Latest Symptoms and Control Methods of Lotus Root Brown Blight
Lotus root brown blight, named because after infection the lotus leaves show round to irregular chlorotic large yellow spots or brown necrotic spots, is also known as black spot disease or lotus leaf spot disease. Although it only damages the leaves, in severe cases it causes the entire leaf surface of the whole lotus field, except for the leaf veins, to be covered with lesions, with half or whole leaves drying up as if scorched by fire. This disease is currently one of the main diseases in lotus root production. When it occurs severely, it reduces the commercial value of the entire lotus field by more than 25%, greatly affecting the interests of lotus root growers. Let us learn about it together below: the symptoms of lotus root brown blight and how to control it.
I. Symptoms
This disease mainly harms lotus root leaves, with fewer symptoms showing on the petioles. On the upper surface of lotus root leaves, pinhead-sized round yellowish-brown small spots initially appear, and they are especially obvious on the underside of the leaves. They later gradually expand into round or irregular brown large spots or dark brown necrotic lesions 0.5-2.0 cm in diameter. The lesions on the underside of the leaves are slightly lighter in color than those on the upper surface, appearing as chlorotic large yellow spots. The lesions are slightly sunken, with or without obvious brown concentric rings, with a clear boundary between diseased and healthy tissue, surrounded by a narrow chlorotic yellow halo. Lesions on the upper surface of floating leaves are mostly dark brown, and in the middle and late stages the diseased parts easily rot; when touched by hand, the upper epidermis easily falls off, but without perforation; on detached leaves, although the upper epidermis of the lesions has already necrosed, it does not easily fall off. In the middle and late stages, several lesions often expand and merge into large irregular scorched lesions. In severe cases, except for the leaf veins, the entire leaf surface is covered with lesions, causing half to whole leaves to dry up and die. When the disease becomes epidemic in contiguous fields, the affected area of the lotus field looks scorched from a distance.
II. Transmission Routes and Conditions for Onset
The pathogen survives and overwinters as mycelium and conidial clusters on diseased plant residues or on seed lotus plants, produces conidia the following year, and spreads through wind and rain for primary infection. After a 2-3 day incubation period, disease develops, and the diseased parts produce conidia again for secondary infection. In Hubei, this disease begins in mid-May and becomes prevalent from July to September during the hot and rainy season. It is especially likely to infect after storms or when lotus root growth is weak. When the water temperature in the lotus field is higher than 35°C, or when nitrogen fertilizer is applied excessively, or when aphids are rampant, the disease is severe.
III. Control Methods
1. Crop rotation and changing crops, avoiding continuous cropping
Implementing rotation between paddy and dryland crops is the best method to control this disease. Affected fields must undergo crop rotation for more than 3 years. Especially for lotus fields with severe disease in the previous year, rotating with gramineous crops such as rice or with cash crops such as garlic and celery can effectively reduce the accumulation of the pathogen in the field and lessen infection damage.
2. Select varieties with stronger disease resistance and tolerance according to local conditions
E Lian No. 4 and E Lian No. 5 have relatively strong disease resistance, good quality and high yield. Diseased-free seed lotus must be selected, or seed lotus retained from disease-free fields. For seed lotus, choose whole lotus roots with 2-3 plump front sections, because they are rich in nutrients and the lotus leaves can extend above the water surface earlier, which helps enhance disease resistance and reduce opportunities for primary infection by the pathogen.
3. Clean the lotus field early
During the middle and late stages of lotus root growth, pick diseased leaves at any time and take them out of the lotus field for centralized deep burial or burning. However, be careful not to break the lotus leaf petioles, so that rainwater or pond water does not enter the petiole air channels and cause rot of the underground stems. Remove diseased leaves before harvesting lotus root. After harvesting lotus root, remove as much lotus root residue from the field as possible and immediately take it outside the lotus field for centralized deep burial or burning. In addition, after Start of Winter (November 7 to 8), disinfect the lotus field soil. For each mu, use 2 kg of 50% carbendazim wettable powder mixed evenly with 100 kg of fine dry soil and broadcast it, then plow deeply and leave the field to freeze during the winter fallow period.
