What causes purple broccoli heads?

1.1 Downy mildew mostly occurs at the rosette stage. On the lower leaves, yellowish-brown angular or irregular spots with indistinct edges and bounded by the veins appear; when humid, white mold forms on the underside of the leaves. It generally becomes severe under high temperature and heavy rain, large day-night temperature differences, insufficient sunlight, waterlogged soil and high plant density.

1.2 Brown spot mainly damages leaves, heads and seed pods. The lower old leaves are affected first, with round brown spots initially appearing on the upper or lower surface of the leaves, slightly marked with concentric rings; when humidity is high, black mold grows on the affected parts. It generally becomes severe in the middle and late growth stages, during continuous rainy weather, or when fertility is insufficient.

1.3 Gray mold mainly damages the flower heads, and also affects stems and leaves. The affected tissue is light brown and water-soaked, then soft-rots, and is covered with gray mold. The pathogen spreads through airflow, rain and farming operations, and invades the plant through wounds or senescent parts. It occurs easily when seedlings grow poorly, density is high and relative air humidity is too high.

1.4 Sclerotinia rot mostly occurs at the heading stage and the flowering and pod-setting stage. It generally starts on the stems and leaf stalk bases near the ground; the lesions are initially brown and water-soaked, and after the leaf stalks are affected, the water supply is cut off, causing the leaves to wilt and droop. Under high humidity, the affected parts are covered with white cotton-like sclerotia. This disease is transmitted through the soil and occurs easily under warm, humid conditions.

1.5 Black rot damages leaves and stems. In mature plants, it generally starts from the lower leaves. The spots mostly form "V"-shaped dead areas extending inward from the leaf margins; the pathogen spreads downward along the veins, forming larger necrotic areas or irregular yellowish-brown large spots, with the leaf tissue at the spot edges pale yellow. After the pathogen enters the vascular bundles of the stem, it gradually spreads to the stem or the veins and leaf stalks, causing the vascular bundles to necrose and turn black, and finally the plant dies, but without odor; under dry conditions the stem develops a black core. In the field, the pathogen is mainly spread through diseased plants, fertilizer, wind and rain, or farm tools. The disease occurs easily when seeds carry the pathogen, under high temperature and heavy rain, with improper fertilizer and water management, continuous cropping with cruciferous crops, and severe pest outbreaks.

1.6 Soft rot mostly occurs at the heading stage and the pod-setting stage. The pathogen mostly invades through wounds, and water-soaked spots initially appear on the outer leaves or the base of the head; when the damage is severe, the whole plant rots, exuding slimy pus, and the affected parts give off a foul odor. The pathogen spreads through rain, irrigation water, contaminated fertilizer, insects and so on. The disease occurs easily when seeds carry the pathogen, under high temperature and high humidity, with excessive watering, many underground pests, and continuous cropping.

1.7 Viral disease can affect plants at both the seedling and mature stages. When seedlings are infected, chlorotic, nearly round spots 2-3 mm in diameter appear on the leaves, after which the whole leaf becomes paler or develops a mottling of dense and light patches. When mature plants are infected, in addition to uneven dense and light mottling on the young leaves, black necrotic spots also appear on the underside of the old leaves, and the diseased plants head late and loosely. In production, the disease becomes severe with early sowing, abundant virus sources or aphids, and in addition improper management, low-lying terrain, poor ventilation, or lack of water and fertilizer.

2 Integrated control

2.1 Reasonable site selection: choose slightly acidic soil that drains well and is loose and fertile.

2.2 Seedbed soil disinfection: the nutrient soil used for raising seedlings must be composted for a long time and sealed and disinfected with formalin.

2.3 Seed disinfection: soak the seeds in warm water at 50-55°C for 20 minutes, stirring constantly.

2.4 Practice crop rotation: rotating with non-cruciferous vegetables can effectively reduce the occurrence of diseases and pests.

2.5 Strengthen seedling management: thin and transplant seedlings in good time, ventilate promptly to reduce air humidity, and promptly remove diseased seedlings.

2.6 Strengthen water and fertilizer management: broccoli likes fertilizer and water, and timely topdressing and watering in stages is the key to a high yield. It is recommended to mainly apply fully decomposed organic fertilizer, and compound fertilizer can be used for topdressing. In the rainy season, open ditches in good time to drain water and prevent soil waterlogging, so as to avoid root rot.

2.7 Improve growing conditions: clean the garden in good time; promptly remove diseased and old leaves; after harvest, promptly clear the remaining plants and rotten leaves from the field; and disinfect disease pits to reduce sources of inoculum. Use high-ridge cultivation to effectively regulate soil temperature and moisture and improve light, ventilation and drainage conditions. When cultivating and weeding, avoid injuring leaves and breaking roots, and promptly destroy diseased plants found to reduce the routes of spread.

2.8 Chemical control: to control bacterial black rot and soft rot, agricultural streptomycin or chloramphenicol 5000-fold solution can be used; to control fungal diseases such as downy mildew and sclerotinia rot, at the early stage of the disease, 50% carbendazim 500-fold solution or 75% chlorothalonil 600-800-fold solution can be used, sprayed once every 7-10 days for 2-3 consecutive sprays, which can effectively control the occurrence of the diseases. Spray insecticides regularly to kill aphids and various other pests and cut off the routes of spread.