Corn Disease Update

Trey Price and Boyd Padgett, LSU AgCenter Plant Pathologist

Holcus spot/paraquat drift

There have been reports of Holcus spot and paraquat drift in northeast and central Louisiana. The two maladies display round to oval, light tan to white spots with or without yellow halos and 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 syringae pv. syringae. Both issues are usually of minor concern, with the exception of paraquat drift heavy enough to affect corn stand and yield.

Common rust

Common rust is being observed in northeast and central Louisiana. Symptoms initiate in the lower canopy. Common rust may be found early to mid-season when temperatures are cool (60-77^oF) and leaf wetness of 4-6 hours.Pustules of common rust are brick red to dark orange in appearance, somewhat elongated, and will appear on both leaf surfaces (Figure 2).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

There are no confirmed reports of southern rust at the time of this newsletter. Symptoms of southern rust usually initiate in the mid-canopy. Southern rust pustules are orange and oval, and almost always appear on the upper surface of leaves (Figure 3).The disease usually appears mid to late season and is favored by temperatures in the upper 70’s to 90^oF and leaf wetness of 6 hours. Fungicide applications may be warranted depending on the hybrid, when the disease initiates in corn, and prevailing environment.

Northern corn leaf blight

We started finding northern corn leaf blight (NCLB) during the month of May in CENLA and NELA (Figure 4).Symptoms initiate in the lower canopy and appear as small oval lesions. If conditions favor disease development, these lesions will elongate (4-6 inches) and may coalesce resulting in blight. Disease development is favored by warm temperatures (65-80^oF) and leaf wetness (6-18 hours). This disease will first appear in susceptible hybrids in fields following corn with reduced tillage.NCLB will progress slowly during dry weather, and more quickly during regular rainy periods.Most of the time, fungicide applications are not needed for NCLB as most hybrids have some degree of resistance, and the corn crop usually fills out before the disease is severe enough to impact yield.However, severe disease may occur in susceptible hybrids.These are the fields that need to be watched closely and that may require treatment.

Southern corn leaf blight

Low levels of southern corn leaf blight (SCLB) have been observed in corn at Dean Lee Research Station. The disease is not common in Louisiana. Like NCLB, symptoms initiate in the lower canopy. Lesions are smaller than NCLB (1/8 to 1/4 inch in width and up to a 1 inch long). Disease development is favored by warm temperatures (70-80^oF) and high moisture. Most hybrids have some degree of resistance to SCLB; however, moderately susceptible hybrids may require treatment.

Fungicide considerations

Fungicide application decisions should be carefully considered field by field based on:disease severity (Figure 5), 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. Ground applications using at least 15 GPA are significantly more effective.

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.

Figure 1.Paraquat drift on corn.

Figure 2.Holcus spot and common rust of corn.

Figure 3. Southern rust of corn.

Figure 4.Northern corn leaf blight.

Figure 5.Northern corn leaf blight severity scale.

Cotton Update

Matt Foster, LSU AgCenter Cotton Specialist

It’s been a difficult year for cotton in Louisiana. As I write this on June 14, approximately 90% of the crop has been planted and most growers will finish within the next few days. Most of the cotton crop was planted in late May and early June. A significant amount of growers planted their intended cotton acreage to soybeans. A few growers have said “This is the first time in 40 years that I haven’t planted cotton.” Several cotton gins have stated that they don’t plan on operating this year.

Even though wet conditions have made nitrogen applications following emergence and weed management very difficult, the cotton crop throughout the state looks good. Insect pressure from thrips was heavy enough in some areas to justify foliar insecticide applications. Approximately 20% of the crop is squaring. Since squaring began, insect populations of aphids, fleahoppers, and plant bugs haven’t reached treatable levels in most areas of the state. As more of the crop begins to square, managing plant height with plant growth regulator (PGR) will be an important objective. Late-planted cotton often grows more vigorously compared to an early-planted crop, so a timelier PGR approach is often needed. Once cotton reaches match head square stage, plant growth, environmental conditions, and square load should be monitored weekly. A few factors to take into consideration when planning for pre-bloom applications of PGR include variety growth habits, soil type, irrigation, insect pressure, and total nitrogen available to the crop. PGR applications should be based on current plant growth characteristics and the anticipated growth rate based on expected growing conditions for the next 7 to 10 days.

