Go to Table 1
Go to Table 2
Go to Table 3
Go to Table 4Peanut Disease and Nematode Control Recommendations
Updated April 2006 by Jason Woodward, Extension Plant Pathologist, Mark C. Black, Extension Plant Pathologist, TCE,
TAMU AREC Uvalde
Table of Contents
Stem, Pod, and Peg Fungal Diseases
All peanut producers experience loss annually from one or more diseases. Refer to the Peanut Disease Atlas (B-1201), available from your County Extension Agent for help with disease diagnosis. Peanut diseases often can be controlled or minimized with appropriate preventative practices. Potential economic benefit of these control suggestions is dependent on each grower’s ability to adapt them to his production system and prevailing environmental conditions.
Foliar fungicides may be applied with ground or air equipment in spray formulations. Chemigation is permitted on several fungicide labels. Any method that evenly deposits the protective fungicide on the entire leaf surface is satisfactory. Apply in a band with a ground rig in early season to reduce costs. With aerial application, equipment must be properly adjusted and operated. Flagging, marking, or GPS positioning insures even distribution and correct swath widths. Stop application if temperatures are above 90 F. and relative humidity is below 45 percent to avoid spray droplets drying before hitting target plants.
Seeds and seedlings are vulnerable to various pathogens on the seed surface, in seed skins, inside the seeds, in crop debris and in soil. High quality seeds are less likely to have seed and seedling diseases. Peanut seeds are relatively fragile and short lived compared to most other crop seeds and seed quality is influenced by many factors. Contributions to seed quality start more than 12 months before planting when the seed crop is initiated. Seed crop production factors that contribute to high quality seed include:
seed production where the growing season consistently allows full crop maturity,
no Sclerotinia blight or CBR in the seed field; minimal levels of other plant diseases (including nematodes), weeds, animal damage, and insect problems,
adequate soil calcium that supplies Ca needs of developing kernels,
no severe stress from drought or heat (stress retards Ca uptake),
mature pods when vines were dug,
no freeze injury after digging, and
good harvest conditions (no extended delays due to rain, moderate temperatures, moderate relative humidity, moderate seed moisture).
Seed crop, postharvest, and planting season factors for high quality seed include:
a. drying after harvest, if any, is slow (usually with forced air only),
b. during transfer, storage and handling, the pods are handled gently and proper storage climate is maintained,
c. seeds are gently shelled shortly before planting and fungicide seed treatment (Table 1) for multiple fungi is applied not long before the planting date,
d. representative samples are tested for germination % and vigor to confirm high seed quality,
e. seed bags are distributed with gentle handling for immediate planting and the grower avoids short-term storage at high temperatures or in full sun,
f. seeds are planted in a rotated field with site/soil prepared for planting,
g. soil temperature (average 70 F or more at a 2-inch depth at 7 a.m. for 3 consecutive days) and moisture allow rapid germination (seedling diseases increase with planting in dry soil, often with long delays before irrigation reaches some parts of a field),
h. planters are adjusted for seed size, target seeding rate and appropriate planting depth for the site and soil type,
i. planting dates are reasonable (seedling diseases increase with late planting because of high soil temperatures and increased time lapse between shelling/treating and planting),
j. herbicide(s) are applied correctly,
k. soil insects do not attack seeds and seedlings, and
l. postplant rainfall, irrigation, and temperatures are conducive to rapid germination and emergence.
Early Leaf Spot and Late Leaf Spot
Combine chemical (Table 2) and cultural practices for more consistent control. Rotation with other crops reduces over wintering populations of leafspot fungi in the soil and makes chemical disease control more effective and profitable. Shorter intervals and maximum rates become necessary when disease pressure is greatest and weather conditions favor additional infection. Early detection of leaf spot requires close observation. Be aware that different fungicides perform in different ways under varying weather and irrigation conditions. Always read and follow labels carefully.
