Root Rots of Green Beans and Lima Beans

Vegetable Disease Information Note 7 (VDIN-007)
Charles W Averre, Extension Plant Pathologist

Root rot, damping-off, and seed rots are major diseases of green beans and lima beans in some years and in some soils. These diseases are caused by a complex of soil-borne fungi; they are not seed-borne. These diseases tend to be spotty in fields, and their occurrence is unpredictable. On the average, two crops out of five are likely to be affected. In some locations it is possible to have a complete loss of stand in a field and then to reseed and have no problem. This variation is due to changes in weather and soil conditions.

A total plant disease control program is necessary to reduce losses from these soil-borne diseases. Some practices will reduce the intensity and the likelihood of occurrence of these diseases.

For example:
(1) Seed only in soils that have been well prepared, with pH around 6.5, fertilized according to the North Carolina Soil Testing Section analyses report, and treated for nematodes if recommended by the N.C. Nematode Advisory Service.
(2) Seed only during favorable weather in warm soils one inch or less deep and on top of a bed to avoid drowning.
(3) Use high-quality seed, treated with a protectant fungicide.
(4) Use recommended fungicides as an in-the-row soil treatment at seeding time (see Table 1).
(5) Avoid problem fields and those with excess legume crop residue.

There are no resistant varieties, though some may be more tolerant than others. Crop rotation is of little value since the fungi are widespread and survive in soils for a long time.

Fusarium Root Rot

Fusarium Root Rot, or Dry Root Rot, is the most common and important root rot of beans in North Carolina. Green bean is the main host but lima bean, southern pea, and garden pea are also affected. It occurs mostly in hot weather in acid and poorly fertilized soils. The disease tends to be evenly distributed over a field.

The disease is caused by the fungus Fusarium solani f. sp. phaseoli. The fungus can live in the soil several years in the absence of beans. Infection usually occurs when the seed germinate and the fungus penetrates the hypocotyl. A slightly reddish discoloration on the taproot occurs as early as 1 week after the plant emerges and gradually increases in intensity and extent. It may also occur in streaks that extend to the soil line. The red color may change to a dark brown and the lesions frequently crack longitudinally. The small lateral roots are usually killed, and clusters of roots may develop above the lesions and just below the soil line. Losses with this disease are usually more severe than the other root rots.

Because continuous bean culture may result in a buildup of the fungus in soil, long-term rotations (4 to 5 years) with nonleguminous crops reduce the severity of the disease. No suitable resistant varieties exist; but some varieties are more tolerant than others. Subsoiling to break up hardpans and bed shaping to improve drainage help to control the disease. Shallow cultivation reduces injury to the root system. Nematodes must be controlled.

Rhizoctonia Root Rot

Losses from Rhizoctonia root rot vary greatly from year to year with a loss of 5 to 10 percent are not unusual. This condition is more prevalent in warm weather; however, it is likely to be found under cooler conditions than those favorable for Fusarium root rot.

The disease is caused by the cosmopolitan soil-inhabiting fungus, Rhizoctonia solani.
On young bean plants, the fungus causes damping-off and attacks the stem below and above the soil surface. The young succulent plants die soon after infection. On older plants, as the stem becomes more woody, reddish-brown cankers extend longitudinally along the stem near the soil surface. The plants at this stage may show little indication of disease, except the yields may be reduced considerably. The fungus may enter the pith where it causes a brick-red discoloration. Under moist conditions, the fungus mycelium can be seen as a brownish tuft on infected plant parts; small (0.3 to 0.5 mm), brown or tan sclerotia may develop later in diseased tissue.

Because R. solani is parasitic on a large number of crop plants, crop rotation is of little value. Shallow seeding and cultivation (soil not placed against the seedling) reduce the severity of the disease. Chemical seed treatments and in-furrow treatments reduce this disease (see Table 1).

Pythium Root Rot

Pythium root rot, damping-off, stem rot, or hollow stem can cause serious losses on beans throughout North Carolina. Infection is often very rapid; it is apt to occur during wet weather, either hot or cold. The fungi which produce this disease remain in the soil for several years and attack a number of different crop plants.

