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Agronomy Fact Sheet Series 1997-2
Department of Agronomy and Range Science
University of California, Davis

Rice Blast: A New Disease in California

California Situation
Blast, a fungal disease of rice, was identified on California rice for the first time in September 1996. Currently, the disease appears to be confined to Glenn and Colusa Counties. A UC survey of 508 fields in five Sacramento Valley counties identified 33 blast infected fields, with 27 in Glenn County and 6 in Colusa County. The disease was not found in any other counties. Blast was observed on M-201, M-202, M-204, M-103, M-401, S-102, L-204, Calmochi-101 and several proprietary varieties. Local observations indicate that California varieties are susceptible to the disease.

Importance of the Disease
Many rice scientists consider blast to be the most important disease of rice worldwide. This is because the disease is widely distributed (85 countries) and can be very destructive when environmental conditions are favorable. Disease occurrence and severity vary by year, location and even within a field depending on environmental conditions and crop management practices. Yield loss estimates from other areas of the world have ranged from 1-50%.

Symptoms
The fungus can infect and produce lesions on most of the shoot including the leaves, leaf collar, stem, nodes, panicle and grains, but not the leaf sheath:

Neck rot and panicle blast
Infections just below the panicle, usually at the neck node, cause a typical "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.

Node infections
Stem nodes may be attacked causing the complete death of the stem above the infection. Diseased nodes are brown or black.
Collar rot, neck rot and node infections were commonly observed on California rice in 1996. Lesions on panicle branches and glumes were also observed, but less frequently. Leaf symptoms were observed, but were not confirmed as leaf blast.

Causal Organism
Rice blast is caused by the fungus Pyricularia grisea. 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.

Disease Cycle and Development
Overwintering
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 these overwintering sources, rice straw and stubble are probably the most important.

Multiple Disease Cycles
A disease cycle begins when a blast spore infects and produces a lesion on the rice plant and ends when the fungus sporulates and disperses many new airborne spores. When conditions are favorable, a single 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. This high inoculum level can be very injurious to a susceptible rice crop. The disease may progress through several phases starting with leaf blast and followed by collar, panicle and node blast. Leaf blast usually increases early in the season, then declines later in the season as leaves become less susceptible.

Environmental Conditions Favoring Blast
The disease is favored by long periods of free moisture, high humidity, little or no wind at night and night temperatures between 63-73°F. Leaf wetness from dew or other sources is required for infection. Spores are produced and released under high relative humidity (RH) conditions, with no spore production below 89% RH. Sporulation increases with increasing RH above 93%. The optimum temperature for spore germination, lesion formation and sporulation is 77-82°F. Lesions produce spores for longer periods at 61-77°F than at 82°F. It is clear that weather in the Sacramento Valley in 1996 was very conducive for blast. More normal weather should be less conducive.

Other Factors Favoring Blast
Blast is favored by excessive nitrogen fertilization, aerobic soils and drought stress. High nitrogen rates and nitrate nitrogen increase rice susceptibility to the disease. Ammonium nitrogen is converted to nitrate when fields are drained and aerated. This may explain why rice is more susceptible on nonflooded aerated soil. Extended drain periods encourage the disease by aerating the soil, converting ammonium to nitrate and by causing drought stress to rice.

Control Strategies
Successful control of blast usually requires an integrated management program including the use of resistant varieties, cultural practices and chemical control. Since resistant varieties may be many years away, cultural practices must be used and chemical controls sought. Important blast control methods are discussed below.

Crop Residue
If possible, destroy diseased straw and stubble by burning. This is an important control measure, but will not provide complete control by itself. Burning diseased crop residue will reduce overwintering inoculum in a given field and region, but will not protect the field from other inoculum sources.

Clean Seed
Use clean seed whenever possible. Use proper seed sampling and testing to identify and avoid the use of blast infested seed. Using clean seed will help control the disease, but will not protect fields from other inoculum sources. Seed treatment to eliminate blast from seed is under study at UC Davis.

Fertilizer Management
Avoid using excess nitrogen fertilizer, since it encourages the disease. Use the optimum nitrogen rate for maximum yield, but no more. Fertilizer spills and overlaps must be avoided since these areas enhance disease development and increase inoculum levels for the surrounding area. Nitrate nitrogen is more conducive to disease development than ammonium nitrogen.

Cultural System
Water seeding is recommended to reduce or eliminate disease transmission from seed to seedlings. Drill seedzing is not recommended, since it allows seed transmission, nitrate formation and may result in drought stress.

Water management
Continuous flooding is recommended to limit blast development. Avoid field drainage, especially for extended periods, since it allows the formation of nitrate and may cause drought stress. Also, drainage immediately after planting (less than 2 days) may allow some seed to seedling disease transmission. Since shallow water favors the disease more than deep water, moderate water (4-5 inches) and deep water (6-8 inches) are suggested for early and late season, respectively.

Chemical Control
No fungicides are currently available in California to control blast. In the southern U.S., benomyl (Benlate®) is registered to control the disease. Typically, two applications are made just before and after heading. These treatments reduce disease severity, but do not provide complete control. Quadris®, a new unregistered fungicide from Zeneca, has efficacy on blast and other rice diseases. Studies in the southern U.S. show it to be as good or better than benomyl.

Resistant Varieties
California varieties are susceptible to blast and M-201 appears to be the most susceptible. It will take much time and effort to develop resistant varieties and they may not always be completely effective. Never-the-less, they are an important, inexpensive way to control the disease. Plans to screen and breed for resistant varieties are in progress.

References:

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Modified: 2 Jan 2003 Comments to jayoung@ucdavis.edu