All aboveground parts of the rice plant are attacked by the fungus. With leaf blast, the lesions are elliptical or spindle shaped with brown borders and gray centers. Under favorable conditions, lesions enlarge and coalesce; eventually killing the leaves. Leaf blast usually increases early in the season, then declines later in the season as leaves become less susceptible.
Collar blast occurs when the pathogen infects the collar that can ultimately kill the entire leaf blade. The pathogen also infects the node of the stem that turns blackish and breaks easily. This condition is referred to as node blast. In some upland varieties, internodal infection also occurs at the base of the plant, which causes white panicles similar to that induced by yellow stem borer or water deficit. Nodal infection is more common than internodal infection.
Neck blast occurs when the pathogen infects the neck of the panicle. The infected neck is girdled by a grayish brown lesion and the panicle falls over if the infection is severe. If neck blast occurs before the milk stage, the entire panicle may die prematurely, leaving it white and completely unfilled. Later infections may cause incomplete grain filling and poor milling quality.
The pathogen also causes brown lesions on the branches of the panicles and on the spikelet pedicels, resulting in panicle blast. Infection of the neck, panicle branches, and spikelet pedicels may occur together or may occur separately.
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