Daniel Fromme, Price, III, Paul P, Padgett, Guy B., Parvej, Md Rasel, Moseley, David O, Copes, Josh, Dodla, Syam
In this article:
|Tassel ears in corn|
|Northern corn leaf blight and southern rust update|
|Soybean disease update|
|Management of in-season potassium deficiency in soybeans|
|LSU AgCenter specialists|
By Dan Fromme, LSU AgCenter corn specialist
A “tassel ear” is an odd-looking affair found almost exclusively on tillers, or “suckers,” of a corn plant along the edges of a field or in otherwise thinly populated areas of a field (Figure 1). It is very uncommon to find tassel ears on the main stalk of a corn plant.
Tassel ears occur when the tassel and the ear are present within the same structure. The tassel ears tend to appear at the top of the plant. The physiological explanation of this syndrome is not firmly known, but it seems to be related to the environment. This phenomenon seems to be more frequent in tillers, in plants close to the borders of the field and under very low plant density levels with more isolated corn plants. The husks are absent, so the ear is not protected from the weather, birds, insects and diseases. Consequently, harvestable, good-quality grain from tassel ears is rare.
Some folks lump the tassel ear symptom into the same category as the malformed tassel symptom of the so-called “crazy top” disease. These two odd tassel symptoms are not related and, in fact, look totally different. The “crazy top” disease is caused by infection of young corn plants during ponding events by the soil-borne fungus Sclerophthora macrospora, which eventually expresses itself by altering normal tassel development (and sometimes ear shoot development) into a mass of leaf tissue. (Reference: R.L. Nielsen, Purdue University)
Figure 1. Tassel corn ears. This phenomenon takes place when the ear appears in the same organ as the tassel and they are produced in the terminal position of the stalk. Photo by Rogers Leonard
By Trey Price and Boyd Padgett, LSU AgCenter plant pathologists
We have received several field calls concerning NCLB. Frequent rains during May resulted in light to moderate disease development. A few fields planted in susceptible hybrids following corn with reduced tillage required fungicide treatment. In the vast majority of cases, fungicide treatment is not necessary because of low disease levels and resistant hybrids. With the current weather pattern, NCLB will progress very slowly, allowing the crop to outrun the disease in the vast majority of cases. That being said, some younger corn in the state should be scouted weekly for the presence of NCLB (Figure 1).
Fungicide application decisions should be carefully considered field by field based on disease severity (Figure 2), crop stage (Table 1), hybrid susceptibility (link), fungicide efficacy (link), tillage regime, prevailing environmental conditions, previous experience and the probability of a return on the investment. If applications are warranted, apply at labeled rates using maximum (5 GPA by air) water volume is recommended. There are many fungicide options for NCLB management.
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. Consider all of the leaves when determining percent % defoliation.
Southern rust was found during the first week of June in the southernmost production areas of Louisiana. At that time, disease incidence and severity were very low. Since then, the disease has progressed slowly enough to allow most of the crop in central Louisiana to reach dent stage without losses. There have been unconfirmed reports of southern rust in northeast Louisiana. We are planning visits to suspected fields. Current temperatures (77 to 90 degrees) are optimal for southern rust development; however, the pathogen prefers leaf wetness of nine to 16 hours with light rain and light wind favoring infection. Conversely, heavy rains with high winds do not favor infection, as infecting spores are washed from corn leaves. We do not expect southern rust to develop quickly given our current weather forecast. Southern rust (Figure 3) may be confused with common rust (Figure 4), which can be found in just about every field if you look hard enough.
Just because southern rust appears in a corn field does not mean that a fungicide application is necessary. As stated in the previous section, fungicide application decisions should take in to account a number of factors disease severity, crop stage (Table 1), prevailing environmental conditions, fungicide efficacy, previous experience and the probability of a return on the investment. There are many fungicide options for southern rust management (link). As the corn crop matures, the probability of a return on fungicide investment steadily declines.
By Trey Price and Boyd Padgett, LSU AgCenter plant pathologists
Up to this point, we have received very few reports of soybean diseases in the state. In northeast Louisiana, taproot decline and southern blight have been observed in several fields.
