1, the cause of rice damping-off
Rhizoctonia solani usually overwinters in soil or plants, and infects rice grains and weak seedling bases when conditions are suitable. Alkaline soil is easy to get sick, and the lowest temperature of seedbed is below 10℃, the high temperature is above 35℃, or the temperature fluctuates.
The pathogen of Rhizoctonia solani can be divided into two kinds, one is fungal Rhizoctonia solani, and the other is physiological Rhizoctonia solani, also known as bacterial wilt: both of them often occur in seedbeds at the same time.
(1) Rhizoctonia solani is an infectious disease caused by fungal harm. Due to incomplete disinfection of seeds or bed soil, coupled with poor seedling growth environment, such as low temperature, low temperature, improper management, unhealthy seedlings, weakened disease resistance, virus invasion, leading to disease.
(2) Ralstonia solanacearum, also known as Ralstonia solanacearum, is a physiological disease caused by low temperature and rainy duration and improper management measures at seedling stage, which makes seedlings grow thin, stems and leaves grow excessively, and roots develop poorly. When the weather suddenly clears up, the roots can't absorb water to meet the requirements of leaf transpiration, resulting in serious water loss of leaves. When the weather is clear, the seedlings wither and die quickly.
2. Symptoms of rice damping-off disease
(1) Rhizoctonia solani: It can be divided into bud rot, basal rot and yellow rot.
Bud rot: the disease occurred before and after the seedling was unearthed, the bud roots turned brown, and the sheath leaves were brown spots or twisted and rotted. The seeds or roots have pink molds, which are distributed in patches on the seedbed.
Basal rot: mostly occurs during the period from vertical needle to two leaves. The leaves of diseased seedlings wither, sometimes there are brown spots on the leaf sheaths, the roots are yellow or brown, and the stem base gradually turns gray and decays. When using portable seedlings, the stems and roots are easily broken and irregularly clustered on the seedbed.
Yellow: diseased seedlings mostly occur before the third leaf, and the leaves are light yellow with irregular brown spots. Sick seedlings are shorter than healthy seedlings, with curly heart leaves, no water drops at the tip in the morning, and dry up and die in the later stage, which can occur in pieces on the seedbed.
(2) Physiological cataplexy: It mostly occurs after the third leaf, the leaves turn blue at the early stage of the disease, there is no water drop at the tip in the morning at the middle stage of the disease, the leaves roll at noon, the heart leaves return to normal in the morning and evening, and the seedlings wilt and die at the late stage of the disease. When using portable seedlings, it can be uprooted, and it will become a piece or a bed on the seedbed, which is very harmful.
3, comprehensive prevention and control technology
1. Agricultural control methods
(1) is to choose a place with flat terrain, sunny ventilation and smooth irrigation and drainage.
(2) The choice of bed soil. Garden soil or dryland soil without atrazine used in dryland last year and high-quality decomposed agricultural fertilizer should be selected. Soil and agricultural fertilizer should be evenly mixed according to the ratio of 3: 1, and then sieved. We should apply sufficient fertilizer, reduce the amount of nitrogen fertilizer, increase the amount of phosphorus and potassium fertilizer, and improve the disease resistance of seedlings.
(3) Cultivating strong seedlings and improving the disease resistance of seedlings are the key to control rice damping-off. It is advocated to cultivate strong seedlings by sparse sowing, that is, the sowing amount per square meter of seedbed is 250-300 grams in terms of dry seeds; Adopt the open-close double-film plastic film floppy disk seedling raising technology to achieve the goal of strong seedlings, and uncover the film in time when more than 80% seedlings emerge; Early ventilation, early hardening of seedlings, early hardening of seedlings, control of temperature and humidity, prevention of excessive growth of seedlings, promotion of root system development and robust growth of seedlings. Keep the temperature of one leaf to two leaves at 25-30℃, do not water the bed surface, and water as little as possible to promote the development of rice root system. Generally, it is advisable to water at 8-9 am. It is not advisable to irrigate the seedlings directly with well water, and it is better to use a watering can instead. The temperature is maintained at 22-25℃ from the second leaf to the third leaf, and at about 20℃ after the third leaf, so that the seedlings can gradually adapt to the external climate and enhance the stress resistance of the seedlings. When the daily average outside temperature is stable above 12℃, the plastic film can be removed.
2. Chemical control methods
(1) Strictly disinfect seeds and bed soil. Seed disinfection can be done by soaking seeds with drugs such as seed soaking spirit and strong chlorine essence. Sterilization of seedbed soil can be achieved by uniformly mixing multifunctional rice (which integrates the functions of increasing fertilizer, adjusting acid, sterilizing and chemical control) with nutrient soil, and using the dosage according to the instructions of seedling strengthening agent, or pouring 30 kilograms of water and 50 grams of 25% dixone into a 20-square-meter seedbed to control the pH value of seedbed soil between 4.5 and 5.5.
(2) Pesticide control at seedling stage. At seedling stage, 2.5 kg water was poured on the seedbed with 35% metalaxyl 1 g/m2 and 15% hymexazol 10 ml/m2 to prevent the occurrence of damping-off disease.
(3) In case of continuous low temperature and rainy weather, ordinary farmers dare not ventilate seedlings on cloudy days, which is easy to induce rice bacterial wilt. Therefore, in order to properly ventilate the seedbed, measures can be taken to blow a small wind and reduce the ventilation time, so as to uncover the cover late and keep the temperature in the bed. Once the disease is found, we can spray 40 grams of water per bag with 35% damping-off agent at the early stage of the disease to prevent the occurrence and development of bacterial wilt.