Field Crop Diseases in Arkansas
- Soybean Diseases
Soybean diseases can be a significant economic factor in Arkansas. On average, diseases reduce yields in the state by an estimated ten percent, although in individual fields, and with certain diseases, losses may be much higher. Because soybean pathogens are common statewide, accurate disease identification and an awareness of the potential for disease losses to occur are essential for the continued success of Arkansas soybean production. The following symptom descriptions and color photographs of common diseases in Arkansas should help you identify and manage soybean diseases before they become a yield-limiting problem.
Bacterial Diseases Foliar Diseases Soilborne Diseases Nematodes Viral Diseases Bacterial Blight Anthracnose Charcoal Rot Soybean Cyst Soybean Mosaic Virus Bacterial Pustule Brown Spot Pod and Stem Blight Root-knot Bean Pod mottle Virus Cercospora Leaf Blight & Purple Seed Stain Stem Canker Reniform Tobacco Ringspot Virus Downy Mildew Sudden Death Syndrome Management Soybean Vein Necrosis Virus Frogeye Leaf Spot Phytophthora Root Rot Aerial Blight Southern Blight Asian Soybean Rust Target Spot
Asian soybean rust:
Soybean Disease Management Links:
Fungicides for Soybean Diseases (MP 154)
Fungicides for Edamame Diseases (MP 154)
Travis Faske, PhD Extension Plant Pathologist firstname.lastname@example.org
- Rice Diseases
There are at least 20 diseases of rice routinely observed in Arkansas with most being caused by fungi and a handful considered major problems to rice production. Major diseases include the following:
Sheath blight is the most prevalent rice disease in the state of Arkansas affecting at least 750,000 acres each year. It is worse on semi-dwarf long grain varieties, with most of these fields treated with fungicides. Read more about Sheath blight on pages 2-5.
Blast is a very important disease in certain areas each year and in favorable years can be devastating, causing up to 100% yield loss. The disease may affect leaves, stem nodes, collars, and panicles. It is managed using resistant varieties, water depth, planting date, and fungicides. Read more about Blast Rice Diseases on pages 1-2, 6-7.Straighthead is a disorder of unknown cause. Symptoms can be induced with arsenic under artificial conditions. It is the oldest known rice disease in Arkansas, occurs on certain fields and can cause heavy losses approaching 100% if these fields are not drained and dried at the right timing. Some varieties are more susceptible than others. Read more about Straighthead on page 12.Kernel Smut has been an increasing problem for many years in Arkansas. Infected kernels turn to black powder inside causing quality problem, especially if the rice is used for parboiling. The disease is suppressed by reducing nitrogen fertilizer and applying fungicides containing propiconazole at the right rate and timing. Read more about Kernel Smut Rice Disease on pages 9-10.False smut was first reported in Arkansas in 1997 and has been widespread in all rice producing counties. Similar to kernel smut, false smut spores replace the developing rice kernel and greatly affect grain quality. It is more common in fields with high nitrogen rate and on later planted rice. It is managed using resistant varieties, adequate nitrogen rate and spraying fungicides containing propiconazole. Rate and timing are important. Read more about False Smut Rice Disease on pages 10-11.
Bacterial panicle blight has been a threatening problem for rice production in recent years in Arkansas. Most commercial cultivars are susceptible (S) or very susceptible (VS). Jupiter and the hybrids show moderate resistance level. A yield loss up to 50 percent can be caused in susceptible varieties that flower in late hot season. There are no recommended pesticides. Early planting and managing nitrogen fertilizer may reduce disease incidence and severity. Read more about Bacterial Panicle Blight on page 11, Rice Disease, Early planting for BPB, FSA7580
Autumn decline of Hydrogen sulfide toxicity (Akiochi) occurs in roots of rice that are grown on highly anaerobic flooded soils. Rice roots turn black and eventually die and rot. Some rice fields across the state of Arkansas suffer from this disorder. H2S odor is often noticed in affected fields. Draining fields at the right time to allow oxygen into soil enhances new root growth. Read more about Autumn decline or Hydrogen sulfide toxicity on page 13 , Akiochi.
Rice seedling rots and seedling diseases are consistent problems on much of the April planted rice in Arkansas and on certain minimum tillage fields. While reducing stand considerably, rice yields may be unaffected by seedling diseases if the stand is uniform, because modern rice varieties have the ability to tiller to fill available space and compensate for early stand loss. Read on seed rots and seedling diseases, Managing seedling diseases, Rice seedling fungicides in the MP154.
Other well known rice diseases are sporadic, localized or minor in the state. They include stem rot, black sheath rot, scald, scab, aggregate sheath spot, bordered sheath spot, leaf smut, brown spot and narrow brown spot among others.
In rare instances, any of these diseases have the potential to cause yield loss under ideal circumstances. White tip nematode is seed-borne and lives inside rice plant and feeds on developing leaf tissue resulting in distinctive whit tip on affected leaves.