An effective disease management strategy incorporates the following components: disease identification, cultural practices, genetic resistance and fungicides. Proper disease identification is crucial for effective management. This will determine which cultural practices are implemented, which varieties are selected, and the choice of fungicide and application and timing. We have included a fungicide efficacy table here for your reference if applications are warranted.
Below are some diseases commonly observed in early to mid-season soybeans.
Aerial blight can spread rapidly in soybeans if not properly managed. This disease is caused by the fungus Rhizoctonia solani and is favored by warm overcast days and extended periods of leaf wetness. This is the same fungus that incites sheath blight in rice. Initial symptoms appear as reddish-brown (sometimes water-soaked, greasy) blotches on leaves, usually in the lower to mid-canopy (Figure 1). As the disease progresses, white cottony fungal mycelia may cause adjacent leaflets to adhere together (Figure 2). If favorable conditions persist, the foliage becomes brown and blighted, and pods may have reddish-brown lesions. If the disease continues to progress, pods may be aborted. Aerial blight is usually evident during early reproductive stages of growth and later. The potential for risk increases when soybeans are rotated with rice. This disease can spread rapidly and should be managed immediately upon detection if the crop is in the late vegetative or reproductive growth stages. In some areas of the state, QoI resistance has developed in the pathogen population requiring the use of SDHI fungicides (link) for management.
Bacterial pustule is caused by the bacterium Xanthomonas axonopodis pv. glycines. Symptoms of this disease are very similar to soybean rust. The disease, while found most years, is not a major disease of Louisiana soybeans. Symptoms begin as small pale green, water-soaked spots with elevated centers on the upper and lower leaf surfaces. As the lesions (spots) mature, they turn brown with elevated volcano-like pustules on the lower leaf surface and are easily confused with rust (Figure 3). Careful examination with a dissecting microscope or a 20X hand lens is required to distinguish between these two diseases. Pustules are dry in appearance and may be found on pods in susceptible varieties. Disease development is favored by temperatures between 85 and 90 degrees Fahrenheit and wet weather. The bacterium, which may be seedborne, survives on surface crop residue, on wheat roots and in weed hosts such as red vine. The bacterium may be dispersed by splashing water or windblown rain.
Brown spot is caused by the fungus Septoria glycines. Symptoms of this disease are variably-sized (specks to 1/8 inch) dark brown spots with irregular borders on the upper and lower surfaces of leaflets (Figure 4). The disease starts low in the canopy progressing upward during periods of warm, rainy weather and may prematurely defoliate plants. Late-season lesions appear rusty brown and may have chlorotic halos. Lesions also may appear on stems, petioles, and pods. Soybean varieties vary in susceptibility to the pathogen. Rotation and tillage may reduce initial inoculum. Some fungicides may be effective on brown spot; however, it is likely that QoI resistance has developed in the pathogen population, and reduced efficacy may occur (link).
Cercospora leaf blight (CLB) is an annual issue in Louisiana. The disease is caused by at least three different Cercospora species. The fungus can infect seedlings, resulting in plant death, or possibly remain latent and not produce symptoms until R5. Initial symptoms appear as small chocolate brown or purple lesions on the petioles near the base of the leaflet (Figure 5). As the disease progresses, foliar symptoms are expressed as a reddish-brown, tan or purple discoloration on the upper leaf surface in the upper canopy. Leaves may have a leathery appearance, and the fungus can sporulate in older lesions resembling ashes (Figure 6). Advanced stages of this disease result in premature defoliation, discolored pods and reduced seed quality. The seed phase of this disease is evidenced by purple-stained seed at harvest. Seedling infection is favored by moderate temperatures (70 to 80 degrees) and extended periods of leaf wetness (eight to 16 hours). However, petiole and leaf symptoms during late reproductive growth stages, including defoliation, are favored by hot conditions. The pathogen may be seedborne and survives on plant debris in the soil. The fungus also has been isolated from multiple weed hosts and cotton. Over the past few years, some products containing SDHI and/or newer triazole materials have been efficacious on CLB (link).
