The 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.
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
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.
Comments on the Disease
Rice blast was first identified on California rice in 1996. The disease is favored by long periods of free moisture, high humidity, little or no wind at night, and night temperatures between 63 and 73°F. Leaf wetness from dew or other sources is required for infection. The optimum day timetemperature for spore germination, lesion formation, and sporulation is 77 to 82°F. Sporulation is also greatest when relative humidity is above 93%.
Shortly after the fungus infects and produces a lesion on rice, fungal strands called conidiophores grow out of the diseased rice tissue and produce spores called conidia. These conidia are dispersed in the air and under favorable conditions may cause new infections. When conditions are favorable, a single disease cycle can be completed in about a week. In addition, a single lesion can produce hundreds to thousands of spores in one night and may produce them for more than 20 days. Under favorable moisture and temperature conditions, the fungus can go through many disease cycles and produce a tremendous load of spores by the end of the season.
The blast fungus can overwinter from one season to the next on diseased crop residue and seed. Weeds have been shown to be alternate hosts for the disease in the greenhouse, but their role in nature is unclear. Of the overwintering sources, rice straw and stubble are probably the most important.
Rice blast management in California requires implementing a variety of cultural practices (destruction of infested residue, use of noninfestedseed, water seeding, continuous flooding, and avoiding excess nitrogen). Environmental conditions in California appear to be permissive for blast development but are usually not conducive. In other areas of the world where conditions are not conducive, the disease is most easily controlled by resistant cultivars, but resistant cultivars are not yet available in California. Fungicides are available, but are only needed when unusually wet conditions occur during the California growing season.
Blast is favored by excessive nitrogen fertilization, aerobic soils, and drought stress. High nitrogen rates and nitrate nitrogen increase rice susceptibility to the disease. Extended drain periods may also encourage the disease by aerating the soil, by converting ammonium to nitrate, and by causing drought stress to rice.
Use proper seed sampling and testing to identify and avoid the use of blast-infested seed in areas where blast is not a problem. This may help limit the introduction of the disease into noninfested areas.
Water seeding is recommended to reduce or eliminate disease transmission from seed to seedlings. Drill seeding is not recommended because it may allow seed transmission, nitrate formation, and result in drought stress.
Continuous flooding is recommended to limit blast development. Avoid field drainage, especially for extended periods because it allows the formation of nitrate and may cause drought stress. Some studies in other areas suggest that shallow water is more favorable to blast development than deeper water.
California varieties are susceptible to blast, and M-201 appears to be the most susceptible followed by M-104 and M-205. Of the widely grown varieties, M-202 appears to be more tolerant than others and exhibits some field resistance to leaf blast. Blast has been observed on M-201, M-202, M-204, M-205, M-103, M-104, S-102, L-204, Calmochi-101, and several proprietary varieties.
Monitoring and Treatment Decisions
Monitor to determine the need for treatments. Throughout the season, examine plants in several locations throughout the field for the presence of leaf lesions; intensify monitoring as plants approach the boot stage. If blast lesions are present and increasing just before the boot stage, a treatment may be justified. When making a treatment decision, consider disease progress, crop growth stage, environmental conditions, and rice variety. For example, there is a greater risk of neck and panicle blast infections occurring when growing one of the more susceptible cultivars and long periods of leaf wetness and warm night temperatures occur. Use a protectant fungicide so that the panicles is protected as it emerges from the boots. Because rice blast is a multiple cycle disease, fungicide applications to control leaf blast early in the season are generally ineffective in reducing the incidence of neck blast and yield losses.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Quadris)||12.3–15.4 fl oz||4||28|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)|
|COMMENTS: Follow label directions. Limited studies have shown a single application at the lowest labeled rate to be effective, but results may vary under different conditions. Can be applied as a preventive treatment for blast control and applied before favorable conditions for blast development. For panicle blast, application can be made at mid-boot to boot-split, but before full head emergence. Under heavy disease pressure and conditions favorable for disease development, use maximum label rates; a second application may be applied when panicles are about 60–90% emerged from the boot (7–14 days later). Water holding period is 14 days.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different Group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action Group number.|