Rice seedling-stage diseases require careful prevention and control, especially during periods of abnormal temperatures

Seedling-stage diseases are the general term for various physiological and infectious diseases during rice seedling raising, also called seedling rot. Physiological seedling rot refers to symptoms such as seed rot, bud rot, black root, greens wilt and yellow wilt death of seedlings caused by adverse environmental conditions; infectious seedling rot refers to seedling death symptoms caused by the harm of cottony leak and damping-off. During the seedling-raising period in northern rice regions, low temperatures and cold wave weather strikes are common, and the annual incidence rate of seedling rot disease is 10%~23%.

Symptom characteristics 1. Physiological seedling rot: 1. Seed rot, refers to seeds rotting before germinating; 2. Bud rot, refers to newly sprouted young buds with poor development curling into a fishhook shape, dying before rooting, or sinking into the mud and rotting to death, or forming floating seedlings; 3. Black root, refers to seedling roots turning black and rotting, with leaves gradually withering and dying; 4. Greens wilt, refers to seedlings suddenly withering and dying while still green without yellowing, with the base not rotting, and the seedlings not easy to pull up; 5. Yellow wilt, refers to seedlings slowly turning brown, withered and yellow before dying, with roots rotting, and diseased seedlings easy to pull up. 2. Infectious seedling rot: 1. Cottony leak, can occur 5~6 days after sowing in water-raised seedling fields, with severe occurrence when seedlings are about 3 cm tall, harming young roots and young buds. A small amount of milky white gelatinous substance appears at the cracked seed shell or bud base, gradually growing radial white flocculent mycelium, gradually becoming rusty, green-brown or dirty brown; seeds rot and cannot germinate, or the base of diseased seedlings rots and dies. Disease occurrence spreads from spots to patches, and in severe cases patches wither and die. 2. Damping-off, commonly seen in moist seedling raising, dry seedling raising and film seedling raising fields, with symptoms as follows: (1) Bud rot, young buds and young roots turn brown, twist and rot to death before emergence, with pinkish-white mold layer growing later; (2) Needle rot, from the needle stage to the 2-leaf stage the central leaf withers and yellows, the base turns brown, sometimes brown spots appear on the leaf sheath, mold layer grows at the stem base, and seedlings are easy to break when pulled; (3) Yellow wilt, occurs from the 1-leaf-1-heart stage to the 3~4-leaf stage, lower leaves begin to wither, yellow and wilt, root hairs are sparse, and later the base turns brown and soft-rots; (4) Greens wilt, mostly occurs around the 3-leaf stage, the central leaf or upper leaves of seedlings curl into a willow-leaf shape and appear grayish-green, root hairs are sparse, and later the base turns brown and soft-rots, prone to occurring in clusters.

Pathogens and occurrence patterns Physiological seedling rot is mostly caused by poor seed quality, uneven heating and cooling during sprouting, poor sowing quality, excessively deep water storage, oxygen deficiency and suffocation, and sudden clearing after cold during the seedling stage or too large temperature differences; black root is caused by excessive fertilizer, excessively deep water storage, and the production of large amounts of reducing substances such as hydrogen sulfide and iron sulfide that poison the seedlings. The cottony leak of infectious seedling rot is mainly caused by infection by fungi such as Saprolegnia and Pythium; damping-off is mainly caused by infection by imperfect fungi such as Fusarium and Rhizoctonia solani. Pathogens can live in the soil for a long time, overwintering as mycelium, oospores and sclerotia in the soil or in diseased residues, and spread by water flow and air flow; Rhizoctonia overwinters as mycelium and sclerotia in diseased residues and soil, spreads by flowing water, and mainly invades through wounds and young tender tissues.

Prevention and control measuresFor preventing and controlling rice seedling-stage diseases, comprehensive measures should be adopted: improving seedling-raising methods, improving seed quality and sprouting techniques, strengthening seedling field management, supplemented by chemical control. Adapt measures to local conditions by using plastic film seedling raising, soft tray seedling raising and dry seedling raising, and preferably not water seedling raising. Select seedling fields that are sheltered from wind and sunny, flat in terrain, medium in fertility, and convenient for drainage and irrigation. Control the stages of seed retention, seed storage, seed sunning and seed selection to improve the seed germination rate. Use warm water seed soaking (55°C for 10 minutes) or chemical seed soaking (50% carbendazim 800-fold solution for 3 days). During sprouting, keep the temperature stable above 10°C, prevent it from being too high or too low, ensure uniform, and strong sprouting, and enhance the cold resistance of seedlings. Strengthen seedling management, prepare the land finely, ensure a level bed surface, bed temperature and permeability; sow when the temperature is stable above 10°C; irrigate scientifically, using water to control temperature and protect seedlings; fertilize reasonably, pay attention to combining nitrogen, phosphorus and potassium, increase organic fertilizer, avoid excessive nitrogen, and enhance seedling disease resistance. For chemical control, apply pesticides promptly at the early stage of disease, sprinkling 75% dexon 1000-fold solution, or 50% amobam 800-fold solution, 5.5% seed-soaking agent 3000~5000-fold solution, 58% metalaxyl mancozeb 1500-fold solution, 2~3 kg per square meter.