Peanut Disease and Nematode Control Recommendations
All peanut producers experience loss from one or more diseases that occur annually on their crop. Refer to the Peanut Disease Atlas(B-1201, available from your County Extension Agent) for help with disease diagnosis. These diseases can be controlled by utilizing appropriate preventative practices. Control suggestions made in this publication have been well documented in field tests over a period of years and have been shown to produce economic benefit when appropriately utilized. Potential economic benefit is dependent on each grower’s ability to adapt them to his production system and prevailing environmental conditions.
Seed Rot and Seedling Disease Control
Plant high quality seed treated with a suggested seed protectant fungicide. Seedling disease is less severe when soil temperatures average 70 F. or more at a 2-inch depth at 7 a.m. for three consecutive days. Suggested dry seed treatment fungicides include Captan + DCNA 60-20, Gus – 4 way (Captan + Maneb + Terrazole + PCNB), Granox (Captan + Maneb), TOPS 90 (90% Topsin), Vitavax PC (Vitavax + PCNB + Captan). Planter box treatment with appropriately labeled fungicides is suggested where seedling disease is a consistent problem.
Vitavax used on Spanish peanut seed may cause marginal leaf burn and reduced seedling vigor. Runner peanuts have not been affected. Increased vigor usually is associated with Vitavax on runner peanuts.
Foliar Disease Control
Early Leaf Spot and Late Leaf Spot
See Disease Atlas
Combine chemical and cultural practices for more consistent control. Rotation with other crops reduces overwintering 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. Be aware that different fungicides perform in different ways under varying weather conditions. Always read and follow labels carefully.
Chemical control methods are:
- Irrigated Peanuts
- 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.
- 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.
- 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 dew favor disease development. Early detection of leaf spot requires close observation. 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.
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 overwinter in Texas, and each year spores must be blown 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 effective against rust at shortest intervals at the first sign of rust in fields or in nearby fields. Southern Runner is the only variety with resistance to rust.
Spanish and Valencia market type peanuts are more susceptible than runner and Virginia types to web blotch. However, runner types in West Texas can experience severe damage. Several foliar fungicides are effective in control.
Foliar fungicides may be applied with ground or air equipment in spray formulations. Any method that evenly deposits the protective fungicide on the entire leaf surface is satisfactory. Use three hollow-cone nozzles per row spaced for optimum coverage. Make the first three applications in a band with ground equipment to control foliar diseases and reduce early season cost. If a three-nozzle arrangement is used (one nozzle at the top and two on the sides), plug the side nozzles for the first application and use only the top one. Use two nozzles on larger peanuts 10 to 14 days later by plugging the top one and using the two side nozzles. For the third and subsequent applications, use all three nozzles even though this may damage some vines. Ground spray equipment should apply the suggested amount of fungicide in 10 to 25 gallons of water per acre, depending on vine size. Careful use of ground equipment has little or no adverse effect on yield. When applying fungicides by air, use at least 5 gallons of water per acre.
Demonstrations under field conditions show that foliar fungicides applied through sprinkler irrigation systems give control equal to those applied by air and ground equipment. Continuous agitation of fungicide-water combinations is required during the hours the center pivot system circles to prevent fungicide settling. This is not a problem with side-roll injection systems.
Aerial application of foliar fungicides provides good control when equipment is properly adjusted and operated. Adequate flagging or marking insures even distribution and avoids swath widths that are too wide. Stop application if temperatures are above 90 F. and relative humidity is below 45 percent to avoid spray droplets drying before hitting target plants. A visible blanket of spray mixture will appear behind the aircraft when the 5-gallon-per-acre-rate is used.
Control of Pod, Peg and Stem Fungal Diseases
Cultural methods for control of southern blight include:
- 1. Rotate crops to avoid peanuts following peanuts.
- 2. If peanuts are planted after peanuts bury crop residue 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.
- 3. Plant on a raised bed. Plant dryland peanuts on a slightly raised bed and irrigated peanuts on a bed at least 4 inches high.
- 4. Avoid high seeding rates. Early development of a dense canopy retains humidity that favors the southern blight fungus.
