DISEASE AND OTHER PESTS AND THEIR CONTROL

The Valencia peanut is susceptible to a number of foliage, pod, and root diseases. Each producer must develop a disease control plan, preferably for each field from past disease and crop records. Recognition of potential disease development plays an important role in the use of effective control measures. Plant diseases are seldom unique to a specific host crop. Knowledge of previous crops and diseases in a field can alert a producer to potential problems.

Certain peanut diseases can greatly reduce both quality and yield. Most of these specific diseases can be controlled by using good sanitation practices and cultural practices that lead to vigorous plant growth.

• Employ good crop rotation practices. Small grains are the best crops to precede peanuts except where crown rot has been a problem. Cotton is a poor crop with which to rotate peanuts. Peanuts should not follow potatoes or other vegetable crops in which diseases common to peanuts have been present. Heavy soils with high clay content should not be planted to peanuts for more than two consecutive years.
• Shortly after harvest, turn under as much of the crop residue as practical.
• Test soil for nematodes, and treat with a nematicide if a problem exists.
• Obtain a soil analysis and fertilize accordingly. Apply foliar iron chelate or iron sulfate in areas with a history of iron chlorosis.
• Prepare a seed bed free of crop residue and large clods.
• Avoid early plantings in cold soils.
• Use high-quality seed with good germination tests. Treat seed with a recommended fungicide.
• Plant seed at a uniform depth and avoid deep placement of seed.
• Irrigate when necessary to ensure the deep roots of the plant are exposed to adequate water.
• Flat cultivate so soil and trash are not pulled toward the plants.
• Harvest should be timed to achieve a 70%(+) sound mature kernel score. Overmaturation can result in decreased yields, as pod stem strength deteriorates upon pod maturity.

Seedling Diseases

Several fungal organisms can attack the seed or young seedlings. Stand reduction is the most obvious field symptom of seedling diseases. Plants may become diseased soon after germination and never emerge from the soil, or they may die any time during the early growing season. Symptoms of seedling diseases vary because of different causal organisms, cultural practices, and climatic conditions.

When the fungus Aspergillus niger is involved in seedling loss, a young stand can appear healthy, then plants wilt suddenly. Wilting plants may show signs of stunted growth when compared to uninfected plants. Large skips frequently occur in parts of the field where plants are infected. Characteristically, crown tissue and roots become rotted and shredded in appearance. Black spore masses may be present in these diseased areas.

Damping-off, caused by Rhizoctonia solani, is a seedling disease that causes sunken cankers at or near ground level. The cankers weaken the developing stem to the point of collapse. When Pythium myriotylum is involved in damping-off, seedling disease is characterized by wilting. Infected small plants may topple over at the ground line. Examination displays rotting plant and stem tissue at the ground line.

Several factors may contribute to seedling disease: use of damaged or broken seed, planting too deeply, uneven planting depth, dry spots in the field, planting too early in cool soil, and poor crop rotation sequences. Seedling diseases can be reduced considerably by planting fungicide-treated seed and by using good cultural practices. Seed should be treated with a fungicide or fungicide combination following label directions. Some producers have found furrow sprays or granules of Terraclor Super X®, and Ridomil® applied at planting are helpful where known problems with seedling diseases exist. (Note: Terraclor controls Rhizoctonia spp. and Ridomil controls Pythium spp.)

Table 2. Weed control (herbicides)

