Symptoms and Control Methods of Lotus Root Brown Blotch Disease
Lotus root brown blotch disease, because after infection the lotus leaves show circular 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 the entire leaf of the whole lotus field, except for the veins, becomes covered with lesions, and half or the whole leaf dries 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 blotch disease and how to control it.
I. Symptoms
This disease mainly harms lotus root leaves, and symptoms are less commonly seen on the leaf stalks. On the upper surface of lotus root leaves, pinhead-sized circular yellowish-brown small spots first appear, and they are especially obvious on the underside of the leaves. Later they gradually expand into circular or irregular brown large spots or dark brown necrotic lesions 0.5-2.0 cm in diameter. The lesions on the underside of the leaf 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. The lesions on the upper surface of floating leaves are mostly dark brown, and in the middle to late stages the diseased parts readily rot; when touched by hand, the upper epidermis easily falls off, but the leaf does not become perforated. For lesions on emerged leaves, although the upper epidermis has already necrotized, it does not easily fall off. In the middle to late stages, several lesions often expand and merge into large irregular scorched lesions. In severe cases, except for the veins, the entire leaf surface is covered with lesions, causing half to the whole leaf to dry up and die. When the disease becomes epidemic in contiguous fields, the diseased area of the lotus field looks scorched from a distance.
II. Transmission Routes and Conditions for Disease Onset
The pathogen survives and overwinters as mycelium and conidial clusters on diseased residues or on seed lotus root plants, and produces conidia the following year, which spread by wind and rain for primary infection. After a latent period of 2-3 days, disease develops, and the diseased parts again produce conidia for secondary infection. In Hubei, this disease begins in mid-May and occurs heavily in the high-temperature and rainy season from July to September. It is especially likely to infect after storms or when lotus root growth is weak. When the water temperature of the lotus field is higher than 35°C, or when nitrogen fertilizer is applied excessively, or when aphid damage is rampant, the disease is severe.
III. Control Methods
1. Rotation and changing crops, avoiding continuous cropping
Implementing paddy-upland rotation is the best method to control this disease. Fields with disease must be rotated for more than 3 years. Especially for lotus fields with severe disease in the previous year, changing to rice and other grass-family crops or to cash crops such as garlic and parsley can effectively reduce the accumulation of the pathogen in the field and lessen infection damage.
2. Select varieties with relatively strong 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 seed lotus roots must not be used; seed lotus roots should be selected and retained from disease-free fields. For seed lotus roots, whole lotus roots with 2-3 plump, large front sections should be chosen, because they are rich in nutrients and their lotus leaves can extend out of the water earlier, which helps enhance disease resistance and reduces 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, but be careful not to break the lotus leaf stalks, so that rainwater or pond water does not pour into the stalk air pores and cause rot of the underground stems. Remove diseased leaves before harvesting lotus root, and after harvesting lotus root try to remove the lotus root residues in the field and immediately gather them outside the lotus field for deep burial or burning. In addition, after Lidong (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 soil to freeze in clods during the winter fallow.
4. Sow at the proper time and plant at a reasonable density
Do not increase the planting density per unit area solely to raise yield, otherwise it easily causes field shading and triggers disease. Only reasonable dense planting can give the field good ventilation and light transmission and ensure vigorous growth of lotus plants. After sowing, it is advisable to irrigate with shallow water to help the temperature rise and make the seed lotus roots 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 growth 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, the temperature is relatively low and there are few standing leaves, so the water layer should be shallow; in the middle stage during the high-temperature season, the water layer should be deep; in the late stage, the water layer should be appropriately shallower again, which helps lotus root formation and oxygen exchange. Note that the water temperature should be controlled below 35°C. Before typhoons and storms arrive, irrigate the lotus field fully and deeply in time, and drain promptly after the wind and rain.
6. Disinfect seed lotus roots
Before planting, disinfect the seed lotus roots by soaking them in a 700-1000 times dilution of 50% carbendazim wettable powder for 20-30 minutes. You can also spray the seed lotus roots with a 1000 times dilution of 50% carbendazim wettable powder + 75% chlorothalonil wettable powder, then cover and seal them with plastic film for 24 hours, and plant after drying.
7. Prevent disease in seed lotus root fields
For lotus fields that had disease the previous year, before planting, use 0.5 kg of 50% carbendazim wettable powder per mu, add 10-15 kg of dry fine soil, mix evenly and broadcast 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 the upper surface of the leaves; in the middle and late stages, emerged leaves are mainly damaged, so spray both the upper and lower surfaces of the leaves. At 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 with water to 800 times for spraying; or spray 0.5:0.5:100 Bordeaux mixture; or spray 77% copper hydroxide (Kocide) wettable micro-particle powder at 700 times, 50% prochloraz wettable powder at 1500 times, 70% fosetyl-Al · mancozeb wettable powder at 500 times, 20% difenoconazole microemulsion at 2000 times, 25% azoxystrobin suspension concentrate at 2000 times, 40% flusilazole emulsifiable concentrate at 5000 times, 58% metalaxyl · mancozeb (Ridomil · mancozeb) wettable powder at 600 times, or 64% oxadixyl · mancozeb (Sandofan) wettable powder at 600 times for 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 it rains after application, respray in time.
The above are the causes, symptoms and related control measures of lotus root brown blotch disease. From the control methods, it can be seen that this type of disease still focuses mainly on prevention. Before planting lotus root, seed selection, lotus field cleaning, sowing, and fertilizer and water management are all introduced, and there are also treatment measures at the early stage of disease onset. We hope this can provide some reference and help to everyone.