Comprehensive control methods and technical key points for broccoli black spot disease

The morphological characteristics and growth habits of broccoli are basically similar to ordinary white cauliflower (cabbage). It belongs to the cabbage species of the Brassica genus in the Brassicaceae family, and is a 1-2 year herbaceous plant. It grows vigorously and has strong heat tolerance and cold resistance. The plant is tall; depending on the variety, after about 20 leaves have grown, it produces a flower stalk, with clusters of flower buds at the top. These tightly cluster into a curd-like head, hemispherical in shape, with green flower buds, hence it is called green cauliflower.

Broccoli growth habits

Light requirements: Broccoli is not very strict about light, but during growth it prefers sufficient light. When light is adequate, the plant grows robustly and can form a strong vegetative body, which is beneficial for photosynthesis and nutrient accumulation, and makes the curd compact and dense, with a bright green color and good quality. Excessive sunlight in midsummer is also unfavorable for the growth and development of broccoli.

Temperature requirements: Within the range of 5—20℃, the higher the temperature, the faster broccoli grows and develops. The optimum germination temperature is 20—25℃, and the optimum growth temperature during the seedling stage is 15—20℃. Broccoli has strong cold resistance and heat tolerance. The optimum growth temperature during the rosette stage is 20—22℃, and the optimum temperature for curd development is 15—18℃. When the temperature is higher than 25℃, curd quality tends to deteriorate, but as long as it is not damaged by freezing, the curd can still grow slowly at low temperatures of 5℃ or even below. Different varieties and different seedling ages have relatively large differences in the temperature requirements needed to complete vernalization, so the selection of varieties is very important for year-round cultivation.

Water requirements: Broccoli requires a relatively large amount of water throughout its growth process, especially during vigorous leaf growth and curd formation, when water shortage must not occur. Even short-term drought will reduce yield. Excessive rain during the seedling stage or overly high soil moisture can easily cause diseases such as black rot and black spot. During curd formation, soil moisture at about 70—80% of field capacity is needed to meet growth requirements.

Soil requirements: Broccoli is not strict about soil conditions, but overly poor soil leads to poor plant development and low yield and quality, while overly fertile soil can cause loose curds and hollow flower stalks. Broccoli is suitable for planting in loam and sandy loam with good drainage and irrigation, deep plow layer, loose and fertile soil, and strong water and fertilizer retention. It adapts to a soil pH range of 5.5—8, but 6 is best. Broccoli requires sufficient fertilizer throughout its growth process, with an N, P, K ratio of 14:5:8. During the seedling stage, the demand for nitrogen is relatively greater; after flower bud differentiation begins, the demand for P and K relatively increases.

Note that during curd development, broccoli also requires relatively large amounts of trace elements such as boron, molybdenum, and magnesium. Therefore, applying appropriate foliar fertilizer with trace elements before budding will significantly increase yield and quality.

Comprehensive control of broccoli black spot disease

Symptoms of broccoli black spot disease:

Broccoli black spot disease, also called alternaria leaf spot, mainly damages leaves, curds, and seed pods. Infected leaves form nearly black circular spots with concentric rings, 1-10 mm in diameter, with inconspicuous rings, but the black mold produced on the lesions is often more abundant and obvious than on Chinese cabbage. When there are many lesions on the leaves, the lesions merge into large spots, and the leaves turn yellow and dry early; when stems and petioles are infected, the lesions are longitudinal and striped, with black mold; when flower stalks and seed pods are infected, dark brown long spindle-shaped striped spots appear.

Pathogen of broccoli black spot disease:

Alternariabrassicicola (Schw.) Wiltshire, called Alternaria brassicicola, belongs to the deuteromycete fungi. The fungal colony grows on both sides of the leaf, dark olive to dark light brown, lobed; conidiophores are olive brown, size 70×5-8 micrometers; conidia are mostly borne in chains of more than 20, sometimes branched, light to dark olive brown, with 1-11 transverse septa, a few longitudinal septa, size 18-130×8-20 micrometers. In addition, Alternaria oleracea M. Ibrath is also a pathogen of this disease.

Transmission routes and disease conditions of broccoli black spot disease:

The pathogen overwinters on diseased residues, on seed-producing plants, and in the soil. When conditions are suitable, it infects broccoli. After disease onset, the conidia produced on diseased parts spread in the field by wind and rain for multiple secondary infections. Generally, after rain, with temperatures of 25-31℃, the incubation period is short and disease is likely; insufficient fertilizer, weak growth, and poor management lead to severe disease.

Comprehensive control methods for broccoli black spot disease:

1) Rotate with non-cruciferous vegetables, use ridge cultivation or high-bed cultivation, drain promptly after rain, and strictly prevent moisture retention.

2) After harvest, promptly remove diseased residues, promptly deep plow, use formula fertilization technology, and when the curd grows to the size of a fist, appropriately control watering and increase phosphorus and potassium fertilizer. For example, topdressing with superphosphate, plant ash, bone meal, etc. can enhance disease resistance.

3) When the curd of seed-producing plants grows to 2/3 size, cut off the middle part that is not easy to bolt, leaving only 3 side flower branches, and spray the cut with 65% zineb wettable powder 500 times dilution for antisepsis.

4) When planting broccoli in protected areas and plastic greenhouses, focus on ecological control. In early spring at planting time, the temperature difference between day and night is large, 20-25℃ during the day and 12-15℃ at night, and relative humidity is as high as above 80%, making condensation easy and favoring the occurrence and spread of this disease. Focus on adjusting the temperature and humidity in the greenhouse, especially in the early planting period. The time of closing the greenhouse should not be too long, to prevent excessive humidity and excessively high temperature in the greenhouse, and achieve an organic coordination of water, heat, and ventilation to slow the occurrence and spread of the disease. For example, use the dust method to sprinkle 5% chlorothalonil dust at the early stage of disease, 1 kg per 667 sq m, once every 9 days, for 3-4 consecutive treatments. You can also use 45% chlorothalonil smoke agent or 15% procymidone (Sumilex) smoke agent, 200-250 grams per 667 sq m. Fumigate overnight.

5) In open fields or greenhouses, start spraying 50% iprodione wettable powder 1000-1500 times dilution, or 75% chlorothalonil wettable powder 600 times dilution, or 80% mancozeb wettable powder 600 times dilution before disease onset. The key to the effectiveness of the above protective agents lies in how early they are used. Where spraying is started before disease onset when no lesions are visible, the control effect is above 70%; although medication after disease onset has a certain inhibitory effect, it is not ideal. Therefore, it is emphasized to start control before disease onset, reduce early inoculum sources, and keep the disease under the economic damage threshold. Stop using the drug 7 days before harvest.