4. Sow at the proper time and plant at a reasonable density
Do not increase the planting density per unit area solely to increase yield, otherwise it easily causes field shading and triggers disease. Only reasonable dense planting can provide good ventilation and light transmission in the field and ensure robust growth of lotus plants. After sowing, shallow water should be irrigated to help raise the temperature and allow seed lotus to sprout earlier.
5. Strengthen fertilizer and water management
Choose fields with good soil quality and convenient irrigation for planting lotus root. Apply sufficient compost fermented with enzyme bacteria or fully decomposed organic fertilizer as base fertilizer. During the lotus root growing period, topdressing should be timely and appropriate, combining organic fertilizer with chemical fertilizer, appropriately increasing phosphorus and potassium fertilizer, and avoiding excessive nitrogen fertilizer.
Manage the water layer according to the growth and development stages of lotus root. In the early growth stage, when temperatures are relatively low and there are few standing leaves, the water layer should be shallow; in the middle stage during the hot season, the water layer should be deep; in the later stage, the water layer should be appropriately shallower again to facilitate root growth and oxygen exchange. Note that the water temperature should be kept below 35°C. Before typhoons and storms arrive, irrigate the lotus field with sufficient and deep water in time, and drain promptly after the wind and rain.
6. Disinfect seed lotus
Before planting, disinfect the seed lotus by soaking it in a 700-1000 times dilution of 50% carbendazim wettable powder for 20-30 minutes. Alternatively, spray the seed lotus with a 1000 times dilution of 50% carbendazim wettable powder + 75% chlorothalonil wettable powder, then cover and seal with plastic film for 24 hours, and plant after drying.
7. Prevent disease in seed lotus fields
For lotus fields that developed disease the previous year, before planting, use 0.5 kg of 50% carbendazim wettable powder per mu, mix with 10-15 kg of dry fine soil, and broadcast it to disinfect the soil; or use 100-150 kg of quicklime per mu for disinfection.
8. Chemical control
Floating leaves are generally damaged first, so spray on the upper surface of the leaves; in the middle and late stages, detached leaves are mainly damaged, so spray on both the upper and lower surfaces of the leaves. In the early stage of disease, promptly choose 70% thiophanate-methyl wettable powder + 75% chlorothalonil wettable powder mixed evenly at a 1:1 ratio and diluted 800 times with water for spraying; or spray Bordeaux mixture at a 0.5:0.5:100 ratio; or use 77% copper hydroxide (Kocide) wettable micropowder at 700 times dilution, 50% prochloraz wettable powder at 1500 times dilution, 70% fosetyl-Al · mancozeb wettable powder at 500 times dilution, 20% difenoconazole microemulsion at 2000 times dilution, 25% azoxystrobin suspension concentrate at 2000 times dilution, 40% flusilazole emulsifiable concentrate at 5000 times dilution, 58% metalaxyl · mancozeb (Ridomil · mancozeb) wettable powder at 600 times dilution, or 64% oxadixyl · mancozeb (Sandofan) wettable powder at 600 times dilution for spray control. When spraying, spray the plants in the middle of the field first, then the plants around the edges. Because lotus root leaves have a lot of wax, it is best to add a small amount of laundry detergent when spraying to enhance the adhesion of the spray liquid. It is best to choose a windless sunny day for spraying. Spray once every 7-10 days, for 2-3 consecutive applications. If rain occurs after application, respray promptly.
The above are the causes, symptoms and related control measures of lotus root brown blight. From the control methods, it can be seen that this type of disease is still mainly prevented. Before planting lotus root, seed selection, lotus field cleaning, sowing, and fertilizer and water management are all introduced, and there are also treatment measures in the early stage of disease onset. We hope this can provide some reference and help to everyone.