Disease management critical in successful production of peanuts

EDITOR'S NOTE — The following article was adapted from material in the 2005 North Carolina peanut information manual, Peanut Disease Management Chapter written by North Carolina State University plant pathologist Barbara Shew.)

An overview:

Foliar and soil-borne diseases must be managed to have a successful peanut crop Successful peanut production means wise management of disease. The basic elements are long rotations, resistant cultivars, early disease detection, weather-based disease prediction, scouting, and proper pesticide selection.

Identification and early detection are critical.

For many diseases, management decisions have to be made before the next crop of peanuts is planted. Others, such as leaf spot and white mold, require attention immediately before or soon after they appear.

Two different fungi cause peanut leaf spot. In the field, it can be difficult to distinguish between late and early leaf spot.

Early leaf spot usually causes brown spots surrounded by a yellow halo. These spots usually appear mid-July, although they can be on the plant as early as 30 days after planting.

Late leaf spot causes darker spots that usually appear in August. The best way to tell the two leaf spots apart is to use a good magnifying glass to observe spore production. Early leaf spot produces silvery, hair-like spores on the top of the leaf. Late leaf spot produces dark brown, velvety spores, usually on the underside of the leaf. Some fungicides are less effective against late leaf spot.

Fungicide application is usually required for control of leaf spot, either by using a set 14-day calendar schedule or by using a weather-based leaf spot advisory. The first spray should be applied at the very early pod stage.

Fungicides for the control of early and late leaf spot include chlorothalonil, tebuconazole, propiconazole plus chlorothalonil, propiconazole plus trifloxystrolin, azoxystrobin, pyraclostrobin and boscalid.

Another foliar disease is web blotch. It produces large, dark patches or blotches on the upper surface of the leaf. Later, lesions turn dark brown and become more circular. An entire field can be affected in a short time. Most leaf spot fungicides control web blotch.

Irregular or funky leaf spot is a relatively new problem of unknown cause. Symptoms appear early in the growing season on lower leaves. Lesions look like early leaf spot, but do not produce spores. Irregular leaf spot differs from phytotoxicity in that lesions are scattered over the surface. Some defoliation may occur, but yield losses have not yet been determined. Poor leaf spot control observed within 45 days of planting usually can be attributed to funky leaf spot.

Tomato Spotted Wilt Virus is spread through the feeding of thrips. Symptoms include 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.

Sometimes, entire plants may wilt and have root rot, resembling Cylindrocladium black rot (CBR).

Use preventative measures to reduce the risk of TSWV. Cultivar choice and plant stand have the greatest effect on TSWV risk. In-furrow use of phorate may reduce disease incidence more than aldicarb. Phorate, however, doesn't control nematodes. A mid-season planting date will help disease suppression. Minimum-tillage appears to reduce the incidence of TSWV.

Use the Tomato Spotted Wilt Risk Index to assess overall risk of TSWV in a given field.

All soil-borne diseases have a pod rot phase. Therefore, management of soil-borne pathogens also helps reduce pod rot. Soil borne diseases are very difficult to control and most can survive in soil for years.

Prevent the build-up of diseases by using at least a three-year rotation to non-host crops.

Nematodes cause stunting, wilting and yellowing of the above-ground portion of the plant. Damage is often seen within clusters in the field. Depending on the species of nematode, root systems may be stunted. Nematode damage can increase susceptibility to CBR.

Check pods immediately after digging for possible root knot nematode problems. Fields to be planted to peanuts should be sampled for nematodes the previous fall. Fumigating with metam sodium for CBR control or applying aldicarb in-furrow at planting for foliar insect control may help reduce nematode populations.

Many fungi can cause seed and seedling rot. Seeds may not germinate, germinate but not emerge, or die shortly after emergence. The result is a poor stand with skips, which can lead to more TSWV. Rots often develop after seeds and seedlings are weakened by environmental problems or poor seedbed conditions.

Cold soils, less than 65 degrees Fahrenheit, retard germination and increase the chance for rots. Use high-quality seed coated with a good chemical seed treatment fungicide. Replanting is the only remedy for stand loss due to seedling disease.

Aspergillus crown rot causes pre- and post-emergence damping off and sometimes kills older plants. Rotation is effective for this disease. Azoxystrobin can be applied preventatively at planting. Seed treatments also inhibit crown rot development.

White mold or Southern stem rot symptoms include wilting of individual stems, brown stem lesions, shredded stems and pegs and plant death.

The disease is characterized by white, stringy fungus and tan to brown birdshot-sized balls (sclerotia) on the lower stems and leaf litter. Damage can occur with few visible signs.

Fields with heavy vine growth and excessive moisture are prone to stem rot. The disease is most active during the hottest part of the season, especially following rain. In drier seasons, the fungus is most active underground. Southern stem rot is often found along with CBR.

Fungicides used to control leaf spot, such as azoxystrobin, tebuconazole, propiconazole plus flutolanil and pyraclostrobin also control stem rot. Most soil fungicides work best when applied just prior to disease onset. 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 Southern stem rot or white mold. Rhizoctonia limb rot does not produce stringy growth or tan sclerotia.

Dark- or grayish brown lesions are usually found on the bottoms of stems where they touch the soil. Lesions have a purple border and may have a target-like appearance. Rhizoctonia pod rot affects pods at all stages of development. Management practices and fungicides are the same for white mold.

Sclerotinia blight starts by killing individual limbs. It requires careful scouting. Vines must be pulled back to reveal cottony growth of Sclerotinia on straw-colored stems. The end portion of infected limbs may remain green and look healthy for several days before wilting is evident. Small black sclerotia that are irregular in shape can be seen both on and in infected tissues. The disease favors cool and wet conditions and is more severe on injured vines.

Avoid vine injury by cultivating early. Rotate with corn or cotton. Winter weeds serve as hosts for the disease. Use moderately resistant cultivars.

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.

CBR turns plants light green or yellow. The plants wilt and die. A blackened 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. If these fruiting structures are not visible, late-season wilting and root rot symptoms of CBR and TSWV can be confused.

Using resistant cultivars and fumigating with metam sodium usually controls CBR. Field with a history of 1 percent to 10 percent CBR should be planted to a resistant variety.

To avoid CBR being transmitted by seed, use seed produced in clean fields.