Chemical control methods are:
Irrigated Peanut off the Cap Rock, Central Texas, South Texas--
a. Spanish and Valencia types - Spanish and Valencia types - Begin fungicide applications 35 to 40 days after planting and continue at recommended intervals until 20 to 21 days before harvest, depending on the fungicide used, weather conditions, and disease development.
b. Runner and Virginia types - Runner and Virginia types - Begin applications 50 to 55 days after planting. Follow the Spanish recommendations given above if late leaf spot occurs during the early stage of plant development.
Irrigated peanut above the Cap Rock--
a. Due to less rainfall and much lower relative humidity’s experienced in peanut fields above the natural escarpment known in Texas as the Cap Rock less leaf spot pressure is experienced there. Foliar fungicides may not be profitable in some seasons. Where a 3 or 4 year rotation is used, leafspot is usually not detectable. Pepper spot (Leptosphaerulina leafspot) is the primary problem causer. It only presents a problem between about July 15 and September 15 after a more favorable environment has developed within a full canopy. This is a more serious problem in the more shallow high pH soils where canopy yellowing is experienced. Consequently, growers in this area should consider zero to three foliar fungicide applications. The most important one is in mid August. The second most important one is August 1 and the third most important is September 1. One of the multipurpose fungicides (labeled for foliar and soilborne pathogens, compare Tables 2,3) used for pod rotting diseases at this time is sufficient for both problems.
For dryland peanuts, follow the same recommendations as for irrigated peanuts if rainfall is sufficient for continuous plant growth and disease development. In years of low rainfall and low humidity, begin fungicide applications at first evidence of either leaf spot disease or when rains or dews favor disease development. Continue applications at suggested intervals through periods suitable for leaf spot development. Dew formation is most consistent in the fall, beginning in September, but may occur anytime.
Rust
The occurrence of peanut rust is usually geographically limited and sporadic except in South Texas where it occurs annually. The fungus has not been observed to over winter in Texas, and each year spores apparently blow in from the Caribbean area. Rust is typically found in South Texas peanuts in mid-July. Once established, rust can develop rapidly during humid wet weather. Late planted peanuts in South Texas are most vulnerable because rust spores produced in nearby early planted fields are carried on prevailing winds to other fields. Apply fungicides highly effective against rust (Table 2) at shortest intervals at the first sign of rust in fields or in nearby fields.
Web Blotch
Spanish and valencia market type peanuts are more susceptible than runner and virginia types to web blotch. However, runner types in West Texas can have problems with this disease. Several foliar fungicides are effective in control (Table 2).
STEM, POD, AND PEG FUNGAL DISEASES
Positive disease identification is necessary to get economic returns from chemicals (Table 3). Signs and symptoms can be similar for two or more diseases, and effective fungicides may differ greatly for cost.
Southern Blight
Southern blight, caused by the soilborne fungus Sclerotium rolfsii, has coarse, initially persistent white fungus strands that develop at a moderate rate on all plant parts and on the soil surface, often in a flat-fan pattern. Nearby plant tissue becomes desiccated due to digestion by the fungus, and the mycelium disintegrates gradually over several days or weeks. On this white fungus growth, mostly-round sclerotia (seed-like long-term survival structures) age from white to tan to black and are never found inside stems, pods, or seeds. Southern blight is favored by warm weather. Control methods include:
Rotate to avoid peanut after peanut if possible. Peanut after peanut should have crop residue buried with a mold-board plow deep enough to avoid bringing it back up during land preparation and cultivation. There may be no advantage in burying residue from non-peanut crops.
Plant irrigated peanuts on a raised bed at least 4 inches high. Plant dryland peanuts on a slightly raised bed.
Use a variety with partial resistance if available (runner, Tamrun 96).
Avoid very high seeding rates in problem fields. Early development of a dense canopy retains humidity that favors the southern blight fungus.
Do not throw soil onto peanut plants during cultivation.
Control foliar diseases with fungicides to prevent leaf shed. Leaf litter at the crown is a nutrition source for the southern blight fungus.
Several fungicides can contribute to southern blight control (Table 3). Multiple applications as preventative treatments in problem fields are suggested rather than single applications or rescue treatments after southern blight injury has occurred.
