Rice blast disease
he rice blast fungus may infect and produce lesions on most of the shoot but usually not the leaf sheath. From the seedling stage through plant maturity, rice blast progresses through several phases starting with leaf blast, followed by collar, panicle and node blast. Leaf Symptoms. Lesions that occur on the leaf are usually diamond-shaped with a gray or white center and brown or reddish brown border and are 0.39 to 0.58 inch (1.0-1.5 cm) long and 0.12 - 0.2 inch (0.3-0.5 cm) wide. Newly formed lesions may have a white or grey-green center and a darker green border. Their shape, color, and size can vary depending on varietal resistance, age of the plant, and lesion age. Leaf blast may sometimes cause the complete death of young plants up to the tillering stage. Leaf blast usually increases early in the season then declines late in the season as leaves become less susceptible. Leaf Collar Symptoms. Infection at the junction of the leaf blade and sheath results in the typical brown "collar rot" symptom. A severe collar infection may cause the leaf to die completely. Node Symptoms. Stem nodes may be attacked as the plant approaches maturity, causing the complete death of the stem above the infection. Diseased nodes are brown or black. Panicle and Grain Symptoms. Infections just below the panicle, usually at the neck node, cause a "neck rot" or "rotten neck blast" symptom that can be very injurious to the crop. If neck rot occurs early, the entire panicle may die prematurely, leaving it white and completely blank. Later infections may cause incomplete grain filling and poor milling quality. Other parts of the panicle including panicle branches and glumes may also be infected. Panicle lesions are usually brown, but may also be black.