Product*	Rate per treated acre	Time of application	Weeds controlled
Alachlor Lasso (4EC) (chloroacet-amide)	3-4 lb. ai./A (3-4 qt. 4EC/A)	Pre-plant; pre-emergence surface	Annual broadleafs & annual grasses. Incorporation must be ABOVE seed. Use under sprinkler irrigation most effective.
Basagran (bentazon)	0.5-1 lb. ai/A (1-2 pt./A)	Early Post-emergence	Broadleafs including annual morning -glory, ragweed; sedges:yellow nutsedge
Blazer (acifluorfen)	0.125-0.375 lb. ai/A (0.5-1.5 pt./A)	Post-emergence	Annual broadleafs including pigweed, morning-glory, ragweed, bindweed. Annual grasses including Johnsongrass & volunteer small grains
Cadre (imazameth)	3-4 oz./A	Early post-emergence	Annual broadleafs, spurred anoda, pigweed,morning -glory; grasses & sedges:Johnsongrass, yellow nutsedge
2,4-DB (phenoxy-herbicide) various formulations	0.23-0.4 lb. ai/A (0.9-1.6 pt. 200/A)	Post-emergence	Annual broadleaf weeds, morning-glory, cocklebur
Dual (chloroacet-amide)	1.5-3 lb. ai/A (1.5-3 pt. 8E) (2.25-5.4 lb. DF) (6-12 lb. 25G)	Preplant incorporated. Pre-emergence. Post-emergence grasses	Broadleafs including pigweed;and sedges; yellow nutsedge
Glyphosphate (Roundup)	0.25-1.5 lb. ai/A (0.25-1.5 qt./A) Perennial weeds: 2-5 lb. ai/A (2-5 qts./A)	Pre-emergence or spot treatment	Emerged annual and perennial broadleaf weeds and grasses
Paraquat	0.125-0.25 lb. ai/A (11-22 fl. oz./A)	Post-emergence from ground crack to 28 days post cracking.	Contact herbicide used to control or surpress a broad spectrum of emerged weeds. Restricted use pesticide.
Poast Plus (sethoxydim)	0.14-0.28 lb. ai/A	Post-emergence	Annual & perennial grasses. No (1.5-2.2pt./A) activity on broadleafs or nutsedge
Prowl (dinitro-aniline)	0.5-1 lb. ai/A (1.2- 2.4 pt./A)	Pre-plant incorporated	Controls broadleafs including kochia and pigweed, and grasses
Pursuit (imida-zolinone)	0.063 lb. ai/A (4 oz. 2AS/A or 1.44 oz. DG/A)	Pre-plant, pre-emergence, post-emergence	Wide range broadleaf control;grasses including Johnsongrass and nutsedges.
Sonalan (ethal Furalin)	0.5-1.1 lb. ai/A (1.5-3 pt. HFP/A) (1.44 oz.DG/A)	Pre-plant incorporated	Broadleafs, including kochia, pigweed, Russian thistle; grasses
Storm (Premix) (bentazon & aciflourfen)	0.75 lb. ai/A (1.5 pt./A)	Post-emergence	Broadleafs: morning-glory, ragweed, pigweed, spurred anoda, cocklebur.
Tough (pyridate)	0.47-0.94 lb. ai/A (1-2 pt. EC/A)	Post-emergence	Broadleafs: kochia, pigweed, Russian thistle,nightshade suppression; sedges: nutsedge
Vernam (vernolate)	2-2.5 lb. ai/A (2.33-3 pt./A)	Pre-plant	Broadleafs: morning-glory, ragweed, pigweed, spurred anoda, cocklebur.

* Consult label for restricted entry interval and protective clothing requirements.

Blackhull

As its name indicates, the blackhull disease causes dark discolored areas on the peanut hull. These discolored areas may be large or small and may discolor the whole surface of the hull or appear as black speckled areas. In severe cases, discoloration extends into the kernels. Peanuts with high percentages of discolored hulls are sold at a discount. Use of recommended cultural practices such as rotation offer the best means of control.

The only fungicide currently labeled for control of blackhull is Benlate. Label recommendation is at the rate of 3 lb/A of Benlate (in sufficient water to facilitate) application or in the furrow at planting. Do not graze or feed treated vines, hay, or hulls to livestock.

Not all hull discoloration is caused by Thielaviopsis basicola, the blackhull organism. Some non-pathogenic (saprophytic) fungi may grow on the surface of the hulls causing discoloration. This is especially true in wet harvest years. Treatment with broad-spectrum fungicides is usually not possible when late-season saprophytic fungi cause discoloration problems, because most fungicides have label-defined application times prior to harvest that prevent their use.

Conditions often associated with blackhull disease are:

• Low temperatures late in the season.
• Heavy textured, alkaline soils.
• Cool, damp soils with poor drainage.
• High population of the blackhull fungus, Thielaviopsis basicola, in the soil.
• Peanuts following cotton or peanuts.
• Delayed harvest.

Verticillium and Fusarium Wilt

Verticillium and Fusarium diseases are caused by soil-borne fungi that enter plants through the roots. They cause problems to the plant by growing in the water-conducting xylem tissue, causing it to become plugged. Verticillium wilt is a major problem in cotton and has been found in potatoes; Fusarium more commonly appears in vegetable crops. Once a field has become infected with either Fusarium or Verticillium, the infectious spores remain present for years. Crop rotation practices do little to reduce the number of infective spores. Consequently, infected fields may not be useful for peanut production in future years. There are no available fungicides that adequately treat infected plants for either of these diseases, or reduce the number of spores present in soil.