AG Chem Information Services for Growing Companies- Site that hosts pesticide labels, including fungicide labels
Greenbook- Site that hosts pesticide labels, including fungicide labels
Extension Plant Pathologist
University of Arkansas
Division of Agriculture
Cooperative Extension Service
Rice Research and Extension Center
2900 HWY 130 E. Stuttgart, AR 72160
Phone:Office 870-673-2661 Ex. 224
- Corn Diseases
Several diseases are economically important for corn production in Arkansas. Most of the major corn diseases are foliar, which vary from year to year because they are strongly influenced by weather conditions. Thus, accurate identification and an awareness of potential disease losses are essential for continued success of corn production. Seedling disease, southern rusts, gray leaf spots, stalk rots and aflatoxin contamination are among the most common corn diseases in Arkansas. The following symptoms description and photographs of common diseases should help identify and manage corn diseases before they become a yield-limiting problem.
Corn seeds and seedlings may be attacked by numerous soilborne fungi (e.g., Pythium, Diplodia, Fusarium, Penicillium, etc.) that cause seed rot or seedling blight. Significant stand loss can occur especially in poorly drained, excessively compacted fields or when cold and/or wet soils conditions persist for a long time after planting. A soft rotting of stem tissues near the soil, yellowing, wilting and death of leaves are common symptoms of blighted seedlings. To manage seedling diseases, plant quality, fungicide-treated seed into warm soil (consistently above 55°F) with sufficient moisture for rapid seedling emergence.
Southern rust is one of the most important foliar corn diseases in Arkansas, which is reintroduced each year from southern states. When southern rust arrives late in the growing season on dough or dent growth stages it causes minimal losses. However, the risk of yield loss is much higher when infection occurs at tasseling or silking.
Symptoms: Southern rust is caused by the fungus Puccinia polysora, which only infects corn. Pustules of southern rust are small (0.2 to 2 mm in diam.), circular to oval, and light cinnamon brown to orange (Fig. 1) that frequently form on the upper leaf surface and very infrequently on the lower leaf surface (Fig. 1). Southern rust pustules may occur on leaves, stalks, sheath, and husks (Fig. 2).
Figure 1. Early season infection of Southern rust on upper and lower corn leaf surfaces.
Figure 2. Southern rust on corn sheath and leaf.
Rust spores are windblown from diseased areas progressively northward during the growing season. Typically, early season scouting for rust is easily overlooked by the casual observer because initial infections are typically only one to two pustules per plant (Fig. 1). The absence of sporulation on the lower leaf surface is in an indication of southern rust. In contrast, rust sporulation on the lower leaf surface is frequently associated with common rust, a minor disease. Early season pustule development is most frequently observed in the field at mid canopy level (4 to 5 ft. from ground) along the edge of the field. Free moisture as dew or light rain is necessary for rust spores to germinate and infect. Symptoms appear about 3 to 6 days after infection and by 7 to 10 days the pustules may rupture to expose more rust spores. Thus, new infections can occur very rapidly after the initial infection when conditions favor disease. Conditions that favor disease development consist of high temperatures (80ºF to 90ºF), high relative humidity, and frequent rainfall. Even in hot summer conditions with temperatures above 100ºF pustules continue to sporulate, hence the name southern rust.
Management: Corn hybrids vary in susceptibility to southern rust and the least susceptible hybrid should be planted in areas with a history of early season rust development. Since rust spores are windblown from southern states and initial infection at later stages of corn development is less likely to impact disease, early planted corn may avoid significant disease pressure. Fungicides are effective at suppressing southern rust though there is no economic threshold for a fungicide application. Producers should consider yield potential, hybrid susceptibility, growth stage, and the long range forecast when southern rust threatens. A fungicide application at tasseling or silking when southern rust has have been observed on a susceptible hybrid with good yield potential may be the most beneficial at suppressing disease development; however, additional application may be needed for season long crop protection. Field corn within two weeks from physiological maturity (i.e. black layer) is very unlikely to benefit from a fungicide application.
Common rust has been reported in all corn production regions of the US. Though this rust frequently occurs in Arkansas yield losses are negligible because environmental conditions in the summer are not favorable for common rust.
Symptoms: Common rust is caused by the fungus P. sorghi. As with southern rust, this rust only infects corn and not sorghum as the scientific name suggests. Pustules of common rust are circular to elongate, golden brown to cinnamon brown and found on upper and lower leaf surface (Fig. 3). As the pustule matures, cinnamon brown urediniospores are replaced by golden brown to black teliospores, which causes the pustules to darken to a brownish black.
Figure 3. Common rust pustules on upper and lower corn leaf surfaces.
Rust spores are windblown from tropics and move progressively northward during the growing season. Conditions that favor infection consist of moderate temperature (61ºF to 77ºF) and high relative humidity (>95%). At least 6 hours of free moisture is required for rust spore germination and infection. The latent period is 7 days with optimum conditions for disease development. Common rust is typically found in the lower canopy (1 to 2 ft. from the ground) and younger leaves are more susceptible than fully expanded mature leaves.
Management: In contrast to southern rust, common rust is considered a minor disease in Arkansas and typically requires no management practice. Resistant hybrids are effective and available; however, environmental conditions during the summer are often unfavorable for common rust to be a significant disease issue. Fungicides are effective, but not economically practical.