- 5. Use herbicides to prevent development of weed and grass residues that may serve as a food source for the southern blight fungus.
- 6. Do not throw soil onto peanut plants during cultivation.
- 7. Control foliar diseases with fungicides to prevent leaf shed. Fallen leaves are a food source for the southern blight fungus.
- 8. Dig when mature.
- 9. Partially resistant Southern Runner variety is an option in South Texas.
Chemical control of southern blight is possible with Vitavax, Terraclor (PCNB), Tilt or Folicur when used correctly. Vitavax stops an epidemic quicker but has a relatively shorter “active life.” Multiple applications of Tilt and Folicur as preventative treatments in problem fields are suggested rather than single applications or rescue treatments after southern blight damage has occurred. Consider these characteristics when selecting a chemical. Vitavax, Terraclor, and Tilt are labeled for application through sprinkler irrigation systems in Texas and show acceptable levels of control when used in this manner. Producers must be aware of strict regulations now existing regarding “chemigation” as it relates to the potential for water contamination.
Positive disease identification is necessary to get economic returns from chemicals. For example, all four above mentioned products are effective against the southern blight fungus but will not control the Pythium pod rot fungus.
Do not feed hay treated with Terraclor, Vitavax, Ridomil PC, or Folicur to livestock and do not allow “hogging” of peanuts.
Sclerotinia Blight, caused by the fungus Sclerotinia minor, was observed for the first time in Texas peanuts in 1981. Additional outbreaks of the disease have been identified in numerous Texas counties. The disease is characterized in early stages by small white tufts of cottony-like growth on the stems near the ground line at leaf axils. The fungus spreads rapidly. 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 that are approximately the size, shape and color of mouse droppings. The distinguishing field diagnostic symptom is rapid plant death, accompanied by stem shredding. At first glance some may confuse this disease with southern blight, caused by the fungus Sclerotium rolfsii. This mistake can be devastating because chemicals that control southern blight have no effect on the Sclerotinia fungus. Research from several states has shown the Sclerotinia fungus can be seed borne. The sclerotia may also be spread by diggers, combines, or vehicles that might carry infested soil or crop residue. Research at Stephenville has shown that sulfur (applied as a foliar fungicide) significantly increases the severity of Sclerotiniablight.
The only labeled product for Sclerotinia blight control is Rovral. The label recommends up to 6 pounds of product in 2-pound increments beginning at first signs of the disease and repeated as needed. Rovral by ground requires large volumes of water (40-60 GPA) to obtain maximum effectiveness. A multi-year rotation, in conjunction with deep burial of crop residue, is also helpful. Sclerotinia blight is more severe on runner than spanish varieties, supposedly because of quicker, more complete ground cover with the runner types. Among the runner varieties, tests have shown losses from Sclerotinia blight to be particularly heavy on Southern Runner and Langley. Tamspan 90 has significantly more resistance to the fungus than other available spanish and runner varieties. Keeping soil moisture below field capacity for the final 45 days allows soil temperature to increase which helps control the organism. Planting early to avoid cool fall temperatures that are conducive to the disease is suggested where possible.
Botrytis blight is caused by a species of the fungus Botrytis. It has only been a problem in far West Texas. Since symptoms so closely resemble Sclerotinia blight, a lab diagnosis is necessary to separate the two. Benlate, labeled for leaf spot control in peanut, is effective against Botrytis blight.
Pythium and Rhizoctonia Diseases
Diseases caused by these two groups of fungi can occur alone but more often occur together. Pythium fungi cause pod rot and root rot. Rhizoctonia fungi cause disease on pods, pegs, limbs, leaves and roots. Pod rots are difficult to control and cultural practices should be adjusted before considering a fungicide. Cultural recommendations for southern blight control are helpful for Rhizoctonia and Pythium pod rot control.
1. Avoid excessive irrigation.
2. Rotate with unrelated crops. If possible, summer fallow during rotation. Use small grains as a winter cover crop. Turn this under deeply with other crop residue in the spring. Plant on a raised bed.
