce desired biomass.
Anyone planting cover crops under EQIP or any other contracts with seeding rate specifications must remember to use seeding rates specified by the contract. Planting a cover crop mix with two or more species is based on a percentage of the seeding rate for each species. Before planting, contact your NRCS office for approval of the seeding rate for the mix. Producers not under contract can contact their local County Agent for assistance with seed mixes.
If you purchase seed based on planting next year’s cash crop, such as cotton, and have changed your mind after planting covers to plant corn, you may face less than expected results from the cover crop and future crop’s yield. Selecting a cover crop species is based on specific objectives to be accomplished, and soil characteristics, environmental factors, and management practices of the past cash crop and next year’s crop should be considered. The timing of planting, termination, and residue management must coincide with the future crop’s planting and management practices, and still accomplish your objectives. Otherwise, your time and investment in utilizing cover crops in your production system can be frustrating and of little value.
Due to tremendous amount of biomass production, cereal rye is an excellent choice for suppression/reduction of Italian ryegrass infestations. Higher seeding rates of 70 to 80 pounds per acre are preferred if this is the cover crop objective. Application of residual herbicide such as s-metolachlor once cereal rye has reached 80% germination (spike) has proven to result in 95% reduction in Italian ryegrass tiller production in spring prior to planting.
Another factor in the decision to plant cover crops are the previous crop lay-by and post-harvest herbicide applications that must be considered. Residual herbicides can cause significant issues to germination and growth of some cover crops, so contact your extension agent or consult the herbicide label for tolerances.
Last, but not least, select the species and varieties of covers that are recommended for your area and will fit your production system. Study the species and varieties you plan to plant and contact your extension agent if you have any questions. Do not put yourself in a situation that can cause havoc in your production system. One example has been planting pasture wheat as a cover crop. There have been multiple issues with termination that have caused serious problems with nutrient scavenging, residue management, and planting the following crop.
In conclusion, remember that legumes produce most growth during the spring, so planting early in fall can benefit establishment before winter freezes slow growth. Brassicas grow rapidly in the fall and can be severely injured/terminated by freezing temperatures, and therefore should be established early to benefit from nutrient scavenging and other benefits. Most cereals grow well in fall and spring, so the planting window is broader. Oats germinate faster than most cereals but can be severely injured/terminated during extreme winter temperatures, while cereal rye, triticale, and wheat are more tolerant to colder temperatures.
David Moseley, Daniel Stephenson, Donnie Miller, and James Villegas, LSU AgCenter Scientists
Use of herbicides as a desiccant in soybean has become popular to potentially improve harvest efficiency in Louisiana. For more information, please see the LSU AgCenter factsheet Desiccating Soybean.
A few important things to consider when apply desiccants in soybean as a harvest aid include:
1. Always follow the label.
2. Soybean pods/seed that are not at R6.5 (where the membrane is separated from the seed) will result in smaller seed size if the plant shuts down too early.
3. Control of Red-banded stinkbugs should be considered up until harvest if they are above threshold.
Daniel Stephenson and David Moseley, LSU AgCenter
Join us for an LSU AgCenter Soybean Production School at the Dean Lee Research and Extension Center!
When: Tuesday, September 10th, 4:00 PM - 6:00 PM
Where: Dean Lee Research and Extension Center, beginning at the State Evacuation Shelter.
Learn essential tips and techniques to maximize your soybean production. This training is perfect for agents, industry professionals, and growers.
Stay for a delicious meal!
Don't miss this opportunity to enhance your soybean knowledge.
For more information, contact: Daniel Stephenson, LSU AgCenter Central Region Director, at 318-473-6520 or dstephenson@agcenter.lsu.edu
| Specialty | Crop Responsibilities | Name | Phone |
| 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 |
| Soil fertility | Corn, cotton, grain sorghum, soybeans | Rasel Parvej | 318-435-2908 |
| Corn, Cotton, and Grain Sorghum | Agronomic |
Shelly Kerns | 318-435-2908 |
| Entomology | Field Crops | Dawson Kerns |
