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 seed protectant fungicide (Table 1) . 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.

FOLIAR DISEASE CONTROL

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 conditions. Always read and follow labels carefully.
Chemical control methods are:

Irrigated Peanut – Except above “Cap Rock”

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”

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.  Multiple years of fungicide testing has proven that foliage fungicides often are not profitable at all. Where a three or four year rotation is used, leafspot is usually not detectable.  Leptosphaerulina leafspot (pepper spot) is the primary problem causer.   It only presents a problem between about July 15 and September 15 after there is a full canopy which produces an artificial environment within it.  This is a more serious problem in the more shallow high ph soils where canopy yellowing is experienced.  Consequently most growers in this area budget from 0-3 foliage 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. Most growers find that one of the multi purpose fungicides used for pod rotting diseases at this time is sufficient for both problems. 

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. 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 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 (Table 2) at shortest intervals at the first sign of rust in fields or in nearby fields.

Web Blotch

Application Methods

CONTROL OF POD, PEG AND STEM FUNGAL DISEASES

Southern Blight

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. Do not throw soil onto peanut plants during cultivation.
6. Control foliar diseases with fungicides to prevent leaf shed. Fallen leaves are a food source for the southern blight fungus.
7. Dig when mature.

Chemical control of southern blight is possible with numerous chemicals when used correctly (Table 3) . Multiple applications 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. Fungicides may be 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 five above mentioned products are effective against the southern blight fungus but only Abound helps control the Pythium pod rot fungus (Table 3) .

Sclerotinia Blight

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 Sclerotinia blight.

The first fully labeled product for Sclerotinia blight control is Rovral (Table 3). Rovral applied by ground requires large volumes of water (40-60 GPA) to obtain maximum effectiveness. A full label was issued for the chemical Omega in 2001.   This chemical has been tested for years as Fluazinam from ISK and was marketed in 2001 by Syngenta.  The fungicide Endura from BASF was labeled for the 2004 season and is similar in effect to Omega 500 but is different chemistry.  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. Tamspan 90 has significantly more resistance to the fungus than other available spanish and runner varieties (Table 5) . 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

Botrytis blight is caused by a species of the fungus Botrytis. It has only been a significant problem in far West Texas. Since symptoms so closely resemble Sclerotinia blight, a lab diagnosis is necessary. Benlate, labeled for web blotch control in peanut and Topsin M are both effective against Botrytis blight.  The more expensive Sclerotinia blight control chemicals, Omega 500 and Endura are also very effective.

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 (Table 3) . Cultural recommendations for southern blight control are helpful for Rhizoctonia and Pythium pod rot control.

Practices include:

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

Black mold caused by the fungus Aspergillus niger is a threat to peanut production throughout Texas. Low quality seeds, late plantings and drought and high soil temperature 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 on lesions located just below the ground line is the best field diagnostic symptom. There are no adequate control recommendations. A good rotation program, avoiding late planting and frequent light early season irrigations reduce losses.

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 irrigation and vine shading. Plant small grain rotation crops in problem fields and turn them under long enough before planting to accomplish initial decomposition.

Biological Control of Soilborne Fungi

It is known that certain fungal species in the genus Trichoderma feed on mycelium and sclerotia of Sclerotinia minor. Sclerotium rolfsii and Rhizoctonia sp. All peanut fields in Texas tested to date have a natural population of Trichoderma. For several years, tests have been conducted in Texas using corn meal to stimulate Trichoderma development as a way to control the major soilborne disease fungi. When yellow corn meal is applied to fields in the presence of moist surface soil, Trichoderma builds up very rapidly over a 5 to 10 day period. The resulting high Trichoderma population can destroy vast amounts of Sclerotinia, Sclerotium and Rhizoctonia. This enhanced, natural biological control process is almost identical to the processes that occur when crop rotation is practiced. The level of control with corn meal is influenced by: 1) organic matter source 2) soil moisture, 3) temperature, and 4) pesticides used. Seasonal applications of certain fungicides may inhibit Trichoderma. Testing will continue to determine the rates and application methods that will give consistent, economical control.