Northern Corn Leaf Blight (NCLB) commonly occurs in humid climates where corn is grown. As with most foliar diseases timing of infection is critical to yield losses. When disease development occurs before silking yield losses of 50% can occur; however, yield losses are minimal when disease development occurs after dough or dent growth stages.
Symptoms: Large (1 to 6 inches) gray green, cigar-shaped lesions develop on infected corn leaves (Fig. 4). Mature lesions are tan with dark zones of fungal sporulation. Size and shape of lesion may vary with genes for resistance among hybrids. NCLB is caused by a fungus, Exserohilum turcicum, which can infect sorghum, Johnson grass and Sudan grasses. The fungus overwinters on crop residue left on the soil surface. In the spring, conidia are produced that are dispersed by wind to cause the primary infection. Secondary infection occurs from the abundant conidia produced on leaf lesion. New lesions can produce spores within a week, thus new infection can occur quickly. Disease development is favored by moderate temperatures (64ºF to 81ºF) and prolonged periods of leaf wetness provided by heavy dew.
Management: Resistant hybrids are available and the most economical tactic to manage this disease. Four single dominant genes Ht1, Ht2, Ht3, and HtN confer resistance to NCLB. Resistance is expressed as chlorotic lesions that support limited sporulation or long latent period with a few lesions. Crop rotation and tillage practices can hasten the decomposition of corn residue and subsequent survival of pathogen on crop residue. Fungicides can be beneficial when NCLB is observed at late vegetative or early reproductive stages on susceptible hybrids with good yield potential.
Figure 4. Several large lesions caused by northern corn leaf blight.
Gray Leaf Spot is one of the most common foliar diseases of corn. It is most severe where minimum tillage is a common production practice along with warm, humid conditions.
Symptoms: Early season symptoms consist of small necrotic spots sometimes with a chlorotic halo that are difficult to distinguish from other foliar corn diseases. Mature lesions are tan to gray and expand linearly between leaf veins giving a rectangular shape (Fig. 5).
Gray Leaf Spot is caused by the fungus, Cercospora zeae-maydis. The fungus overwinters in crop residue that remains on the soil surface. In the spring, spores are produced that are dispersed by wind and splashing rain onto corn leaves. Disease development is favored by warm (72 to 86° F) humid (high humidity or leaf wetness) conditions.
Management: Resistant hybrids are available and should be considered in minimum tillage systems. In fields with a history of Gray Leaf Spot, conventional tillage practices that promote crop decomposition and one crop rotation is recommended. Foliar fungicides may be economically beneficial on susceptible hybrids to protect high yield potential with prolonged conditions that favor disease development.
Figure 5. Numerous rectangular lesions caused by Gray Leaf Spot.
Southern corn leaf blightt (SCLB) was considered a minor disease with little economic importance until 1970 when this disease reached epidemic proportion. SCLB destroyed 15% of the U.S. corn crop costing an estimated one billion dollars. Later, it was determined a new strain of the fungus (race T) produced a toxin (T-toxin), which was highly aggressive on corn hybrids with the Texas male-sterile cytoplasm (cms-T). In 1970, the majority (~85%) of the U.S. corn hybrids contained cms-T. Today, resistant corn hybrids are used to manage this disease.
Symptoms: Leaf lesions vary in shape, color and size but are generally oval to elongated, tan with yellow green halos to reddish brown borders. Lesions may occur on leaf blades, sheaths, stalks, and husks. The fungus may also attack ears causing a black, felty mold on the kernels.
This disease is caused by the fungus Bipolaris (Helminthosporium) maydis, which infects many other grasses. There are three physiological races of B. maydis; race O, race T, and race C. Only race O and T have been reported in the U.S. The fungus overwinters in soil and crop debris. Conidia (spores) are produced by the pathogen, which are disseminated by wind and splashing rain onto plants. Warm (86 to 90 F) wet weather conditions favor disease development. The disease cycle can be completed within 2 to 3 days with favorable conditions, thus this disease can spread very rapidly.
Management: Resistant hybrids are the most practical and effective management tactic for SCLB. Tillage and one year crop rotation with non-host crops can reduce the overwinter pathogen survival on crop residue. Fungicides are available but rarely economical except in seed corn production.
Anthracnose leaf blight, top dieback, and stalk rot are important diseases of corn. The leaf blight stage can occur on leaves at any growth stage. Anthracnose top dieback and stalk rot are often observed in the field after pollination resulting in premature plant death and subsequently higher yield losses.
Symptoms: Leaf lesions begin as small water soaked lesions that may enlarge up to ½ inch with tan to brown center and yellowish to reddish brown boarders. Lesions may coalesce, killing large leaf areas or entire leaves of susceptible hybrids. Lesion size and can vary depending upon hybrid making diagnosis in the field difficult. The fungal fruiting bodies (acervuli) develop abundantly on dead tissue and can typically be seen with the aid of a hand lens. Dark black spines (setae) can often be observed protruding from these fungal fruiting bodies. Leaf symptoms are common on lower leaves early in the season with no further disease spread until later in the season after tasseling.