Downy mildew is caused by the fungus Peronospora manshurica. Currently, this disease is not a major threat to soybeans produced in Louisiana. Symptoms may be confused with those produced by soybean rust and initiate as small pale green to yellow spots on the upper leaf surface. Older lesions or spots may turn gray to dark brown and when the disease is active, grayish tufts of fungal mycelium (similar to dryer lint) can be found on the underside of the leaf opposite the yellow spot (Figure 7). The disease develops rapidly when temperatures are between 50 and 77 degrees in the presence of extended periods of high relative humidity. Fungicide applications are not recommended for management.
Frogeye leaf spot (FLS) is caused by the fungus Cercospora sojina. Symptoms are found predominately on the leaves during mid to late reproductive stages but may also appear on the petioles, stems, and pods. Initially, small chocolate brown spots can be found on the leaflets. If the disease continues to develop, mature lesions have light brown to gray centers with a reddish-brown margin (Figure 8) and may be confused with herbicide drift. During favorable conditions, black fungal reproductive structures are produced in the center of the leaf spots. Stem lesions are rare and are elliptical with red centers and dark brown to black margins. Pod lesions are circular to elliptical, sunken and light gray to brown. Disease development is favored by warm, humid weather and frequent rainfall. The pathogen can survive on seeds and in infected plant debris. Most soybean varieties are resistant to FLS. There are many available fungicide options for management in susceptible varieties (link).
Southern blight is caused by the fungus Sclerotium rolfsii. We have observed the disease in several fields in northeast Louisiana, most commonly in heavy soils following soybean. Southern blight can affect soybean at any point during the growing season and is commonly observed during early vegetative stages or during pod fill in conjunction with Southern root-knot nematode. In vegetative stage soybeans, light brown lesions are observed at or near the soil line that will girdle stems resulting in plant death. Infections can spread to nearby plants often seen going down the row in wide-row operations. White mycelial growth is observed near the crown during rain or humid weather. Tan to brown reproductive structures the size of mustard seeds are often found at the soil line. The same pathogen that causes southern blight in soybean causes “white mold” in peanuts, so rotating the two crops is not advisable. Rotating to corn, grain sorghum or rice will keep inoculum levels in check. Fungicide efficacy data are unavailable for managing southern blight in soybeans.
Soybean rust is caused by the fungus Phakopsora pachyrhizi. Symptoms initiate in the lower canopy and begin as small, brown to tan raised (volcano-like) pustules on the lower leaf surface (Figure 9). Spores produced in these pustules resemble grains of sand and are tan when young. Older spores are darker in color. As the disease progresses, pustules may coalesce into blighted leaflets, causing the leaflets to defoliate. Symptoms are usually evident when soybean is in the mid (R3) to late (R6) reproductive growth stages. Pustules can also be present on petioles and pods when disease is severe. Kudzu is another host for this fungus. The disease develops rapidly when temperatures are between 59 and 77 degrees and when leaves remain wet for six to 10 hours. There are many fungicide options available for management (link).
Target spot is commonly observed every year in many fields in Louisiana; however, the disease has not been severe as it was in the northeastern corner of the state in 2016. Target spot symptoms include concentric rings within lesions that are surrounded by yellow halos (Figure 10). Affected leaflets may turn yellow and fall off, with defoliation progressing from the lowest leaves upward. Petioles in the lower canopy may exhibit small CLB-like lesions caused by the target spot pathogen. Susceptibility and symptomology may vary with variety. Target spot can be a problem when very susceptible varieties are subjected to optimal conditions (frequent rainfall events) for disease development. This disease is probably one of the most common foliar diseases of soybean in the southern United States. It is observed annually, usually during late stages (R6 and beyond), but is rarely yield-limiting. Based on limited data, fungicides, particularly SDHI compounds (link), may be effective on target spot; however, applications must be made prior to canopy closure to deliver the product low enough in the canopy to slow upward progression. Based on ongoing research in other states, it is very likely that resistance to strobilurin fungicides exists in this pathogen population. The odds of a return on investment are low when treating for target spot, as severe disease pressure is the exception rather than the rule.
Anthracnose is caused by the fungus Colletotrichum truncatum. Early infections can result in pre- and postemergence damping off. Foliar symptoms include petiole cankers, leaf rolling, necrosis of the laminar veins and premature defoliation. The fungus can produce acervulli (fruiting bodies that resemble black specks) on stems and pods (Figure 11). These bodies occur randomly — not in linear rows, as is the case with pod and stem blight. If the disease continues to develop on pods, seed quality will be compromised. The disease is favored by high relative humidity. Infection occurs throughout the growing season, and the fungus overwinters in crop debris and infected seed. Many fungicide products are effective on anthracnose (link).
