Diseases of Crucifers
Cabbage, cauliflower, turnip, broccoli are some of the important cruciferous crops
Club root (Plasmodiophora brassicae)
The above ground symptoms are, yellowing of leaves, stunting and later the diseased plant die. Young plants die with in a short time while older plants fail to produce marketable heads. On roots and root lets, spindle shaped with thick centre and tapered ends giving a club shaped malformation is developed. The club shaped structered cells are invaded by the secondary,weakly parasitic organisms and the cells are disintegrated which produce toxins and the plants are killed.
Avoiding infected field
Black rot (Xanthomonas campestris pv.campestris)
On leaves ‘V’ shaped chlorotic to yellow lesions develop from the leaf margins. They become black later. The veins and veinlets become black.Systemic infection spreads to the root. The vascular bundle in the stem becomes black followed by integration of fleshy stalk. The attack of soft rot organisms cause further detoriation. The cabbage heads and cauliflower curds are invaded and become discolored.
Use of healthy seed Spraying with Agrimycin 200 ppm
Alternaria leaf spot/ Black spot/ Brown spot (Alternaria brassicicola)
Small dark coloured lesions up to 1.0 cm in dia appear on leaves. In humid condition conidiophores appear on the lesion in concentric rings. Linear spots also appear on petioles, stems and seed pods. On cauliflower curds brown discoloration occurs. In case of A.brassicae the spots are smaller in dia and lighter in colour.
Hot moist weather, temperature 26° C. Dew or rain for 9 hrs is essential for infection.
Hot water treatment of seeds at 500C for 30 minutes. Crop rotation, avoiding overhead irrigation.
Wire stem gets its name from symptoms that occur on the stem at the soil level. A dark, watersoaked lesion initially appears on the stem. Later stems become wiry and slender at the point of the lesion. Diseased crucifer plants transplanted to the field grow poorly, are stunted, and may eventually die, especially if there is inadequate moisture shortly after transplanting. If infected plants remain alive, the stem becomes tough and woody. Plants that survive usually mature late and fail to produce a marketable head.
Bottom rot is a disease of mature cabbage. After cabbage transplants become large enough to begin to shade the ground, the disease appears on the lower side of the head leaves that are in contact with the soil. The midrib is often the first part of the leaf attacked. Resulting lesions are sunken, black, and sharply elliptical with the long side of the lesion parallel to the side of the midrib. Lesions may dry out and become papery brown in appearance if the weather becomes dry. The surface of the lesion may be covered with a sparse, weblike mycelium. Eventually a general black decay occurs at the base of the leaf. The tip of the leaf then turns yellow and then the entire leaf wilts. Infected leaves are shed and only a naked stalk, capped with a small head, remains. Bottom rot becomes a head rot if temperatures are warm and the relative humidity is high. A weblike mycelium develops between diseased leaves. Infected leaves are eventually covered with small, brown, fungal structures called sclerotia, which can persist on plant debris in the soil for long periods of time. The stem remains unaffected, thus, the head remains upright.
Choose a fertile, well-drained site for the plant bed. Poor drainage favors seedling disease. Avoid excessive amounts of nitrogen fertilizer. Succulent plants are more susceptible to infection. Seed crucifers when the soil temperature reaches 69°F (21°C) and seed as shallowly as possible so that germination and emergence are rapid. Discard transplant seedlings that show symptoms of wire stem. Avoid banking or throwing soil up around plants during field cultivation. Harvest cabbage heads early. Cabbage leaves become more susceptible the longer they stand in the field.
Sources of the fungus include infested seeds, cruciferous weeds, and residues of cruciferous crops remaining in or on the soil Survival in crop residues in soil for 3 years.
Use of disease free seed Rotation with non cruciferous crops for 4 years where the disease is severe Providing adequate drainage facility. Soil fumigation with chloropicrin etc.
Cabbage yellows / Fusarium wilt of cabbage (F. Oxysporum conglutians)
The plant become yellow in colour. The wilting of leaves may be more prominent on one side of plant that the other. Leaves dries up. When the infected stem is cut open brown discoloration will be seen.
Use of disease free seed.Crop rotation, field sanitation . Use of resistant varieties.
Ring spot of cabbage (Mycosphaerella brassicicola)
Outer leaves are severely affected. Brown to tan spots of 1-2 cm dia. appear on leaves. Central portions are grey in colour with numerous fruiting bodies appear in concenric rings. The spots are surrounded by a green band which remains great even after the whole leaves turn yellow. Diseased leaves fall off. When the spot affects head, marked value is reduced.
Collection and destruction of plant debris Hot water treatment of seeds 45° C for 20 min
Stalk rot (Sclerotinia sclerotiarum)
In cabbage water soaked spots appear on stem and leaves near the ground level. The leaves wilt and plant collapses with in 10-15 days. When head is infected cottony white fungal growth is seen with numerous hard black sclerotia. In cauliflower yellowing starts from tip of the leaves to down wards till the entire leaves are covered. The leaves shed prematurely. The rotting progress to stem and the stem girdles and the stem rots upto the curd region. Curd are also affected. White fluffy mycelial growth with numerous sclerotia in the affected portions can be seen..
Crop rotation with paddy