Anthracnose top dieback is commonly associated with yellow or dead flag leaf scattered across the field any time after tasseling. Fungal fruiting bodies can be observed on stalk and leaf sheath below the tassel. These blighted top may top-lodge above the ear before harvest. These symptoms can be helpful to differentiate Anthracnose top die back from top dieback caused by environmental stress.
Anthracnose stalk rot is relatively easy to diagnose in the field late in the growing season by shiny black, linear streaks or blotches that appear on the surface of the lower stalk. Stalk pith tissue is often gray to brown in color and the stalk can be easily crushed or lodged with little pressure. Severely diseased stalks are weak and my lodge before harvest. In contrast to top dieback, premature death of the entire plant is an indicator of stalk rot.
Anthracnose of corn is caused by the fungus, Colletotrichum graminicola. The fungus overwinters on crop reside left on the soil surface. In the spring, spores are produced that are dispersed by wind and splashing rain onto leaves causing the primary infection. Anthracnose is favored by warm temperatures, heavy dew and long periods of cloudy, wet weather.
Management: Resistant hybrids are the most economical management practice; however, resistance to leaf blight may not be resistant to anthracnose stalk rot. Also, hybrids with resistance to other stalk rots may not be resistant anthracnose stalk rot. Hybrids with good stalk strength, yield potential, and resistance to appropriate phase of anthracnose should be selected. Since anthracnose overwinters in corn residue complete burial of residue and one-year rotation from corn will significantly lower in-field inoculum. Avoid plant stress by using a balanced soil fertility program and control insect feeding root worms. Finally, if top dieback and/or stalk rot are prevalent harvest should be scheduled earlier to prevent additional losses.
Charcoal rot is a common stalk rot disease that frequently occurs with prolonged drought conditions. Severe yield losses may result when hot, dry weather persists during and after tasseling.
Symptoms: Infection can occur at any stage from seedlings to plants nearing maturity. Early symptoms of charcoal rot include brown, water-soaked lesions on the root and lower stalk that later turn black. Late in the season, small black microsclerotia can be observed speckled within affected stalks, causing the stalk to appear gray black. Severely diseased stalks are weak and dry rotted causing them to lodge a few inches above the soil line before harvest.
Charcoal rot is caused by a fungus, Macrophomina phaseolina, which has a wide host range that includes many agronomic crops (e.g. soybean and sorghum). The fungus overwinters in the soil as black microsclerotia (0.05 to 0.22 mm) and may infect roots and lower stems at any time during the growing season. Charcoal rot is favored by soil temperatures of 86 to 100 °F with widespread severity during years that are hot and dry.
Management: Charcoal rot losses can be minimized with adequate irrigation during the growing season. Excessive nitrogen rates and low potassium dramatically increase charcoal rot severity. Thus, timely irrigation, combined with a well-balanced fertility program, can effectively reduce charcoal rot. Resistant varieties are not available through planting hybrids with good stalk strength is encouraged to reduce lodging.
Other Stalk Rot diseases of corn may be caused by a variety of soilborne fungi; most important are species of Gibberella, Fusarium, and Pythium. The exact causal organism can be difficult to identify because several fungi may invade tissue once symptoms are obvious. Stalk rots cause yield losses by increasing lodging of the crop or by cutting off the supply of water and nutrients from the roots. Stalk rots are usually increased by drought stress, hail damage, leaf diseases, high stand density, excessive nitrogen fertilization, low soil potassium levels, lack of rotation, possibly minimum tillage and stalk feeding insects.
Symptoms: Stalk rots normally begin soon after pollination and become more severe as the plant matures. Premature death of the entire plant is an indicator of stalk rot. Rotting affects the roots, crown and lower internodes. Various discolorations of the pith, including whitish-pink to salmon, are common as is stalk breakage and premature decline.
Management: Plant strong-stalked hybrids to reduce lodging. Practice balanced fertilization based on recent soil tests and avoid excessive nitrogen. Avoid narrow rows and excessive seeding rates if possible. Control stalk insects and harvest as early as possible, especially when stalk rot is persistent or the field has a history of stalk rot.
Aspergillus Ear Rot and Aflatoxin
The single most serious disease threat to Arkansas corn production is aflatoxin - a chemical contaminant of grain produced by certain Aspergillus fungi. Aspergillus flavus and A. parasiticus are weak and opportunistic plant pathogens that cause Aspergillus ear rot. These fungi are ubiquitous and commonly found in soils across Arkansas. Though Aspergillus ear rot commonly occurs at the tip, infection can occur anywhere on the ear. Yellowish to green spores can be seen with unaided eye at infection points (Fig. 6); however, a lab assays is necessary to quantify the level of aflatoxin contamination. Aflatoxin can be produced by these fungi in the field or during storage. Predisposing factors that contribute to field contamination include; drought conditions from tasseling to grain-fill, high temperatures at flowering, insect damage and nutrient (nitrogen) stress. Aflatoxin is produce when kernel moisture is between 20 to 16%, thus delayed harvest or rain during drying can contribute to higher level of aflatoxin. Similarly, contamination in storage is commonly associated with improper drying and storage conditions. In these situations the kernel moisture level increases and dormant fungi begin to grow and produce aflatoxin. Thus, timely harvest and proper storage conditions are key considerations to minimize aflatoxin contamination.
