What are the symptoms of black rot in greenhouse broccoli?

At present, broccoli is mostly grown in open fields, and not much is cultivated in greenhouses. This is mainly because during greenhouse broccoli cultivation, the high-temperature and high-humidity environment favors the occurrence of diseases, especially black rot. Once the disease occurs, it may cause reduced yield in mild cases and possibly total crop failure in severe cases. In response to this situation, we conducted research on integrated control techniques for black rot in greenhouse broccoli, and the integrated control techniques are summarized as follows.

I. Identification of black rot symptoms

The disease can occur from the seedling stage to the adult plant stage. When seedlings are affected, the cotyledons become water-soaked, then quickly die or the disease spreads to the true leaves. When adult plants are affected, the main damage is to the leaves, first forming at the leaf margins, then extending inward to form "V"-shaped dead spots. The lesions are yellowish-brown with pale yellow edges. When the roots are affected, the vascular bundles turn black, the interior becomes dry rot, and eventually the whole plant wilts and dies. When the curd is affected, a lesion spreads gradually from one point on the surface of the head, and in severe cases the entire curd turns black and rots.

The pathogen is mainly spread through diseased plants, fertilizer, wind and rain, or farm tools. This disease is generally likely to occur under conditions such as seed carrying the pathogen, high temperature and frequent rain, improper fertilizer and water management, continuous cropping with cruciferous crops, and severe pest infestation.

II. Disease characteristics

Black rot is a bacterial disease, and high temperature and high humidity favor its occurrence. The growth temperature of the pathogen is 5-39°C, with the optimum temperature being 25-30°C. A relative humidity above 95%, especially dew on the leaves and guttation at the leaf margins, favors pathogen invasion. In greenhouse broccoli cultivation, the high-temperature and high-humidity environment inside the greenhouse is more favorable for the occurrence of black rot. During the heading stage, once water droplets on the greenhouse film drip onto the flower buds, black rot is very likely to occur.

III. Control measures for black rot

① Select disease-resistant varieties.

② Rotate with non-cruciferous vegetables for 2-3 years.

③ Strengthen cultivation management: deep plowing and sun-drying the soil, and growing on raised beds. By sun-drying the soil, field sources of the disease are reduced. Growing on raised beds can improve water drainage in the field; practice rational close planting, manage fertilizer and water well, and keep plants vigorous. When transplanting, use wide and narrow row planting; adjust the temperature and humidity inside the greenhouse properly. First, do a good job with seedbed management. For early spring cultivation, the temperature is relatively low, so mainly pay attention to the frequency and amount of watering in the seedbed to prevent the occurrence of black rot. For other cultivation methods, the temperature during the seedling period is relatively high, especially for winter broccoli cultivation, where the seedling-raising time is mid-to-late August, so the main focus is on lowering the temperature by covering the seedbed with a shade net.

Next, do a good job with management after transplanting. After transplanting, water is generally given once every 7-10 days. After budding, heavy fertilizer and heavy watering are needed, and at this time the frequency and amount of watering should be increased. After each watering, ventilate heavily to reduce humidity inside the greenhouse. Ventilate promptly when the temperature inside the greenhouse exceeds 25°C. Generally, after 8:00 on sunny days, open the greenhouse door for ventilation, and after 9:()0 open the side skirts of the greenhouse on both sides for ventilation to reduce humidity inside the greenhouse, then cover everything again around evening. During the bud stage, the upper leaves of the broccoli can be used before dusk on the first day to cover the flower buds, preventing water droplets on the greenhouse film in the morning from dripping

onto the flower buds and causing black rot. When there are no water droplets on the greenhouse film in the morning, move the covering leaves away.

At the same time, avoid excessive drought and waterlogging, promptly remove early diseased leaves, and thoroughly clear away diseased plant residues at harvest to reduce the initial inoculum sources in the field.

④ For chemical control, generally starting after emergence, every 10 days use 72% agricultural streptomycin soluble powder 15g + 2 tubes of benzyl iodine (15mU per tube) mixed with 13kg of water for spray control. A total of about 10 treatments are needed throughout the entire growth period.