Sclerotinia Blight
Sclerotinia Blight, caused by the soilborne and seedborne fungus Sclerotinia minor, has been increasing in importance in Texas peanuts since 1981. Annually, additional outbreaks of the disease occur, sometimes in counties with no previous diagnosis. The disease is characterized in early stages by non-persistent small white tufts of cottony-like growth at leaf axils on the stems near the ground line. The fungus spreads rapidly during cool wet weather. Later stages of the disease show up as severe stem shredding, almost as if the stems had exploded, accompanied by the production of many small, black, irregular-shaped sclerotia (seed-like long-term survival structures) that resemble mouse droppings in size, shape and color. Sclerotia also form inside stems, pods, and occasionally, seeds. Confusion of this disease with southern blight, caused by the fungus Sclerotium rolfsii, can be costly because chemicals that control southern blight have little if any effect on S. minor. Sulfur applied as a foliar fungicide may significantly increases the severity of Sclerotinia blight.
The Sclerotinia fungus can be seed borne, so infested fields should not be used for seed production. Some seed treatment fungicides can reduce seed transmission (Table 1). Equipment-related movement of infested soil and crop residue (diggers, combines, vehicles) can spread the fungus. Sclerotia may also survive the digestive processes of cattle and migratory birds.
Contributions to control include:
multi-year rotation with non-hosts (for example, small grains) with deep burial of crop residue,
keep soil moisture below field capacity for the final 45 days to allow soil temperature increases during daylight,
plant early in south Texas to avoid disease-conducive cool fall temperatures,
use spanish varieties because of more open plant canopies (Tamspan 90 has lower losses than other spanish varieties); among runners, Tamrun 96, Tamrun OL 01 and Tamrun OL 02 have lower disease ratings than most other varieties, perhaps because their fewer stems and more open crown area are less favorable to S. minor; among virginias, Perry and VA-C92R are more resistant than others.
e. Three fungicides are labeled for Sclerotinia blight control (Table 3) in 2006. Rovral, on the market the longest, must be applied by ground in large volumes of water (40-60 GPA) for maximum effectiveness, but control is sometimes not adequate. Omega (labeled in 2001) and Endura (labeled in 2004) provide more control than Rovral.
Botrytis Blight
Botrytis blight is caused by a species of the fungus Botrytis. It has been observed statewide, but has only been a significant peanut problem in far West Texas. Symptoms may closely resemble Sclerotinia blight (both produce delicate white mycelium and sclerotia), so a laboratory diagnosis is suggested. With time in the field, Botrytis produces conidia on mycelium and lesions, but S. minor does not. No fungicide label currently mentions control of the Botrytis fungus on peanut. Thiophanate-methyl is labeled for three other foliar diseases (Table 2) and is effective against Botrytis blight. The more expensive Sclerotinia blight control chemicals, Omega 500 and Endura (Table 3) are also very effective.
Pythium and Rhizoctonia Diseases
Diseases caused by these two groups of fungi can occur alone or together. Pythium fungi contribute to seed rot, seedling disease, root rot, wilting, stunting, plant death, and pod rot (pod breakdown). Symptoms of Pythium infection may include a wet black decay sometimes covered with a loose white fungus mat; sloughing outer root layer, and greasy dark brown-black pod lesions. Rhizoctonia fungi cause disease on seed, seedlings, roots, lower stems, pods, pegs, limbs, and leaves. Symptoms of Rhizoctonia infection may include sunken red-brown dry-textured lesions on the hypocotyl (stem below cotyledons), stem (girdled seedlings), and limbs, and dry dull-surfaced light/dark brown pod lesions.
Pod rots are poorly understood and difficult to control. Soil nutrition, physical soil factors, and soil fauna (insects, nematodes, mites) can be involved. Large-seeded varieties tend to have more pod rots than small seeded varieties. The Texas A&M peanut breeding program used parents with partial pod rot resistance, and Tamrun 96 often holds up better than some other varieties. Cultural practices should be addressed rather than managing the problem solely with fungicides (Table 3). Cultural recommendations helpful for Rhizoctonia and Pythium pod rot control include:
avoid sites and soil types with known histories of these diseases,
rotate with unrelated crops; if possible, summer fallow during rotation; use small grains as a winter cover crop (terminate and leave in place for wind protection or turn under deeply with other crop residue in the spring),
avoid excessive fertilizer, especially unneeded K and Mg,
plant on a raised bed,
improve drainage in low areas,
avoid excessive irrigation,
where salinity is a problem, check for and break up hard pans to allow leaching of salts by rains and irrigation, and
apply gypsum (a water-soluble calcium source) at flowering/pegging, especially in areas where sodium salts accumulate in the soil from low quality irrigation water; large seeded varieties often require more calcium than small seeded varieties.