Figure 6. Aspergillus ear rot of corn.
Aflatoxin levels are regulated by the Food and Drug Administration (FDA) beginning at 20 ppb (parts per billion) for use in food and feed. Thus, corn grain that exceeds 20 ppb is very difficult or impossible to sell or use in current marketing environments. Prevention of this problem is of critical importance each year in the South.
Cultural practices to manage aflatoxin contamination in corn include planting early and maintaining proper irrigation from tasseling through grain fill. Also, maintaining optimum fertility for corn production and utilizing Bt hybrids to manage insect damage. Selection of hybrids adapted to the area as well as those with closed shuck morphology has also minimized contamination. A biological product that contains a non-toxigenic strain of A. flavus (Afla-Guard) has been effective at reducing aflatoxin contamination by 60 to 75% in the field. Green fungal spores can be found in ears of corn in treated fields; however, it is impossible to determine if it is a toxigenic infection without lab analysis. This biological management tactic is probably best suited for field with a history of relatively low levels of aflatoxin rather than a high risk field with a history of high levels of aflatoxin contamination.
Travis Faske, PhD Extension Plant Pathologist email@example.com
- Cotton Diseases
Several diseases are economically important for cotton production in Arkansas. Most of the major cotton diseases in Arkansas are soilborne, so they survive from year to year in the soil within the cotton field. Successful management of these diseases to avoid economic loss depends on accurate identification of the pathogens involved and a systems approach to their control.
Seedling diseases are caused by several soilborne fungi, but the most common are Pythium spp., Rhizoctonia solani, Thielaviopsis basicola, and several species of Fusarium. These fungi cause pre-emergence and post-emergence damping off, and black root rot, especially when cool, wet conditions persist after planting. Seedlings infected with Pythium spp. rapidly collapse from root decay. Prior to complete collapse lesions on the radicle often have a soft watery appearance. Seedlings infected with R. solani exhibit a dark, sunken lesion at or below the soil line. Similarly, seedlings infected with Fusarium often have black to brown lesions on the hypocotyl and root system. Thus, a laboratory assay is necessary to identify seedling disease fungi. Cotton seedlings infected with T. basicola have a blackened root system and hypocotyl below soil line. Though not all these pathogens will be present in all fields, most cotton fields in Arkansas will have some of these pathogens. There is no host plant resistance for seedling diseases, thus management consist of fungicides and cultural practices. Fungicide seed treatments are the first line of defense in suppressing seedling disease especially when seeds are planted under in favorable soil conditions that promote rapid germination and seedling emergence. Additional applications of fungicides in-furrow at planting may occasionally provide additional seedling protection and assist in stand establishment, but specialized application equipment is necessary. Currently, there is no fungicide with good efficacy for black root rot. Proper crop management with fertilizers and herbicides to avoid injury and promote vigorous growth will aid in good stand establishment.
Blight of cotton (also called Angular Leaf Spot) caused by Xanthomonas axonopodis pv. malvacearum is an important bacterial disease. Foliar symptoms consist of dark brown to black, angular leaf spots (Fig. 1) that may progress along major leaf veins (Fig. 2). As the disease progresses, petioles and stems may become infected, develop black cankers that girdle the petiole or stem and result in premature defoliation. Bolls may also become infected causing boll rots that discolor lint, and rot seeds. Infected bolls have round (non-angular), dark brown to black lesions that may appear water soaked. Environmental conditions that favor disease are high temperatures (86 to 97 °F) and high humidity (>85%). Bacterial blight can be seed transmitted, thus good management begins with using high quality, acid delinted, disease free seed. Fields with bacterial blight should not be cultivated when foliage is wet to minimize spread of the disease. Shredding stalks and incorporating crop residue into the soil after harvest can reduce overwinter survival of the pathogen on crop debris. Fields with documented bacterial blight should be planted with a blight-resistant cultivar the following year or rotated with a non-host crop like corn, grain sorghum, peanut, and soybean.
Figure 1. Bacterial blight of cotton on leaf and cotton boll.
Figure 2. Lesion from bacterial blight coalescing along leaf vein.
Fungal Leaf Spots
In contrast with cotton production areas in the southeastern states where target spot (caused by Corynespora) can be of economic importance, typically, fungal leaf spots of cotton (Fig. 3) are minor diseases in Arkansas and rarely require a remedial action especially when they occur late in the season. Leaf spots are commonly observed when the cotton plant begins to flower as nutrients like potassium move from leaves to developing bolls. Nutrient deficient leaves are susceptible to relatively weak fungal pathogens that can cause leaf spots. Cercospora Leaf Spot and Stemphyllium Leaf Spot are appear similar to one another and cause brown or gray spots with reddish-purple margins. Alternaria Leaf Spot lesion has a targeted pattern and typically forms in the lower canopy. Alternaria Leaf Spot can be confused with Corynespora Leaf Spot, so accurate diagnosis of the disease by the Plant Disease Clinic may be necessary. Fungicides for leaf spot control have not been shown to improve yield in Arkansas. Following recommended nutrient requirements for healthy cotton production is usually sufficient to manage fungal leaf spot of cotton.
