Vegetable Disease Information Note 14 (VDIN-0014)
Charles W Averre, Extension Plant Pathologist
Turnip, mustard, collards, and closely related green crops in North Carolina are subject to several leafspot and soil-borne diseases including nematodes and damping-off. Diseases may cause losses by reducing stands, making plants unthrifty, and by lowering the quality of marketable leaves. Because most greens in North Carolina are grown in relatively small, isolated plantings, diseases are usually not a serious problem. However, diseases of greens can be devastating at times and growers should always take precautions to minimize the risk of disease occurrence and take steps to reduce their severity if a disease does occur. The following steps are suggested for a "Total Disease Management Program":
Anthracnose leafspot is caused by the fungus Colletotrichum higginsianum. The disease can be recognized by the presence of small, rounded spots with dry, generally straw-colored centers on the leaves, petioles and stems. This disease is favored by relatively warm weather and is usually the first disease to appear in the spring.
White spot is caused by the fungus Cercosporella brassicae, and the disease can be recognized by the large, irregular, spots on the leaves, almost white in color. Cotyledons, petioles, and seed pods may also be involved. The disease is favored by cool temperatures, 12.5 to 24.0 degrees C (55 to 75 degrees F). Consult the current edition of the NC Agricultural Chemicals Manual for currently labeled disease control materials.
Alternaria leafspot or brown spot is caused by the fungus Alternaria brassicae. The disease can be recognized by the presence of brown, round spots which often have concentric rings. Usually the spotting is more prevalent on older leaves, and the spots may enlarge to over an inch in diameter. The fungus may also cause damping-off by infecting the small stems shortly after emergence. The disease is most severe during the coolest part of the season.
Downy mildew, caused by the fungus Peronospora parasitica, can be recognized initially by the presence of a white, downy type of mold on the underside of lower leaves. Later, a slight yellowing occurs on the top side of the leaves in poorly delineated spots; eventually the affected area and entire leaves become blighted. Characteristically small, black sunken lesions or streaks can be seen initially in the affected leaf tissue. The disease is favored by cool, wet weather. The disease is very common in North Carolina on all mustard family crops; however, it is only occasionally a major problem.
Powdery mildew, caused by the fungus Erysiphe polygoni, is recognized by the presence of extensive white powdery fungal growth, principally on the upper surfaces of new and old foliage. It is favored by prolonged dry periods with cool nights. Its occurrence is infrequent but is usually sudden. Sulfur sprays are effective.
Black rot is a bacterial disease caused by Xanthomonas campestris and is recognized by "V-shaped," bright yellow to orange areas on the margins of the leaves. The veins become black. The disease is seed-borne, and it is important to obtain seed from areas where the disease does not occur. Once the disease is present, it is important to limit spread by entering the field only when the plants are dry. See Vegetable Disease Information Note No. 16.
Mosaic is usually caused by the Turnip Mosaic Virus, and it is transmitted by winged aphids. Other viruses such as Cucumber Mosaic and Southern Celery Mosaic may be involved. It is recognized by poor growth of plants with a mosaic pattern of light green on the normal green color of leaves, and by crinkling and distortion of leaves. Control is based on destroying the overwintering reservoirs of the viruses by destroying wild and volunteer plants in the mustard family, and by killing the overwintering aphids.
Damping-off is caused by different soil-borne fungi such as Pythium spp., Rhizoctonia solani and others. The disease is recognized by poor emergence and by seedlings that rot at the soil line. The occurrence of this disease is sporadic and is dependent on weather conditions, usually wet weather. The control is based on the use of seed treated with a fungicide by the seedsman, proper seeding depth, and in some cases treating the soil with a multi-purpose soil fumigant.
Wire stem and stem rots are caused by Rhizoctonia solani and soft rot bacteria Erwinia spp. respectively. Wire stem is recognized by the drying up of the stem near the soil; the control is similar to that mentioned under damping-off. Stem rot affects collards and is recognized by a soft, wet, and smelly rot in the pith of the stem. It is favored by hot weather. Because the disease is often associated with burrowing larvae of insects, an effective preventative insecticide program will minimize the disease. Vapam, Vorlex and Chloropicrin soil fumigants may help.
Club-root is caused by the soil-borne fungus Plasmodiophora brassicae. It is recognized by unthrifty plants that often wilt during the day; the presence of large galls or clubs on the roots is diagnostic. Turnips and mustard greens are usually not affected, but collards are quite susceptible. This is a serious disease because once present in the soil, the fungus can survive 5 to 10 years or longer in the absence of a host; it is limited to crops and weeds in the mustard family and possibly to a few other plants (see Vegetable Information Note No 17). The disease is not widespread in North Carolina and is confined to the counties in the northern mountains and to a few isolated farms throughout the state. It tends to be worse if soils are wet after planting. It is extremely important that plants be certified and free from the disease; under no circumstances should any lot of plants be set out if even a trace of the disease is present. In affected areas, incorporating 1500 pounds per acre of "quick lime," "builder's lime" (calcium hydroxide) immediately prior to setting plants will reduce the intensity of the disease. Contaminated fields often can be made safe by keeping them free from any crops and weeds in the mustard family for five years.
especially the root-knot nematode (Meloidogyne incognita and others),
are soil-borne but rarely cause problems because greens are normally grown
when temperatures are cool. Nematode problems are usually recognized by
unthrifty plants and by root systems that are considerably reduced in
size and are brown or have galls or knots. A soil sample should be taken
in the fall for a nematode assay. (See Extension Leaflet 187 or AG-40.
Nematode assays can be obtained through the Nematode Advisory Section,
Agronomic Division, North Carolina Department of Agriculture, Reedy Creek
Road Center, Raleigh, North Carolina 27607. There is a fee for each sample.)
Depending on the nematode problem, certain crop rotations can reduce the
problem. Soils can also be treated with a fumigant-type nematicide such
as Telone, DD, and Nemagon. Nemagon cannot be used for collards.
use agricultural chemicals exactly as stated on the label of the container
and only on the crops mentioned on the label.
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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. 02/91/1000