Charcoal rot is caused by the fungus Macrophomina phaseolina. Infected seed may not germinate, and seedlings may die soon after emergence. Infected plants die prematurely during hot, dry weather. Symptoms occur in dry, sandy areas in a field. The roots and lower stems are deteriorated, and the epidermal and sub-epidermal tissue will be silvery and dotted with black pepper-like sclerotia (survival structures) (Figure 12). Seed from infected plants are usually smaller than normal but are not discolored or shriveled. Disease development is favored by hot, dry weather (82 to 95 degrees). The fungus can survive in the seed coat, host residue or soil.
Pod and stem blight occurs most frequently on pods and stems. The disease is caused by the fungus Diaporthe phaseolorum var. sojae. Infection may occur early in the season; however, signs of the disease are not evident until late season (R7). Pycnidia (fruiting bodies that resemble black specks) occur in linear rows on the stems and pods. If favorable conditions persist, seed quality will be compromised. This disease is favored by warm, wet weather, and the fungus overwinters in crop residue or infected seed. Some fungicides may be effective on pod and stem blight (link).
Red crown rot is caused by the fungus Calonectria ilicicola. Root infections may occur soon after planting, but initial symptoms are usually not evident until soybeans are in mid to late reproductive growth stages. Roots become black with rotted segments, and the base of the stem at the soil line may be covered with brick red reproductive structures, usually most evident when soil is very moist (Figure 13). Foliar symptoms are characterized by interveinal chlorosis and necrosis. Moderate temperatures and wet soil conditions at planting promote disease development. Maximum root infections occur when soil temperatures are 77 to 86 degrees. The fungus may overwinter in the soil and in infested plant debris on and in the soil.
Taproot decline (TRD) is prevalent in northeast Louisiana again this year. Symptoms of TRD may occur at any point in the growing season, with foliar symptoms of interveinal chlorosis or necrosis most evident during reproductive stages (Figure 14). Plants adjacent to those exhibiting foliar symptoms may have died earlier in the season, often unnoticed. When pulled, affected plants may break off at the soil line. When excised, the surfaces of tap and lateral roots will exhibit black, discolored growth (Figure 15). If stems are split at the crown, there is often a white, cottony growth in the centers. In most cases, this disease goes completely unnoticed until pod fill (R5 to R6), where it appears at a distance as early cutout. It is often confused with sudden death syndrome. Rotation to corn, cotton, grain sorghum, peanut, or rice may reduce disease incidence and severity. Tillage also may lower the chances of TRD occurring. Resistant varieties may be available. Research on seed treatment and in-furrow fungicide options is ongoing.
By Rasel Parvej, David Moseley, Josh Copes and Syam Dodla, LSU AgCenter scientists
Potassium (K) is the second-most yield-limiting nutrient in soybeans. Even though nitrogen (N) is the most limiting nutrient, a soybean plant meets its own N requirement through biological N fixation. Therefore, soybeans are mainly fertilized with K and phosphorus (P) fertilizers in soils that have very low to medium K and P levels. Soybeans are more responsive to K than P fertilizer and require a large amount of K to maintain optimum water balance in plants, increase photosynthesis and assimilate translocation from source to sink, reduce transpiration losses of water and improve uptake of other nutrients. A 55-bushel soybean crop requires about 160 pounds of K2O (potassium oxide) per acre, or approximately 2.9 pounds of K2O per bushel harvested.
Potassium deficiency can decrease soybean yield by more than 50% across soil types that range from sandy loam to clay loam. In addition, K deficiency decreases P uptake by soybean plants and reduces soybean seed quality by decreasing seed oil and protein content and increasing purple seed stain. Potassium deficiency can occur in any soybean field that is very low to low in soil test K level and is not fertilized with K. Potassium deficiency, however, often occurs in coarse-textured soils with low cation exchange capacity (CEC of less than 10), such as loamy sand to silt loam soils. Coarse-textured soils are highly prone to K leaching below the root zone. Sometimes, fall applications 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 also are poor in water holding capacity, and drought in these soils often causes K deficiency by decreasing K uptake by plant roots.