Symptoms for both diseases include stunting and yellow leaves that wither and drop early, or wilting lateral growth. Characteristically, the central main stem often remains healthy in appearance. If wilting plants appear randomly scattered throughout the field, suspect Verticillium or Fusarium wilts. Dark brown discoloration in the woody portion of the stem and the upper root portion of the plant helps to confirm the diagnosis of these wilts. Loss can be kept at a minimum if the disease or cultivation history of a field is known, and peanuts are not planted following cotton or vegetable plants including potatoes in crop rotation.

Iron Deficiency (Chlorosis)

Chlorosis is the lack of chlorophyll in plants. The entire leaf may be white, or the veins may be green, while the areas between the veins may be white or light green. Severe chlorosis causes stunting. Chlorosis can be caused by a lack of available iron from soils. Uptake of adequate balanced nutrients during periods of rapid growth after rainfall or heavy irrigation, causes temporary chlorotic conditions not to be confused with unavailability of soil iron. Foliar-applied iron chelates or sulfates will correct iron chlorosis. Follow label directions.

Leaf Spot Diseases

Rainfall or irrigation followed by high humidity during the growing season can contribute to Cercospora leaf spot (Cercospora sp.) and web blotch (Phoma spp.). These fungi are present to some extent in most locations from year to year, but are most troublesome during wet, humid years. Cercospora causes brown-to-black, circular-to-irregular spots on the leaflets, petioles, and stems. The spots are frequently surrounded by a yellow halo. Web blotch causes brown areas on leaves that have distinct webbing or netting patterns.

Leaf diseases can be controlled by applying fungicides when excessive moisture and humidity occur during the growing season. Fungicide applications should begin before leaf spots become established. Most products do not claim to eradicate existing infections, but rather work to prevent infection. In order to prevent or delay the development of resistance by the organisms, two or more types of fungicide should be applied. However, take care not to alternate applications of two different fungicide types. Doing so may prevent buildup of protective residues from either fungicide type and leave plants unprotected. Pre-mixed formulations of fungicides that work together are sold specifically for this purpose.

Fungicides used to control foliar diseases should usually be applied before infectious spores settle on leaf surfaces, and continued according to product labels. Chemigation, aerial, and spray-coupe application have all been successfully used. As plants mature, their canopy may prevent lower leaves from being adequately treated, so use application techniques that allow greater penetration of the plant canopy. Current control efforts focus on an environmental monitoring program established throughout the peanut production area.

The "Blotchcast" software program (developed for Roosevelt County, New Mexico), assists producers by recommending fungicide application times for controlling web blotch. Electronic environmental monitors linked to computers by digital radio communication provide data used by computers to forecast favorable conditions for growth of the web blotch fungus. Throughout the growing season, announcements are made when favorable conditions occur. The most commonly used web blotch control fungicide is Bravo®.

Southern Blight (also known as stem rot)

Southern blight (Sclerotium rolfsii) is a fungal disease that may be prevalent in some fields. It is usually recognizable by thin, sinuous, white fungal filaments (hyphae) on the base of plant stems and nearby surrounding soil. Small white dots called sclerotia accompany the hyphae. Sclerotia, which turn tan to brown with age, serve as overwintering bodies for fungus spores. Peanut stems, pegs, and pods can all be affected. Cool, moist weather is required for southern blight, which usually appears during fall or late summer. Plants wilt and die when they are girdled by the fungus, but it is common to find some surviving branches on each affected plant. Typically, this fungus spreads from infected plants to adjacent ones, so visual examination of an infected field may show small patches or areas of wilting or dying plants. Heavy applications of gypsum, sulfur, or use of other fungicides that reach the soil may prevent southern blight from developing on soil surfaces. Under these circumstances, symptomatic plant wilting and death may not occur and the plants may appear healthy upon visual examination of the field. However, the fungus can still attack developing and mature pods beneath the ground. Producers are cautioned to occasionally pull plants, especially in low areas of the fields where moisture accumulates, and examine them for subterranean infestations. Affected pods are tan to light brown, often covered with white mycelia, and at maturity are usually completely rotted and unharvestable.

Control methods for this disease include deep covering of crop residue, flat cultivation to avoid pulling soil and trash toward the plants, crop rotation with grain sorghum to reduce the numbers of infectious spores, and use of soil fungicides specific for control of this pathogen. Numerous fungicides are registered for southern blight control; many contain PCNB as the active ingredient.

Preventing foliar diseases also is important in controlling southern blight. Foliar diseases often cause leaves of the peanut plant to abscise. Dead leaf tissue on the soil surface provides decaying organic matter for southern blight organism to begin growing.

