Brassica spp.
Black Rot (bacterium – Xanthomonas campestris pv. campestris): Infection usually occurs near margins of leaves. Yellow areas develop along the margins, progressing into the leaf in an inverted-V shape. Veins in the affected areas are black. The bacterium may attack cruciferous plants at all stages of growth. A black discoloration is observed in the stem when it is split lengthwise. Cauliflower is very susceptible to black rot and the interior of the stem may be destroyed by secondary soft rot organisms that attack the black rot infection. It is a seed-borne bacterium which can persist in the soil for only a short time. Some wild plants may act as hosts harboring the bacterium while crucifers are absent from commercial fields. Young plants resulting from infected seed serve as a source of secondary infection. Under crowded conditions in the field and during periods of rain, the bacteria spread quickly to nearby healthy plants. Control is achieved by following a 2 year rotation, by planting disease-free seed, and seed that has been treated with hot water to eliminate seed-borne bacteria.
Fusarium Yellow (fungus – Fusarium oxysporum f. sp. conglutinans): The first indication of the disease is the yellowish-green color of the foliage. Plants appear wilted and stunted. Infected plants usually show a curved mid-rib and the leaf grows on only one side. The disease may be easily confused with black rot since many symptoms are similar. The fungus can live in the soil for a number of years without a host plant. It infects plants through young rootlets and wounds caused by transplanting or insects. It develops in the water conducting vessels, causing a brown discoloration similar to that produced by the black rot bacterium. The disease is checked by very cold or very hot soil tempera- ture. Resistant varieties and long rotations will avoid crop loss.
Downy Mildew (fungus – Peronospora parasitica): The fungus is more noticeable on the underside of leaves as a gray, fluffy, downy growth in well defined spots. Outer leaves of the head will develop small black spots when infected with the mildew fungus. The fungus overwinters in crop refuse. Sanitation, rotation and spraying with a protective fungicide are the most effective control methods. During periods of high humidity and cool temperature, the fungus is difficult to control and is almost always present in cabbage fields.
Black Leg (fungus – Phoma lingam): First symptoms occur on leaves and stems as small spots with ashen-gray centers and black dots. The stem lesions gradually enlarge extending to the roots. In advanced stages the root system develops a dry rot. Wilted leaves tend to remain attached to the stem instead of defoliating like plants infected with yellows or black rot. The fungus is carried on seed and can persist on infected plant residue for one or two years. Spread is dependent on dew, rain or irrigation water. Avoid soils where cabbage or related plants were grown within the last four years. Destroy crop residues by shredding and deep burial of crop residue.
Powdery Mildew (fungus – Erysiphe polygoni): This disease occurs infrequently on cabbage and related plants. A white, powdery growth is observed on the upper surface of leaves. Protective fungicides can be applied if plants become severely infected.
Alternaria Leaf Spot (fungus – Alternaria brassicae): The first symptom is a minute dark spot on seedling stems and on the leaves. These spots enlarge and are marked with concentric rings, giving a bull’s eye appearance. The fungus overwinters on cabbage residue or on seed. Spores are disseminated by wind or water. Hot water treatment, as recommended for black rot, will rid the seed of this organism. Fungicide application will prevent the fungus from developing in the field.
Rhizoctonia or Wire Stem Disease (fungus – Rhizoctonia solani): This disease may appear at different stages of growth. A damping-off phase of the disease will cause young seedlings to die. After seedlings are older, they may be attacked but lesions seldom completely girdle the stems. The infected stems are somewhat smaller than normal and are tough and woody. Older plants can also be attached causing a head rot or a root rot. The causal agent is a common soilborne fungus that attacks many plants. Crop rotation and planting healthy transplants are the most effective means of control.
Internal Tip Burn (physiological): Tip burn causes leaf margins to turn brown. The leaves are buried in the head. Exact nature of the problem has been associated with poor water movement within the plant.
Soft Rot (bacterium – Erwinia carotovora): Soft rot also occurs most commonly when fields become water saturated. Stems become decayed and have a foul odor. During storage and transit, a slimy decay with a foul odor develops. The disease begins in areas that have been bruised, particularly during periods of high temperature and humidity prevail. Proper handling during harvesting, packaging and storing along with keeping temperatures low are the best means of control. Follow a long rotation and plant on raised beds in well drained soil to prevent field infections.
Cabbage Mosaic (virus): Leaves display green and yellow mottled areas. Leaf veins may be lighter in color. Keeping fields and surrounding areas free of weed hosts will help reduce the disease.
Root Knot Nematode: (See Nematodes)
Southern Blight: (See Southern Blight)