Figure 3. Foliar leaf spot of cotton.
Fusarium and Verticillium Wilts
Two vascular wilts occur in Arkansas. Fusarium wilt is caused by Fusarium oxysporum, and Verticillium wilt is caused by Verticillium dahlia. Both pathogens can survive for several years in cotton fields, and both cause a browning and discoloration of the vascular tissue of infected cotton plants. Thus, a laboratory diagnosis is needed to determine the fungal pathogen.
Fusarium wilt can occur at any time during cotton development depending on environmental conditions and host susceptibility. The vascular system of symptomatic plants is dark brown (Fig. 4). Infection generally results in the death of the plant, and foliar symptoms can resemble severe potassium deficiency. Fusarium wilt most commonly occurs at warm temperatures (86 to 90°F) and a pH of 5.0 to 6.5. In Arkansas, Fusarium wilt is generally associated with the presence of the root-knot nematode, and wilt severity and yield loss are highest in fields with a history of Fusarium wilt and root-knot nematodes. Some cotton cultivars are available that are resistant or tolerant to Fusarium wilt, although where root-knot nematodes are severe, wilt resistance may not be fully expressed.
Verticillium wilt can also survive for several years in cotton fields. This disease often occurs during the last 1/3 of the growing season, especially when the growing season is cooler and wetter than normal. Similar to Fusarium wilt, the vascular system of symptomatic plants is dark brown, thus a laboratory assay is needed to accurately identify wilt fungi. Verticillium wilt commonly occurs when soil temperatures are somewhat cool (75 to 82°F), and a pH of greater than 6.0. Verticillium wilt is not associated with or enhanced by the presence of nematodes. High nitrogen levels or potassium deficiencies tend to increase disease severity. Cotton cultivars with tolerance to Verticillium wilt are available, but no highly resistant cultivars are available. Management of the crop for earliness, avoiding excessive planting density, and optimizing plant nutrition will aid in minimizing losses due to Verticillium wilt.
Figure 4. Discolored vascular system caused by Fusarium wilt.
Cotton boll rot occurs when certain fungi and bacteria cause damage to bolls, lint, and seed. Though the frequency of boll rots varies from season to season it is most commonly associated with cool wet weather, rank growth, and boll injury. Though cultural practices can help reduce boll rot potential no single practice will give satisfactory control. Plant growth management practices that minimize boll rots include avoiding excessive nitrogen rates and using growth regulators to shorten plant height. Seeding rates that allow establishment of optimum stands (2 to 3 plants per foot of row), wide row spacing (38 inches), careful use of late-season irrigation, and adequate insect control all will help minimize boll rot. No fungicide or bactericide has been shown to be economically effective to minimize boll rots.
The southern root-knot nematode (Meloidogyne incognita) and the reniform nematode (Rotylenchulus reniformis) are the most economically important nematodes on cotton in Arkansas. Root-knot nematode (RKN) is commonly found in coarse textured soils whereas reniform nematode (RN) is often associated with soils with greater percentage of silt or clay. Both species are widely distributed in the cotton growing areas of the state, and RKN has been reported in every cotton-producing county in Arkansas. The highest RKN population densities have been reported in Ashley, Crittenden, Drew, Desha, Lafayette, Lincoln, Lonoke, Miller, Mississippi, and Phillips counties. Reniform nematodes have been reported in 12 Delta counties with the highest incidence in Monroe, Jefferson, and Ashley counties.
Foliar symptoms of RKN are often mistaken for nutritional deficiencies or soil compaction that results in incipient wilting, thin stands, or stunted cotton plants (Fig. 5). RKN causes diagnostic galls on primary and secondary roots (Fig. 6). Foliar symptoms of RN can be similar to those caused by RKN; however, galls are not produced on cotton roots (Fig. 6). Nematode issues are rarely diagnosed based on plant symptoms in the field, thus a soil samples is needed to identify nematode problems.
Figure 5. Stunted cotton plants and thin stand due to southern root-knot nematode
Figure 6. Moderately galled cotton root caused by southern root-knot nematode (left) and brown egg masses of reniform on young cotton root system (right).
Soil samples should be taken in the late summer or fall (June to November) when population densities are highest after nematodes have reproduced on the existing crop. Collect 15 to 20 core samples with a ¾-in.-diam. soil probe 8 to 10-in deep within in the roots zone of symptomatic living plants. Combine the cores into one composite sample, and remove a one-pint subsample for the nematode assay. This sample should be placed in a plastic bag, labeled (identifying information on the outside of the bag) and tag and sealed. Protect samples from direct sunlight and excessive heat by storing them in an insulated ice chest (without ice). Samples may be held in an ice chest in a cool dry location (shaded floor of an air conditioned building) for a few days prior to shipping if necessary. Ship as soon as possible after collection and avoid delivery on weekends or holidays. Nematode samples should be sent to the Nematode Diagnostic Laboratory at 362 Highway 174 N., Hope, AR 71801. A small fee is required for nematode assays.