Aspergillus Crown Rot
Aspergillus crown rot (black mold), caused by the fungus Aspergillus niger, affects peanut production throughout Texas. This ubiquitous fungus is seedborne and soilborne. The fungus attacks the crown or collar area near the soil line and may girdle and kill the plant at any stage from seedling to harvest. Slightly fluffy fungus growth (with dusty black spores) on stem lesions just below the ground line is diagnostic. A good rotation program, high seed quality, no late planting, good planting moisture, and adequate early season irrigations reduce losses.
Diplodia Collar Rot
Diplodia collar rot, caused by Lasiodiplodia theobromae (Diplodia gossypina) may occur where soil and stem temperatures are high due to insufficient irrigation, skips, non-lapped canopies, or defoliation by foliar diseases, insects, or hail. The fungus survives well on crop debris soil (as a saprophyte) and causes disease on several plants following heat stress or wounding, including a boll rot on cotton. Symptoms include yellow, wilted, or dead plants in patches, blackened limb cankers, and pod lesions. The fungus may be seedborne. Black pycnidia (fungal fruiting bodies) develop on limb lesions in contact with soil. In Virginia, Virginia market type varieties were more susceptible to collar rot than runners.
Cylindrocladium black rot
Cylindrocladium black rot (CBR) is caused by the soilborne and seedborne fungus Cylindrocladium parasiticum. It was diagnosed in west Texas peanuts for the first time in 2004 in one field planted with seed produced in a southeastern state with a history of CBR. No symptoms were seen in 2005 in the region. Symptoms are black decay of roots, lower stem, pods, and pegs that causes plants to yellow and die. Low soil moisture during fallow and deep soil freezing reduce fungus populations in soil. Control strategies include long rotations with non-host crops, sanitation of equipment leaving infested fields, pre-plant seed-bed injections with a fumigant, use of pathogen-free seeds, certain fungicide seed treatments for suspect seed, in-furrow at-plant fungicides, and partially resistant varieties. Infested fields should not be used for seed production.
Black Hull
Black hull, caused by the soilborne fungus Thielaviopsis basicola, is a peanut pod disease with big implications for the in-shell market. The fungus can cause disease on and increase its population on a large number of plants including cotton (black root rot), several weeds, most other legumes, sweet potato, several other vegetable crops, and many ornamentals. It is favored on peanut in West Texas and New Mexico by high soil pH and late season cool wet conditions. Control suggestions include:
use a 3-4 year rotation with non-hosts (small grains),
avoid sites with high pH and histories of black hull or black root rot in cotton,
plant on raised beds to increase drainage in the pod zone and allow daily heating,
select a runner variety with partial resistance (Tamrun 96, etc.) and avoid highly susceptible varieties (AT-120, etc.), and
thiophanate-methyl fungicide has been reported to reduce incidence, but current labels do not list this disease (Table 3).
Aflatoxin (Segregation III)
Aflatoxins are highly toxic chemical compounds produced in peanut seeds by the fungi Aspergillus flavus and A. parasiticus following periods of field stress or high storage moisture. Similar problems in corn and cotton seeds are primarily associated with A. flavus. Aflatoxins may accumulate before digging in drought stressed dryland peanuts, especially where pods are injured by insects or other agents. Segregation III peanuts are usually associated with pre-harvest drought conditions of kernel moisture below 25% and high soil temperatures (80 to 100 F). Aflatoxins can further increase during harvest, curing and storage if drying conditions are poor, kernel moisture is high for extended periods, or the storage site has high relative humidity (seldom in Texas) and facilities that leak during rains. Large-seeded virginia varieties appear more prone to aflatoxin development than spanish or runners under South Texas conditions. Problem loads can only be pressed for oil.