3. Improve drainage in low areas. Where salinity is a problem, check for and break up hard pans to allow leaching of salts.
4. Apply gypsum (a calcium source) at pegging, especially in areas where sodium salts accumulate in the soil from low quality irrigation water. Large seeded virginia type peanuts require more calcium than runner and spanish types.
5. Avoid excessive fertilizer.
Black mold caused by the fungus Aspergillus niger is a threat to peanut production throughout Texas. Low quality seeds, late plantings and drought stress for the first few weeks after planting have been associated with a high disease incidence.
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. The black, slightly fluffy fungus growth at the ground line is the best field diagnostic symptom. There are no adequate control recommendations. A good rotation program, a good foliar fungicide program, avoiding late planting and light early season irrigations may be helpful.
Diplodia Collar Rot
Rotating with non-related crops lowers populations of this fungal organism in the soil. Diplodia has been less severe in plots where leaf spot was controlled with fungicides and where soil temperatures were reduced by vine shading. Plant small grain rotation crops in problem fields and turn them under long enough before planting to accomplish initial decomposition.
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. Root knot is easily diagnosed from 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:
Late maturing varieites have more potential for damage than short-season spanish market types.
Use caution when selecting a nematicide since soil moisture is extremely critical for optimum control. The only labeled fumigant-type nematicide, Telone II, works best when shanked 10 to 12 inches into loose soil that is just a bit on the dry side (but not extremely dry in the top 3 to 6 inches). 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 nematicides work best with good soil moisture conditions. As planting conditions vary from season to season, growers must consider soil moisture in selecting nematicides.
Aflatoxin (Segregation III)
Aflatoxin is a chemical compound produced by the fungi Aspergillus flavus and A. parasiticus. Aflatoxin may accumulate before digging in drought stressed dryland peanuts. Reduce seeding rates in dryland fields to conserve soil moisture. Some soils have a higher population of the fungus than others. If peanuts from a field consistently have this condition, consider rotating with other crops. Irrigate if possible because peanuts under drought stress are more susceptible to field infection by Aspergillus sp. Segregation III peanuts are usually associated with pre-harvest drought conditions of kernel moisture below 25% and high soil temperatures (80 to 100 F). Pod injury from insects or other agents favor infection by these fungi.
Aflatoxin may also accumulate during harvesting and curing if drying conditions are less than ideal. Use inverting diggers to keep pods off the soil surface while curing within the windrow. Adjust combines to prevent pod damage and transport peanuts in vented trucks and trailers to prevent heating. Force air through the truck or trailer and dry as soon as possible.
Aflatoxin may also accumulate during storage in regions with high humidity or in facilities that leak during rains.
Varietal Characteristics Relative to Disease Development
Peanut varieties differ in their susceptibility to disease organisms. Tamspan 90 is less susceptible to Pythium pod rot than the other varieties. Although runner and spanish peanuts are both affected by Pythium pod rot and southern blight, runner types suffer the most damage. Give runner types extra consideration when chemical treatments are required.
Both spanish and runner peanuts can be heavily damaged by root knot nematodes; however, the extra 30 days needed to mature the runner type magnifies their damage potential. Split applications of nematicide may be necessary for runner varieties. With the longer growing season needed for runner peanuts and their partial resistance to early leaf spot, late leaf spot often is the predominant foliage disease. Early leaf spot affects both types but is usually worse on spanish varieties. Spanish varieties are also more susceptible to web blotch. Large-seeded virginia varieties appear more prone to aflatoxin development than spanish or runners under South Texas conditions. Where Sclerotinia blight is a problem, spanish peanut varieties, particularly Tamspan 90, can often be grown without chemical control. Runner types are much more susceptible to the fungus. Consider all these factors when planning a chemical control program.
Yield loss from spotted wilt, caused by tomato spotted wilt virus (TSWV), has occurred in South Texas since the mid-1980’s. Several fields in Mason and McCullough counties had problems in 1994. Thrips insects move TSWV from plant to plant.