NEMATODE CONTROL

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: Texas Plant Disease Diagnostic Laboratory, Texas Agricultural Extension Service, College Station, Texas 77843. There is a $20.00 per sample fee. Nematode sample forms are available at County Extension Offices (Form D-827). Rotation with crops resistant to the nematodes damaging peanuts should be used in a control program. Consider a nematicide when plant parasitic nematodes have previously limited production.

Late maturing varieties have more potential for damage than short-season spanish market types.

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 gallons per acre. 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.

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 (Table 5) . 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.

VIRUS DISEASES

Spotted Wilt

Yield loss from spotted wilt, caused by tomato spotted wilt virus (TSWV), occurs in Southwest and Central Texas. Yield losses may exceed 50% in susceptible varieties. Tobacco thrips and western flower thrips are vectors (carriers).

Impatiens necrotic spot virus (INSV) was detected in peanut in Southwest Texas in 1998 and 1999 as single INSV infections and double infections with TSWV. INSV is related to TSWV, but western flower thrips are more efficient vectors of INSV than are tobacco thrips. The plant host lists are similar and symptoms are probably identical for TSWV and INSV.

TSWV and INSV over wintering sites are not completely understood. Both viruses have large host ranges. Infested tobacco thrips may over winter in some soils. Western flower thrips can be active throughout the year and may spread one or both viruses during the winter among weeds and susceptible vegetable crops. Spinach and potato can harbor TSWV through the winter in South Texas. TSWV is not seedborne in any crop or weed so far as we know.

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 and apparently with INSV often simply yellow, wilt, and quickly die. This is accompanied by brown streaking within the vascular system and deterioration of roots. TSWV can be detected in the crown area of most plants in fields exhibiting these symptoms in Southwest and Central Texas. INSV was detected sporadically in Southwest Texas in 1999.

Risk of spotted wilt is reduced by use of varieties with some level of resistance. Resistant peanut varieties have fewer infected plants and those infected plants have milder symptoms than more susceptible peanut 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. This varies widely from year to year (fall rains usually increase risk for the following season) and from field to field. Peanuts planted in the proximity of TSWV hosts (spinach, potato, spring green bean) and early planted peanut fields may have increased risk. Very early and very late planted fields usually have increased risk. Careful planting date and field selections may allow growers to miss some thrips migrations in some years. The second and more important factor is how fast the virus spreads from peanut plant to peanut plant. Large thrips populations from nearby cotton production may increase spread. The only thing known to slow down this type of spread is to increase the level of variety resistance

Anything that can be done to enhance overall plant health may prolong plant life and increase the chance of making a crop in spite of the virus. It is especially important 4 to 6 weeks before digging infested fields to avoid over watering. This does not constitute virus control, but helps keep infected plants alive.

Efforts continue on developing superior resistant varieties for Texas growers (Table 5) . Variety options for partial TSWV resistance in 1999 include TAMRUN 96, Georgia Green, AT-108, ViruGard, Georgia Bold, Florida MDR-98, and TAMSPAN 90. Georgia Green may not be resistant to INSV.

TSWV-susceptible varieties such as Tamrun 88, Tamrun 98, AT-127, or Florunner increase the risk of spotted wilt wherever they are planted and, because the virus spreads, even in nearby fields of more resistant peanuts.

Insecticides have not provided spotted wilt control. Consult an Extension Entomologist for specific insect control information.

ATMOSPHERIC SCORCH - OZONE

Nitrogen dioxide and hydrocarbons emitted from automobiles, industrial combustion, oil refineries and other 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. Pepper spot caused by a species of the fungus Leptosphaerulina often invades these scorched leaves and enhances the damage. Regular use of a foliar fungicide helps prevent these secondary infections in damaged tissue.

SALT AND BORON DAMAGE

Low peanut yields and severe pod rots are potential problems in soils with a high sodium adsorption ration (SAR). 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. Calcium deficiency can 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 rainfall and irrigation runoff and increase flushing of sodium from soil.

Boron toxicity is a problem in some soils in West Texas, decreasing plant growth and yields. The most common symptom is a yield decrease with little detectable foliage reduction.

Soil and irrigation water should be tested at least annually in areas at risk for high SAR or Boron. Test results should be considered when selecting fields for planting.