EDITOR's NOTE — The following comments were pulled from Barbara Shew's disease management chapter in the 2004 Peanut Production Guide, published by the North Carolina Cooperative Extension Service. Shew is an Extension plant pathologist with North Carolina State University.
Wise management of disease is essential to profitable peanut production. Long rotations, resistant cultivars, early disease detection, weather-based prediction, scouting and proper pesticide selection comprise the basic elements of a disease management strategy.
When it's said and done, the best way to avoid plant diseases is long rotations. Changes in the peanut program have given peanut farmers the opportunity to plant rotational crops that do not host the organisms that cause peanut disease. If a field does have a history of disease, plant cultivars resistant to that disease.
Early detection is also critical in developing effective and low-cost approaches to disease management. Thorough scouting and accurate identification of diseases is necessary for selection of the most appropriate management tactics and crop protection products.
Use cultural and chemical controls to achieve maximum benefits. Rotation and resistant cultivars generally reduce the number of pathogens.
Pesticides should only be used when cultural practices have not been sufficient to reduce pathogen levels below economic thresholds.
In most cases, fungicides must be applied before or soon after diseases appear to be effective. To anticipate disease outbreaks, use weather-based disease advisories, which will minimize unnecessary pesticide use and add precision to necessary applications.
For many diseases, such as tomato spotted wilt virus, Cylindrocladium black rot, Sclerotinia blight, pod rot, and nematodes, management decisions much be made before the next crop of peanuts is planted. Prepare maps to show where these diseases were located in the current crop. These maps will serve as guidelines for future rotations, fumigation and use of resistant cultivars.
Two different fungi, Cercospora arachidicola (early leaf spot pathogen) and Cercosporidium personatum (late leaf spot pathogen), cause peanut leaf spots. Early leaf spot causes brown lesions surround by a yellow halo and aren't normally observed until mid-July. Late leaf spot causes darker spots and usually appears about a month later.
Because the leaf spot fungi attack only peanut rotations with any other crop help to reduce disease. Avoid highly susceptible cultivars. Conservation-tillage also appears to reduce leaf spot slightly.
In addition to cultural practices, fungicide application is usually required for leafspot control. Either apply fungicides on a set 14-day schedule or according to a weather-based leaf spot advisory.
The advisory enables farmers to spray only when it's necessary and avoids tractor damage to vines and spider mite flare-ups.
Fungicides that control early and late leaf spot include cholorothalonil, tebuconazole, propiconazole plus chlorothalonil, propiconazole plus trifloxystrolin, azoxystrobin, pyraclostrobin and boscalid.
Web blotch has been very serious in the past few growing seasons. Large, half-inch dark patches or blotches form on the upper surface of the leaf. Later, lesions turn brown and become more circular. Web blotch is controlled by most leaf spot fungicides, but you may need to spray more frequently and use a more effective fungicide. Pyraclostrobin and boscalid show excellent activity against web blotch.
I need to say a word about fungicide resistance management. Because several of the new fungicides registered for use on peanuts tend to be very effective at low rates, when used properly, will be important disease management tools for many years to come. Unlike older fungicides, however, many of the new fungicides have very specific modes of action. This specificity increases the possibility that rare fungicide-resistant strains of the pathogen will survive fungicide treatment.
To maintain fungicide efficacy:
Mix or alternate spray of fungicides with different modes of action. Do not use fungicides at less than the recommended rates. Do not exceed the total number of recommended sprays. And use broad-spectrum fungicides for the first and last spray of the season.
Irregular (or funky) leaf spot is a relatively new problem of unknown cause. The spots look similar to early leaf spot without sporulation. Irregular leaf spots are scattered over the leaf surface. A wide range of fungicides and bactericides tested against this problem were completely ineffective. It's recommended to confirm the presence of early leaf spot before making unplanned fungicide applications early in the season. Poor leaf spot control observed within 45 days of planting usually can be attributed to irregular leaf spot.
TSWV is found in all peanut counties in North Carolina. It's spread through feeding thrips. Symptoms including stunting, dead terminal buds, pale yellow or white ring patterns on leaves; purple blotches on the underside of leaves; stunted, small and malformed growth; undersized pods; and red seed coats.
