What are the control methods for rice brown spot?

Symptoms Also known as rice sesame leaf blight. It occurs in all rice-growing regions nationwide. Disease can occur from the seedling stage to the harvest stage, and all aboveground parts of the rice plant can be affected, with leaves being the most common. When seeds are affected at the germination stage, the coleoptile turns brown, the sprout fails to emerge, and the cotyledon dies. Leaf and leaf sheath disease at the seedling stage mostly appears as elliptical lesions, the size of sesame seeds, dark brown; sometimes the lesions enlarge and merge into strips, and when there are many lesions the seedlings die. On leaves of adult plants, infection begins as small brown spots, gradually enlarging into elliptical spots the size of sesame seeds, with the center of the lesion brown to grayish white, the margin brown, and around it yellow halos of varying shades; in severe cases they merge into irregular large spots. Diseased leaves dry up inward from the leaf tip, damp brown, and black mold-like growth (the pathogen's conidiophores and conidia) appears on dead seedlings. Lesions on leaf sheaths are initially elliptical, dark brown, with light brown, water-soaked margins, later becoming irregular large spots with a grayish brown center. When the neck and branch nodes of the panicle are affected, the damaged parts are dark brown, causing panicle blight. When grains are infected early, the affected grains are grayish black and the discoloration spreads to the whole grain, resulting in blighted grain. When infected later, the lesions are small and the margins indistinct. Heavily diseased grains are brittle and easily broken. In humid weather, black velvety mold layers grow on the above diseased parts, namely the pathogen's conidiophores and conidia.

Pathogen Bipolaris oryzae (Breda de Haan) Shoem., called rice bipolaris leaf spot fungus, belongs to the Deuteromycotina fungi. Synonym Helminthosporium oryzae Breda de Haan. Conidiophores emerge from stomata in bundles of 2-5, unbranched, slightly curved, with septa. Conidia are terminal, obclavate or long cylindrical, slightly curved, brown, with 3-11 septa, measuring 24-122 x 11-23 (μm). The sexual state Cochliobolus miyabeanus (Ito et Kuribayashi) Drechsler ex Dastur, called Miyabe cochliobolus, belongs to the Ascomycotina fungi, and has only been found on culture media, not produced under natural conditions.

Transmission routes and disease conditions The pathogen overwinters as mycelium in diseased residues or attached to seeds, becoming the primary infection source the following year. Conidia on lesions can survive 2-3 years under dry conditions, and latent mycelium can survive 3-4 years; mycelium turned into the soil loses viability after one winter. After diseased seeds are sown, latent mycelium can directly infect seedlings, and conidia can be blown by wind to seedling fields or main fields, where germinating hyphae directly penetrate or enter through stomata; when conditions are suitable, symptoms appear quickly and conidia form, spreading by wind and rain for secondary infection. Disease is severe under high temperature and high humidity and when fog or dew is present. It occurs in acidic soil, sandy soil, and when phosphorus and potassium are deficient. Disease is severe in dry seedling fields. The temperature range for mycelial growth is 5-35°C, with 24-30°C optimal; the temperature range for conidial formation is 8-33°C, with 30°C optimal. The temperature range for germination is 2-40°C, with 24-30°C optimal. Spore germination requires the presence of water droplets, with relative humidity greater than 925. Under saturated humidity at 25-28°C, infection of the host can occur in 4 hours.

Control methods (1) Deep plowing to destroy stubble and reduce the inoculum source. Diseased rice straw should be disposed of and destroyed promptly. (2) Select seed from disease-free fields for seed retention, or disinfect seeds. (3) Apply more decomposed compost as base fertilizer, topdress in a timely manner, and increase the application of phosphorus and potassium fertilizers; especially the application of potassium fertilizer can improve plant disease resistance. In acidic soil, pay attention to drainage and apply lime appropriately. Irrigate shallowly and frequently, avoiding long-term flooding that causes poor aeration. (4) For chemical control, see rice blast.