Soybean K deficiency symptoms first appear as irregular yellowing on the edges of K-deficient leaves. As the growing season progress, the entire leaf edges turn brown, and eventually the whole leaf dies. Potassium deficiency symptoms can occur as early as at the V3 vegetative stage (three trifoliate leaves) mainly on the lower older leaves (Figure 1). But symptoms often occur on the upper younger leaves during the reproductive stages, especially under severe K deficiency conditions (Figure 2). Soybean fields with K deficiency symptoms early in the growing season are very easy to diagnose and manage. However, most soybean fields suffer from K deficiency and exhibit yield losses without showing any visible deficiency symptoms at all, or at least not until the later reproductive growth stages (beginning seed, R5, to full-seed, R6). This phenomenon, called hidden hunger, is most common in soybean fields that are low to medium in soil-test K level, have not received K fertilization, have high leaching potentials due to low CEC and excessive rainfall, or undergo severe drought conditions. Soybeans grown in low pH (less than 6.0) soils also 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 deficiencies in soybeans. 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 a K deficiency and rescuing yield losses than diagnosis at the later growth stages.
After tissue sampling, tissue K concentration at a particular growth stage is interpreted to diagnose K deficiency. Many current tissue K interpretations used by most of the plant diagnostic labs only allow interpretation of K concentration for soybean leaflet (without petiole) collected at or around the R2 stage. Recently at the University of Arkansas, Parvej et al. (2016) developed critical trifoliate leaflet and petiole K concentrations from the R2 to R6 reproductive stages (Figure 3). These critical K concentrations would allow soybean producers, agronomists and crop consultants to sample either leaflet or petiole or both to diagnose K deficiency across the reproductive growth stages of soybeans.
For proper tissue sampling, 15 to 20 recently matured trifoliate leaves, including petioles from the third node from the top of the soybean plant, should be collected. The date and soybean growth stage should be recorded (Figure 4). Then, the leaflet of each trifoliate 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 3. 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.
For maximum soybean growth and yield, tissue K concentration should be above the critical level across the growth stages. If the tissue K concentration falls below the critical level, especially during the early reproductive stages, soybeans should be fertilized with K to make sure K is not yield liming. 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 applying 60 pounds K2O per acre until the R5 stage or about 5-weeks past the R2 stage. This is because soybeans uptake more than 70% of the total K after blooming and maximize (100%) K uptake near the R6 stage. Therefore, diagnosis of K deficiency followed by an immediate K application early in the growing season would allow soybean plants enough time to actively uptake K from soils or through leaves and recover significant yield losses. However, pre-plant K application is the best way to maximize soybean yield.
Both dry and liquid fertilizers can be used in correcting soybean K deficiency during the growing season. However, dry fertilizer would be more effective and economical for correcting severe K deficiency since a high amount of K would be required. Foliar application of liquid K may be effective for a small amount of the K requirement because K fertilizer has a high salt index that can burn soybean foliage if applied in high concentrations (Figure 5). Therefore, the foliar method requires several applications to correct a severe K deficiency, which would increase application cost. Also, foliar K fertilizer is more expensive than dry K fertilizer. The most effective and economical method is either by top-dressing or flying 100 pounds muriate of potash (0-0-60; 60 pounds K2O) per acre. An in-season K deficiency management experiment for soybeans has been established at the Macon Ridge Research Station in Winnsboro, Louisiana, this year, and the results will be shared during the growers’ extension meeting in the winter.
Figure 1. Potassium deficiency symptoms during the early vegetative growth stages of soybeans.
Figure 2. Potassium deficiency symptoms during the reproductive growth stages of soybeans.
Figure 3. 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)
Figure 4. Steps of soybean tissue sampling during the R2 reproductive stage. Pencil in the picture indicates third node from the top of the plant.
Figure 5. Soybean foliage damage due to the application of high rate of liquid potassium.
|Corn, cotton, grain sorghum||Agronomic||Dan Fromme||318-880-8079|
|Grain sorghum||Agronomic||Dan Fromme||318-880-8079|
|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|
|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|
||Feed grains and cotton||Rasel Parvej||479-387-2988|