The disease is caused by several species of the fungus Pythium, such as P. ultimum, P. debaryanum, P. aphanidermatum, and P. myriotylum. In very young plants, Pythium causes wet rot which soon kills the plant. The stem of the plant may be invaded by the fungus at or above the soil surface, producing a soft rot, ranging from colorless to dark brown. Larger plants may also wilt or die. Often a cottony white growth can be seen on infected stems during periods of high humidity.

Preparation of the seed bed to minimize water standing on the row will reduce the disease. Treat seed and soil with a systematic fungicide (see Table 1).

Other Root Rots

Beans may exhibit seed rot, damping-off, and root rot symptoms due to many other causes.

Sclerotinia sclerotiorum which causes Sclerotinia wilt or white mold also causes a stem rot under certain conditions. The disease frequently occurs after a period of warm, humid weather. It can be recognized by the white fungus growth and large (2 to 5 mm) black bodies (sclerotia) in the pith of the stem.

Southern root and stem rot is caused by the fungus Sclerotium rolfsii and is a hot weather disease. It causes a rot at the base of plants. It can be easily recognized by the white fungus growth and numerous seed-like bodies (sclerotia) that form around the base of the plant. See Vegetable Disease Information Note No. 9.

Ashy stem blight is a root rot disease caused by the fungus Macrophomina phaseoli. It often starts as a dark, sunken lesion at the base of the cotyledon and may extend into the roots. The plants may easily break off when cultivated or are blown by strong wind.

Fertilizer injury causes stem or root rot when excess fertilizer comes in contact with plants. The greatest damage occurs when the fertilizer is placed in the row and the seed is planted at the same time or soon there after. Careless placement of sidedressing may also result in injury. See Vegetable Disease Information Note No. 10.

Other things that injure roots or beans may cause symptoms that may be confused with the parasitic diseases. Examples of these include: mechanical injury, excessive water, and insect (e.g. lesser cornstalk borer) or pest injury.

Table 1. Crop protectants recommended for controlling root rots and related diseases of green beans and lima beans.

White mold (Sclerotinia) Terraclor 75W 2.5 lb per 15 to 20 gal. of water per 8,400 ft. row for pole bean; 14,500 ft. for bush bean Apply as a band spray at seeding. Repeat at 2- to 3-week intervals. Do not apply after first bloom. Do not cultivate after application. Granular and dust formation are also available.
Benlate 50W 1.5 to 2.0 lb per 100 gal of water Spray at 25% bloom, repeat at full bloom.
Topsin-M 70W 1.5 to 2.0 lb/100 gal. Spray at 25% bloom, repeat at full bloom.
Botran 75 W 3.0 lb/100 gal Spray at first appearance.
Rhizoctonia root rot, damping-off, Southern blight Terraclor 75W 1.3 to 2.0 lb per 8-10 gal of water per 14,500 ft. of bush beans, 8,430 ft. of pole bean Spray in furrow at seeding. Use lower rate for lighter soils. Avoid contact on seed. Do not apply after first bloom.
Pythium damping-off Apron 25W; Ridomil 2E 2.0 oz/100 lb; 1.0 pt/13,000 ft. Seed treatment. Apply in furrow or 7-inch band on row, seed.
Root rots, damping off, nematodes, Southern blight Telone C-17
4.4-7.3 gal/A
4-8 gal/A
4-8 gal/A
12-30 gal/A
Inject fumigation with a single chisel 6 inches deep and simultaneously shape the hill with disk hillers. Soil must be moist. When all traces of the fumigant are gone (2 weeks or longer), remove the crown of the hill and seed. Vapam is water soluble and may be applied in irrigation systems.

NOTE: All agricultural chemicals must be used exactly as stated on the label of the package.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. 06/91/1000

Published by
North Carolina Cooperative Extension Service
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race, color, national origin, sex, age, or disability. North Carolina State
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Reformatted Dec. 2000 by Plant Disease and Insect Clinic