Where TSWV overwinters is not completely understood. Infested tobacco thrips may overwinter in some soils. Western flower thrips may spread the virus among weeds and susceptible vegetable crops during the winter. Spinach and potato crops can also carry the virus through the winter in South Texas. Green beans grown in the spring and fall are susceptible. The virus is not seedborne in any crop or weed.
A late season out break of spotted wilt may not cause the typical ring spotting of leaves and stunted plant growth. Plants simply yellow, wilt, and quickly die. This is accompanied by brown streaking within the vascular systems. The problem is significantly worse in fields or areas of fields that are experiencing stress of some type. TSWV can be detected in the crown area of most plants in fields exhibiting these symptoms in South and Central Texas. Various soil fungi including Rhizoctonia, Fusarium, and Pythium can also be isolated from these dying plants as well as healthy plants. A virus is seldom the organism that actually kills a plant. It does weaken the plant to the point that any number of other organisms can invade and actually do the killing. Anything that can be done chemically or culturally to enhance overall plant health may prolong plant life a bit and increase the chance of making a crop in spite of the virus. This would not constitute virus control. It would simply be an increased level of management until such time as an acceptable more resistant variety is available.
The fall of 1994 brought significant rainfall to the South Texas peanut production area. Therefore, area-wide risk in 1995 is expected to be moderate-to-high. Peanuts planted in the proximity of spinach, potato, spring green bean, and early planted peanut fields may also have increased risk. Very early and very late planted fields may have increased risk.
Risk of spotted wilt in South Texas has been reduced but not eliminated by use of varieties with some level of resistance. Resistant peanut varieties may have fewer infected plants and those plants have milder symptoms than more susceptible peanuts varieties under the same conditions. Spotted wilt epidemics are driven by two factors. The first is how much virus is brought into the field by thrips and this varies widely from year to year. Changing the planting date and careful field selection may allow growers to miss some thrips migrations in some years. The second factor is how fast the virus spreads from plant to plant.
Variety options in 1995 include GK-7 with its improved yield characteristics for South Texas and Georgia Runner. Both which have similar moderate resistance to TSWV. Southern Runner displays significant resistance.
Plow up spinach fields right after the final harvest. Avoid planting peanuts near potatoes and green beans.
Avoid planting in areas with a history of severe spotted wilt losses. More susceptible varieties such as Tamrun 88, AT-127, Florunner or Okrun increase the risk of spotted wilt in nearby resistant peanuts. Resistant varieties are expected to be most effective when acreage of more susceptible varieties in the area is near zero. Avoid later plantings downwind from other peanut fields if possible.
Southern Runner matures later than GK-7 and must be segregated from other runners for shelling and blanching. It should only be planted south of San Antonio and preferably by June 1. Later plantings through June and into July have an increasingly greater risk of freeze damage.
Insecticides have not provided spotted wilt control. Consult an Extension Entolomologist for specific insect control information.
Peanut Mottle is caused by peanut mottle virus (PMV) and may occur at low incidence wherever peanuts are grown in Texas. There is a low level of seed transmission of PMV and aphids can transmit it from plant to plant. The virus causes no yield loss in peanut but aphids can carry it from peanut to nearby green beans where it can cause yield and quality loss.
Atmospheric Scorch – Ozone
Nitrogen dioxide and hydrocarbons emitted from automobiles, industrial combustion, oil refineries and many lesser sources react with sunlight to form ozone. Electrical storms produce ozone which can be brought down from the upper atmosphere by strong down drafts. The result on peanuts is a scorched appearance primarily on the upper leaf surface of the youngest leaves. Regular use of a foliar fungicide helps prevent secondary infections in the damaged tissue.
Peanuts have a low tolerance to certain salts. The foliar symptoms that develop after irrigation with saline irrigation water vary from a brown marginal leaflet burn to death of the leaf. Pod rot often increases when the cations sodium and potassium accumulate in the fruiting zone. Sodium and potassium apparently compete for position on soil particles with calcium, a nutrient absorbed in large quantities by the developing pods. A calcium deficiency has been associated with increased susceptibility to pod rot fungi. Supplements of gypsum (land plaster) can decrease pod rot under saline conditions.