Diagnosing In-Season Potassium Deficiency in Soybean

Rasel Parvej, David Moseley, Brenda Tubana, and Syam Dodla, LSU AgCenter Scientists

Potassium (K) deficiency can decrease more than 50% soybean yield across soil types that range from sandy loam to clay loam. Potassium deficiency symptoms in soybean first appear as irregular yellowing on the edges of K deficient leaves and can occur as early as at the V3 vegetative stage (three trifoliolate leaves) mainly on the lower older leaves (Figure 1). But symptoms often occur on the upper younger leaves during the reproductive stages especially in severe K deficient soils (Figure 2). Soybean fields with K deficiency symptoms early in the growing season are very easy to diagnose and manage. However, most of the soybean fields often suffer from K deficiency and exhibit yield losses without showing any visible deficiency symptoms at all or at least until the later reproductive growth stages (beginning seed, R5 to full-seed, R6). This type of phenomenon is called hidden hunger and its most common in soybean fields that are low to medium in soil-test K level, have not received K fertilization, have coarse-textured soils with high leaching potentials due to low cation exchange capacity (CEC) and excessive rainfall, or undergo severe drought conditions. Coarse-textured soils are highly prone to K leaching below the root zone. Sometimes, fall application of K fertilizer in coarse-textured soils results in late-season K deficiency due to K leaching from excessive rainfall during winter and/or spring. Coarse-textured soils are also poor in water holding capacity and drought in these soils often causes K deficiency by decreasing K uptake by plant roots. In addition, soybean grown in low pH (<6.0) soils often suffer from hidden K hunger effects because low pH decreases soil K availability even after fertilization.

Diagnosing hidden K deficiency early in the soybean growing season is very difficult and requires thorough scouting along with additional information such as fertilization history, soil texture, soil pH, soil-test K level, crop rotation, rainfall amount and distribution after fertilization and during the growing season, drought period, etc. Tissue sampling during the growing season is the best and perhaps the only tool to diagnose hidden K deficiency in soybean. Tissue sampling is predominantly conducted at the full-bloom (R2) stage; but can be done at the later reproductive (early pod, R3 to beginning seed, R5) stages. However, diagnosis at the early growth stages would be more effective and economical in correcting K deficiency and rescuing yield losses than diagnosis at the later growth stages.

For proper tissue sampling, 15 to 20 recently mature trifoliolate leaves including petioles from the 3^rd node from the top of the soybean plant should be collected and the date and soybean growth stage should be recorded (Figure 3). Then the leaflet of each trifoliolate leaf should be separated from the petiole and both the leaflet and the petiole or the leaflet only should be sent immediately to the plant diagnostic lab for K concentration. After receiving the results, tissue K concentrations for both the leaflet and the petiole at the specific growth stage can be interpreted using Figure 4. For example, the critical K concentration at the R2 stage ranges from 1.46 to 1.90% for leaflet and 3.01 to 3.83% for petiole and any K concentration below the critical level would be deficient and above the critical level would be sufficient. From the R2 stage, critical tissue K concentration declines linearly with the advancement of growth stage due to K translocation from vegetative to reproductive plant parts (pods and eventually seeds). Therefore, the growth stage at the time of tissue sampling should be recorded to properly interpret the tissue K concentration.

Soybean K deficiency can easily be corrected by applying K fertilizer during the growing season. However, the effectiveness and economics of applying K fertilizer to rescue yield loss depends on soybean growth stage and the severity of K deficiency. The earlier the growth stage for K application the more effective and economic it would be in recovering yield loss. Recently, research conducted at the University of Arkansas suggests that soybean K deficiency can be effectively and economically corrected by top-dressing or flying 60 pounds K₂O per acre (100 lb Muriate of Potash per acre; 0-0-60) until the R5 stage or about 5-weeks past the R2 stage. Foliar application of liquid K would not be effective and economic to correct severe K deficiency since foliar products contain very small amounts of K. Also, foliar products require several applications since K has a high salt index that can burn soybean foliage if applied in high concentrations.

Figure 1. Potassium deficiency symptoms during the early vegetative growth stages of soybean.

Figure 2. Potassium deficiency symptoms during the reproductive growth stages of soybean.

Figure 3. Steps of soybean tissue sampling during the R2 reproductive stage. Pencil in the picture indicates 3^rd node from the top of the plant.