Use preventative measures to reduce the risk of TSWV. Cultivar choice and plant stand have the greatest effect on TSWV risk. Use quality seed, choose a mid-season planting date and use phorate. Minimum-tillage appears to reduce TSWV incidence. Use the TSWV Risk Index to assess the overall risk of TSWV in a given field.
Simply put, soil-borne pathogens are very difficult to control. They can survive in the soil for years. Once again, a rotation of at least three years is the best defense against disease.
Since soil-borne pathogens have limited mobility, mapping can be a useful tool for deciding where certain cultural practices and/or chemical treatments should be applied the next time peanuts are grown. Clean equipment regularly to avoid introducing soil-borne pathogens into un-infested areas.
Nematodes cause stunting, wilting and yellowing of above-ground portions of the plant. Damage is often seen in clusters within a field. Root systems may be stunted and pods and roots may have small lesions. Nematode damage can increase susceptibility to CBR.
Take pod and root samples after harvest in a zigzag pattern and submit the sample to a reputable lab. (In North Carolina, the North Carolina Department of Agriculture and Consumer Services operates a Nematode Advisory Service.
Since nematodes are often found in small areas of fields, it's often unnecessary to treat the entire field. Fumigate with metam sodium for CBR control or applying aldicarb in-furrow at planting for foliar insect control may help reduce nematode treatments.
Seed and seedling rots can be caused by many fungi. These diseases can cause poor stands with skips, which can lead to TSWV. Rots often develop after seeds and seedlings are weakened by environmental problems or poor seedbed conditions. To avoid this problem, plant in warm soil of at least 65 degrees at a 4-inch depth for three consecutive days. Cold soils retard germination. Use high-quality seed coated with a good chemical seed treatment fungicide. Replanting is the only remedy.
Aspergillus crown rot causes pre- and postemergence damping off and sometimes kills older plants. Dark brown discoloration is common on decayed roots and hypocotyls. Seed treatments also inhibit crown rot development. Azoxystrobin can be applied preventatively at planting if crown rot is a concern.
Southern stem rot or white mold is most active during the hottest part of the season, especially following rain. In drier seasons, the fungus is most active underground. White mold is often found along with CBR. White mold is characterized by white, stringy fungus growth and tan to brown birdshot-sized balls on the lower stems and leaf litter. Fields with heavy vine growth are susceptible to white mold.
Some fungicides that control leaf spot, such as azoxystrobin, tebuconazole, propiconazole plus flutolanil, and pyraclostrobin, also control white mold. If flutolanil is used, continue your leaf spot control program with an appropriate fungicide. Treat fields with a history of problems according to the leaf spot advisory between July 15 and the end of August.
Rhizoctonia limb and pod rot is sometimes confused with white mold, but it produces dark or grayish-brown lesions on the bottom of stems where they touch the soil. Lesions often have a purple border. Rhizoctonia pod rots affects pods at all stages of development and is the most destructive aspect of the disease. The disease is most common in moist fields, irrigated fields or where vine growth is thick. Management practices and fungicides are the same as for white mold.
Sclerotinia blight starts killing individual limbs; therefore, scouting is required to see symptoms when they first appear. Signs of the cottony growth of the Sclerotinia are most visible on humid mornings. The end portion of infected limbs may remain green and look healthy for several days before wilting is evident. The disease favors cool, wet conditions and is more severe on injured vines. To prevent Sclerotinia blight buildup, rotate with corn or cotton. Avoid vine injury by cultivating before June 15 or not at all. Fields planted by May 1 tend to have less Sclerotinia blight. The fungicides fluazinam and boscalid are effective against Sclerotinia blight when applied preventatively.
A weather-based Sclerotinia blight advisory can be used to time applications and prevent unnecessary fungicide applications.
CBR causes entire plants to turn light green or yellow, wilt and die. A blackened, rotting root system is characteristic of this disease. The fungus produces numerous brick-red, pinhead-sized structures on crowns, lower stems and pods, especially following moist weather.
Rotate with corn, cotton, sorghum and small grains. Plant fields with a history of CBR on a bed around mid-May. Plant resistant cultivars and fumigate when fields have a history of more than 10 percent. Fumigants must be injected 12 inches deep at least two weeks prior to planting. The effectiveness of the material can be lessened if a rain occurs within a day or two or fumigation. Herbicides can be incorporated prior to bedding and injected. Some fungicides applied in-furrow or to foliage may suppress CBR. Check the label for details.