Management options include:
avoid drought-prone sandy fields,
manage insect and nematode problems to minimize pod damage,
irrigate adequately to minimize drought stress,
reduce seeding rates in dryland fields to conserve soil moisture,
harvest drought stressed fields and sections of fields separately from better fields to avoid mixing with loads that are non-Seg. III, and
adjust combines to prevent pod damage and transport high moisture peanuts in vented trucks and trailers to prevent heating; force air through the truck or trailer and dry as soon as possible.
g. A non-aflatoxin producing strain of A. flavus is available as a biological control treatment to peanut producers (Afla-Guard, Circle One Global, Inc., Cuthbert, GA). This ‘good’ fungus competes with the aflatoxin-producing strains, thereby reducing aflatoxin accumulations in peanuts. A related product is labeled for application to cotton preceding a corn crop to reduce aflatoxin in corn.
Several kinds of plant parasitic nematodes may cause damage but root knot, caused by the peanut root knot nematode Meloidogyne arenaria, is normally the most severe in Texas. Root knot is diagnosed based on galls on roots and usually also on pegs and pods. Other nematodes require soil and laboratory analysis of plant samples for identification. The best time to sample is at or near harvest. Send a soil sample representative of damaged areas, along with peanut pods, if available to: Texas Plant Disease Diagnostic Laboratory, 1500 Research Parkway, Suite A130, Texas A&M University Research Park, College Station, TX 77845. The current fee is $30.00 per sample. Nematode sample forms are available from this address and online at http://plantpathology.tamu.edu/extension/tpddl/forms.asp.
A control program should include:
a. rotation with crops resistant to the nematodes damaging peanuts,
b. late maturing varieties have more potential for damage than short-season spanish market types,
c. NemaTam runner variety has resistance to peanut root knot, and
d. consider a nematicide when plant parasitic nematodes have previously limited production.
Use caution when selecting a nematicide (Table 4) since soil moisture is extremely critical for optimum control. Telone II works best when placed in the ground 10 to 12 inches with a moldboard plow at rates of 6-12 gal./ac. Excessive soil moisture and cold temperatures limit movement of the fumigant in the soil, thus reducing effectiveness and possibly causing plant stunting. This fumigant will cause fewer problems when applied at least 10 to 14 days before planting. Granular contact nematicides work best with good soil moisture conditions.
Spotted Wilt
Yield loss from spotted wilt, caused by Tomato spotted wilt virus (TSWV), occurs in South and Central Texas. Yield losses may exceed 50% in susceptible varieties in certain years. Tobacco thrips and western flower thrips are vectors (carriers). Impatiens necrotic spot virus (INSV) is related to TSWV and is also vectored by western flower thrips. INSV problems in peanut to date have been very low compared to TSWV.
Typical early season spotted wilt symptoms include ring spotting of leaves and stunted plant growth, but late season symptoms of spotted wilt often do not. Older plants that become infected with TSWV often simply yellow, wilt, and quickly die. This is accompanied by brown streaking within the vascular system and deterioration of roots. Avoid overwatering 4 to 6 weeks before digging problem fields because excess moisture accelerates secondary decay of TSWV-infected roots.
TSWV has a large host range and the virus is not seedborne in any crop or weed. Infested tobacco thrips may over winter in some soils. Western flower thrips can actively feed on various plants throughout most of the year and may spread the virus during the winter among weeds, various native annuals and perennials, susceptible vegetable crops, and ornamentals. Green bean and pepper can be a bridge for TSWV in fall and spring. Spinach and potato can harbor TSWV through the winter in South Texas.
Periodic off-season rains allow more non-crop vegetation (wildflowers, cool season weeds) that helps maintain and increase thrips and the virus. Seasons preceded by such rains usually have increased risk for the peanut crop. However, if rains stop by late-winter or early-spring, cool season vegetation may dry down and thrips may disperse well before the first peanuts are planted, thereby avoiding high spotted wilt incidence in peanut. On the other hand, numerous rains in May-June 2004 supported heavy non-crop vegetation that dried down and forced thrips migrations before peanut planting was finished, and late planted peanuts had severe spotted wilt.