Pod Rots

>pythium myriotylum, Rhizoctonia solani, and Sclerotium rolfsii (table 3) are all capable of causing pod rots. When pod rots occur, more than one fungus may be associated with the rotted conditions.

pythium spp. causes a wet pod rot, which provides a characteristic symptom of its infection. pythium spp. are water molds and when infected pods are squeezed a wet pulpy condition is present inside the developing pod. Left to maturity, these pods will completely rot and the pods will be lost from harvest. If a heavy disease condition is observed in the field and loss is judged to be significant, use of Ridomil® will help to arrest the condition and prevent further infection.

Rhizoctonia solani infects developing pods and may penetrate pods to rot the kernels inside. Rhizoctonia most commonly causes ulcers and canker-like indentations on the pod, and pod discoloration. Pods are predisposed to infection after damage by wireworms. Once the pod has been penetrated, Rhizoctonia gains simple entrance to the kernels inside and begins to destroy them. Empty, rotted, light-weighted pods often blow out during harvest, while partially destroyed kernels increase concealed damage at grading. Rhizoctonia, or "Rhizoc" as it is commonly called, can be treated with PCNB-containing fungicides, as it does not respond to Ridomil®.

Pod rot caused by Sclerotium rolfsii (southern blight) can be devastating if an entire field becomes infected. Usually, however, southern blight only infects small areas. As with any fungicide, the degree of infection and potential loss must be weighed against the cost of treatment.

Pod rots do not exhibit noticeable above ground symptoms, therefore occasional pulling of plants throughout the field, especially during pod maturation, is important. It is the only way pod rots can be detected. Healthy vines do not automatically predict healthy pods and peanuts below the soil. Proper treatment, if required, must begin early enough so as to not exceed label application restrictions regarding the number of days before harvest.

Table 3. Guide for distinguishing pod rots.

Item	pythium spp.	Rhizoctonia spp.	Sclerotium spp.
Pod color	Dark brown to black	Dark brown to black	Tan to light brown
Moisture level of affected pods	Water when squeezed	Dry	Moist and slimy but not watery
Most favorable soil moisture levels	Too wet	Normal	Normal to dry
Controlled by PCNB (Terraclor)	NO	YES	YES
Controlled by Ridomi	YES	NO	NO

Sclerotinia Blight

Sclerotinia blight is not currently a problem for peanut producers of eastern New Mexico. The Valencia peanut and the soils of the region, however, are not immune to infection by this pathogen. Constant quarantine practices are advised. Any used peanut-production equipment, purchased where this pathogen exists outside of the Valencia peanut-growing area, should undergo thorough cleaning and washing prior to entering New Mexico. It is highly advised to only use seed grown and produced within the eastern New Mexico growing region. Once this pathogen enters New Mexico, it can easily spread to most peanut-producing fields, and its treatment and prevention with fungicides will greatly reduce the economic gain from peanut crops. Sclerotinia blight is in most adjacent peanut-growing regions, including parts of Oklahoma and Texas.

Nematodes

Plant parasitic nematodes are microscopic worms that live in the soil and infect plants. Two varieties, root-knot and root-lesion nematodes, are known to cause problems in New Mexico. Typically only small areas of fields are affected, but widespread infections can destroy a crop.

Root-knot nematodes cause gall formations on peanut roots, pegs, and pods. Plants severely affected by root-knot nematodes are stunted and may be lighter in color. During field inspection, healthy, normal-sized plants provide a marked contrast to dwarfed and stunted ones affected by nematodes. Small areas within a field are generally involved and plants of different heights are common within the area. Take care to observe and treat infected areas so the organisms are not spread throughout the field during normal yearly field preparation and cultivation. Numbers of root-knot nematodes can be reduced by rotating small grains with peanut crops.

Root-lesion nematodes affect roots, pegs, and pods, and can best be identified by the presence of small spots on pods. These spots are tan with a dark center. Root-lesion nematodes have a wide range of host plants and therefore cannot be controlled by crop rotation where nematode populations are high. Nematicides can be used to control both root-lesion and root-knot nematodes.

Insects

Insects are not usually a major problem for peanut producers in eastern New Mexico. Control of thrips in peanuts is believed to assist the early development of plants, but research has demonstrated little economic improvement from their management. Army worms are an occasional problem in some areas. The moths, active at night, are dark gray with a noticeable whitish spot near the extreme tip of the wings. The larvae feed day and night. Larvae color ranges from dark tan to green to nearly black and they have three yellowish-white lines down their back. Some peanut fields adjacent to grassland are occasionally damaged by grasshoppers. Both army worms and grasshoppers are easily controlled by labeled insecticides.