Management of plant-parasitic nematodes depends on species and population density. Economic thresholds for RKN and RN on cotton are 50 and 1,000 nematodes/100 cc soil, respectively. Nematode population densities at or above these levels would likely cause a problem and management practices should be implemented.
Nematode management on cotton consists of cultural practices, resistant cultivars and nematicides. Cultural practices focus on reducing nematode population densities by using crop rotation with resistant or nonhost crops (Table 1). A two year rotation is best to reduce nematode population densities, which is often unattractive to producers, but can be a cost-effective approach over time.
Table 1. Rotational crops for management of cotton nematodes.
Resistance is the most economical tactic to manage plant-parasitic nematodes. Cotton cultivars, Stoneville 5458B2RF, PhytoGen 367WNR, and Deltapine 174RF are adapted for production in AR and have a moderate level of resistance to RKN. Currently, there is no cotton cultivar with resistance to the reniform nematode.
Nematicides are popular tactics to manage cotton nematodes. Nematicides do not provide season long control of nematodes, thus it is recommended to utilize several management tactics to manage cotton nematodes. A high population density of cotton nematodes is often needed for fumigants to be cost effective. Non-fumigants are applied at-plating in-furrow or seed treatment or foliar application. The granular nematicide, Aldicarb (Temik), was historically widely used in-furrow at-planting. However, the manufacture of Aldicarb was suspended in 2011 and, although the product may be applied on cotton through 2014, supplies are extremely limited. Seed treated with a nematicide provide some suppression of nematode infection during early root development. Seed treatments are most effective when there is a low level of cotton nematodes in the field. Oxamyl (Vydate) has been used as foliar spray early in the season (generally shortly before or at squaring) often in combination with seed treated with a nematicide.
Table 2. Nematicides labeled for use on cotton in Arkansas
Travis Faske, PhD Extension Plant Pathologist firstname.lastname@example.org
- Peanut Diseases
Effective disease management is essential in production of a high-yield, high-grade peanut crop. Nationwide 40% of pesticides sprayed on peanut are for management of foliar and soilborne diseases. Thus, peanuts are prone to several diseases. Currently, Southern blight, Rhizoctonia limb rot, and Tomato Spotted Wilt Virus are important diseases in Arkansas although other diseases will likely occur with continued peanut production. Scroll down for a brief description and symptoms of some diseases that have been observed or are potential threat to peanut production in Arkansas.
Seedling diseases are caused by several soilborne fungi, but the most common are Pythium species and Rhizoctonia solani. These fungi cause pre-emergence and post-emergence damping off, especially when cool, wet conditions persist after planting. Seedlings infected with Pythium spp. rapidly collapse from root decay, often with a sloughed outer root layer. Alternately, seedling infected withR. solani exhibit a dark, sunken lesion just below the soil line. Seed treatments are usually effective at minimizing losses to seedling diseases, but other factors can contribute to a poor stand like herbicide injury, excessive rain, and poor quality, so be sure to determine the cause of the poor stand before implementing management practices.
Leaf spots are commonly caused by two fungal pathogens neither of which has been confirmed in Arkansas. Early leaf spot is caused by Cercospora arachidicola and late leaf spots by Cercosporidium personatum. Early leaf spot lesions are circular, brown to dark brown, usually surrounded by a yellow boarder. Late leaf spot lesions are similar in shape with darker spots and faint or no yellow boarder. Other leaf spots are web blotch (Phoma arachidicola), which have a large, olive-green colored webbing on upper leaf surface. Though none of these have been confirmed in Arkansas it is likely they will become important diseases with continued peanut production.
Southern blight is also called Southern stem rot, stem rot, and white mold commonly occurs at midseason (July or August) when foliage covers the rows. This is one of the most widespread diseases of peanut in the U.S. and can cause significant yield loss. Typically, infected plants or stems turn yellow and wilt (Fig. 1). A white fan-like mat of fungal mycelium can be seen at the base of symptomatic plants along with numerous small round fungal fruiting bodies (called sclerotia) that are about the size of a mustard seed (Fig. 2). Immature, sclerotia are yellow-tan then progress to reddish-brown and finally dark brown at maturity. Southern blight is caused by a soilborne fungus (Sclerotium rolfsii) that has a host range of ~ 200 species. The pathogen overwinters as sclerotia that can remain viable for three to four years near the soil surface. Disease is favored by hot (77-95°F), humid weather conditions.
Figure 1. Wilted peanut plant stems infected with southern blight.
Figure 2. Numerous reddish-brown sclerotia and white mycelium of Sclerotium rolfsii near the peanut crown.
Management of southern blight consists of preventing inoculum buildup by rotation with corn or sorghum. Soybean is a good host for Sclerotium rolfsii and other pathogens that affect peanut, so peanut should not follow soybean. The sclerotia do not persist when buried deeper (6 inches) in the soil, thus deep plowing is a good management practice in some production systems. Though some cultivars are less susceptible than others, do not rely on host plant resistance alone for disease management. Fungicides are effectively used to manage this disease, which are applied preventively beginning approximately 60 DAP or when <1% disease incidence is observed in the field. Recent studies have shown benefits from an in-furrow or early application of a fungicide to manage Southern stem rot.