Figure 4. Critical soybean leaflet and petiole K concentration from the R2 to R6 stages. (Source: Parvej, M.R., N.A. Slaton, L.C. Purcell, and T.L. Roberts. 2016. Critical trifoliolate leaf and petiole potassium concentrations during the reproductive stages of soybean. Agronomy Journal 108:2502-2518. doi:10.2134/agronj2016.04.0234; Y-axis is changed to English unit)

June 2021 Entomology Update

Sebe Brown and Tyler Towles, LSU AgCenter Entomologists

Cotton

With Louisiana experiencing warm days and adequate moisture, much of the cotton has grown out of the thrips susceptibility stage (1-4 leaf cotton) and is beginning to put on squares. Square initiation results in an increased presence of a diverse cohort of plant bugs and below is an adult and immature guide to aid in the identification of plant bugs in cotton. Keep in mind that corn is a suitable host for tarnished plant bugs prior to cotton squaring. When corn silks begin to dry down, tarnished plant bugs migrate out of corn and into more suitable hosts including cotton. Cotton fields that share borders with corn fields often see large migrations of tarnished plant bugs and square retention can be significantly affected. It is important to keep this factor in mind when scouting cotton acreage.

Tarnished Plant Bug

Tarnished Plant Bug Adult: LSU AgCenter

Tarnished Plant Bug Nymph: LSU AgCenter

Tarnished plant bugs (TPB) are 1/4 inch long insects that vary in color from yellowish-brown to green with black markings and a conspicuous triangle located on the dorsal (back) side. Nymphs resemble adults in general body shape and color but do not have wings. Tarnished plant bugs damage cotton from pinhead square to final boll set. Larger square damage affecting anthers, stigma, and styles can cause fertilization problems and fruit shed. The Louisiana threshold for bloom to harvest is 2-3 TPB per 5 ft of black drop cloth, 10 TPB per 100 or sweeps or 10% dirty squares. Pre-bloom threshold levels are 10 -25 TPB per 100 sweeps.

Cotton Fleahopper

Cotton Fleahopper Adult: LSU AgCenter

Cotton Fleahopper Nymph: LSU AgCenter

Fleahoppers are small, 1/8 inch, insects that have an oval-shaped, elongated body. These insects are yellow to green and resemble other Hemipteran true bugs and essentially look like a very small, green TPB. However, unlike TPB nymphs, fleahopper nymphs have dark spots on the hind legs.

Cotton should be scouted for fleahoppers during the first three weeks of squaring. Detection can be difficult due to the flighty nature of these insects. Simply casting a shadow over the pest will often make them take flight. Louisiana pre-bloom thresholds for fleahoppers are 10 to 25 insects per 100 sweeps with pre-bloom treatment levels adjusted to maintain between 70 and 85% first position square retention.

However, scouting small cotton with a sweep net is difficult and produces questionable results. Additionally, detecting small fleahopper nymphs in a sweep net is difficult as well. A better technique is to simply examine the terminal of plants watching for adults taking flight and then examining the terminal very closely for small nymphs. Morning is the best time to scout for fleahoppers and if the wind is blowing, they take shelter in the plant canopy.

Clouded Plant Bug

Clouded Plant Bug Adult: University of Tennessee

Clouded Plant Bug Nymph: University of Tennessee

Clouded plant bugs (CPB) are 3/8 inch long insects that are characterized by a thickened first antennal segment and hind legs that are noticeably larger than the first two pairs. Adult CPB are generally brown in color with mottled patches of black, yellow, or white. Nymphs are often greenish-yellow in color possessing red and white horizontally striped antennae. Larger nymphs can also be identified by a dark spot on the dorsal side of the abdomen. Clouded plant bugs are an occasional pest in Louisiana cotton with pre-bloom and bloom threshold levels being the same for tarnished plant bugs but each CPB should be counted as an equivalent to 1.5 TPB.

Injury by CPB is similar to TPB with square abscission, bloom injury and boll feeding (cat-facing) occurring mid to late season. As with other plant bugs, monitoring square retention and sweep net sampling prior to bloom should be used to determine levels of CPB infestations. Sampling for plant bugs from bloom until harvest is aided by the use of a drop cloth because this technique is more suitable for detecting nymphs.