Peanuts planted in the proximity of TSWV crop hosts (spinach, potato, green bean, pepper) and early planted peanut fields often have increased risk. Very early and very late planted fields usually have increased risk. Careful planting date and field site selections may allow growers to miss peak thrips migrations from other crops and non-crop areas in some years. Large thrips populations from nearby cotton production may increase spread of TSWV in peanut. There is some evidence that cotton can be a weak TSWV host with no long-term symptoms.
Movement of virus-infected thrips into and within peanut fields is very important. Two cultural practices and variety canopy traits may affect thrips migration/landing behavior. A high seeding rate decreases spotted wilt. The twin-row planting pattern decreases spotted wilt. Both practices accelerate ground cover (lapping) and after lapping, produce a flatter peanut canopy with less prominent main stems compared to low seeding rates and single row planting pattern. It’s interesting that varieties with partial TSWV field resistance planted at the same seeding rate and row pattern have less prominent main stems compared to more susceptible varieties. Our hypothesis is that canopy shape can influence thrips behavior enough to reduce spotted wilt. Work is ongoing to explain this phenomenon and use it for improved spotted wilt control strategies.
Peanut varieties planted in areas at risk for spotted wilt should have some field resistance. Resistant peanut varieties have fewer infected plants and those infected plants have milder symptoms than more susceptible peanut varieties under the same conditions.
Efforts continue with plant breeders to develope superior multiple-disease-resistant varieties for Texas growers. Variety options for partial TSWV resistance in 2006 include Tamrun 96, Tamrun OL 01, Tamrun OL 02, Georgia Green, AT-108, ViruGard, and TAMSPAN 90.
Ozone
Temporary high concentrations of ozone can cause "atmospheric scorch" on peanut leaflets. Nitrogen dioxide and hydrocarbons emitted from automobiles, industrial combustion, oil refineries and other sources react with sunlight to form ozone. Lightning during electrical storms produces ozone which can damage the surface of peanut leaflets. A scorched area appears primarily on the upper leaf surface of the youngest peanut leaflets. Pepper spot caused by a species of the fungus Leptosphaerulina may develop in these scorched leaves. Regular use of a foliar fungicide helps minimize injury by this weak pathogen in damaged tissue.
Salts
Low peanut yields, marginal leaflet burn, and severe pod rots are potential problems in soils with a high sodium adsorption ratio (SAR). The foliar symptoms that develop after irrigation with saline irrigation water vary from a brown marginal leaflet burn to death of the leaflet. Pod rot often increases when sodium (Na), potassium (K), and perhaps other cations accumulate in soil in the fruiting zone. Excess cations compete with calcium for position on soil particles and allow the Ca to move below the pod zone. Calcium is a nutrient absorbed in large quantities by the developing pods and essential for high kernel quality. Calcium deficiency in pods may be associated with increased susceptibility to pod rot fungi. Supplements of gypsum (land plaster) can decrease pod rot under high SAR conditions. Water infiltration into soil is decreased in soils with high SAR. Furrow diking can reduce runoff after rainfall and irrigation and increase flushing of sodium from soil.
Boron
Boron is required in very small amounts for peanut kernel quality. However, toxicity is a problem in some soils in West Texas, decreasing plant growth and yields. Yield decrease occurs with few foliar symptoms (reduced canopy size). Soil and irrigation water should be tested at least annually in areas at risk for high Boron. Test results should be considered when selecting fields for planting.
Alkalinity
Alkaline (high pH) soils are a challenge for optimal peanut production. Canopy symptoms may include yellowing and reduced leaflet and canopy size. Supplementation may be necessary for minor elements that are less available at high pH. Nodulation and nodule activity should be monitored because some fields may need Bradyrhizobium inoculant every time peanut is planted. Nitrogen fertilizer may be needed if nodule numbers and activity are not high.
Go to Table 1
Go to Table 2
Go to Table 3
Go to Table 4
Return to Mark Black