Rhizoctonia limb rot is a late season disease that is more severe in irrigated runner peanut or after extended period of cool wet environmental conditions. Limb rot is characterized by an elongated, dark brown target-patterned lesion on stems and lower branches that are in contact with the soil line. Affected stems may wilt under hot conditions and die whereas the remaining plant appears unaffected. Pegs and pods may also be infected. Rhizoctonia limb rot is caused by a soilborne pathogen, Rhizoctonia solani AG-4, which overwinters in crop residue and in soil as scleroita. Crop rotation with corn or sorghum can minimize inoculum for subsequent crop and timely applied fungicides can be effective at protecting yield potential. Fungicides used for Rhizoctonia limb rot are often effective for Southern blight, thus producers can target multiple peanut diseases with some fungicides.
Aspergillus crown rot is caused by Aspergillus niger, which can be found in most peanut fields in Arkansas. The fungus causes seedling blight and also attacks older plants at the crown near the soil line. Plants growing under hot, drought-stressed conditions are most susceptible. Black spores and fluffy fungal growth (Fig. 3) often develop on the diseased hypocotyl or crown. Crown rot is usually of minor importance, but occasionally reduces yield. Crop rotation, seed treatments, and planting in good moisture for optimum plant stand are good management practices.Figure 3. Discolored and rotted root system with black spores near crown caused Aspergillus crown rot.
Tomato Spotted Wilt Virus (TSWV) is widely distributed in Arkansas and can cause 50% yield losses on highly susceptible cultivars. Infected susceptible peanut plants exhibit characteristic ring spotting or mottled leaves and stunted plant growth (Fig. 4 & 5). Advanced symptoms include chlorotic plants that wilt and die. Tobacco thrips and western flower thrips are vectors of this virus, which transmit the virus from TSWV infected plants to peanut plants. TSWV has a wide host range that includes many weed species found within and near peanut fields, vegetable, and some row crops. The most economical and practical management practice is the utilization of host plant resistance, which has been effectively used in Arkansas. Many runner peanut cultivars are available that are resistant to TSWV for commercial production. In-furrow insecticides like Phorate (Thimet) or Admire Pro (Imidacloprid) can suppress thrips feeding, which has been effective at minimizing early season infection in other peanut growing regions. Cultural practices include late planting to avoid heavy thrips pressure and planting during optimum conditions to achieve uniform plant stands and planting higher plant populations have also been effective in minimizing the spread of TSWV in a field in other regions.
Figure 3. Ring spotting of peanut leaf by Tomato Spotted Wilt Virus.
Figure 4. Mottling of peanut leaves by Tomato Spotted Wilt Virus
The peanut root-knot nematode, Meloidogyne arenaria, is most important root-knot nematode of peanut, which was reported in 1989 in Arkansas. However, it has yet to be detected on peanut in the peanut production area of Arkansas. The northern root-knot nematode, M. hapla, and lesion nematode, Pratylenchus brachyurus, have been reported in Arkansas and could be potential issues with continued peanut production. The southern root-knot nematode, M. incognita, is the most common species in Arkansas. Peanut is a poor host to the southern root-knot nematode so it is an excellent crop to use in a rotation in sandy soil where there is a history of southern root-knot nematode.
Pod Rot is a complex disease caused by one or more fungal pathogens. Pythium pod rot is caused by Pythium spp., and characterized by greasy-appearing, brown to black lesion on a soft pod. Rhizoctonia pod rot is caused by Rhizoctonia solani and characterized by dry, brown to dark brown lesion on a firm pod. This complex is difficult to manage. Maintain recommended levels of calcium and crop rotation are good management practices. Fungicides only provide partial control and do not always increase yield or grade. Fungicides for Pythium include mefenoxam and metalaxyl whereas fungicides for Rhizoctonia include azoxystrobin.
General Fungicide Application Practices for Peanut
Fungicides & Nematicides Registered for Peanut (MP 154): Peanut Seedling Diseases, Foliar and Soilborne Diseases, and Nematodes
Though disease pressure is relatively low in Arkansas fungicides are applied proactively in a two or three block program depending on severity of soilborne diseases. Currently, soilborne diseases are the main issues, which are often managed with fungicides because resistance is lacking. It is important to apply the fungicide where infection is occurring, near the soil surface. In some cases a higher volume of water (20 gal water/A) is sufficiently, but more often additional water is needed. Applying a fungicide prior to a rain shower or use of overhead irrigation can distribute the fungicide into the lower canopy. Applying fungicides at night when tetrafoliate leaves are closed can also be beneficial and has been effective in other peanut producing regions. Some fungicides like tebuconazole and propiconazole have soilborne and foliar activity thus these fungicides can provide dual purpose for managing peanut diseases. See MP 154 for other fungicides with both soilborne and foliar disease efficacy. Below is a general timeline of when soilborne and foliar diseases are most likely a threat in peanut (Table 1).
Table 1. General timeline of soilborne and foliar diseases in peanut.
Travis Faske, PhD Extension Plant Pathologist email@example.com
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