|Investigations on Rice
Blast Disease in California
Rice - 99
Project Leader and Principal UC Investigators
Robert K. Webster, professor, Department of Plant Pathology, UC Davis
Chris Greer, Research Associate, Department of Plant Pathology, UC Davis
|Rice blast disease, which is
caused by the fungus Pyricularia grisea, was first identified in Glenn and Colusa counties
in 1996 and has since spread to Sutter and Butte counties. The disease got a late
start in 1999, which may account for the apparently low disease incidence and lack of
severe leaf blast outbreaks. Overall, distribution of the disease was about the same
as in 1998, with it being reported for the first time in Butte County in a field west of
Gridley. Blast was not observed in Sutter County during 1999, although there may be
undetected cases. Spread of the disease is characterized as moderate;
occurrence and severity is sporadic.
This project aims to develop an improved understanding of the disease to give growers help in managing the damage it causes. In 1999 researchers emphasized studies of the pathogens diversity, identification of overwintering sites, monitoring fields for spore production, analysis of weather data and the use of fungicides to control the disease.
The Weather Factor
Prevailing weather conditions in the majority of the rice-growing season in the Sacramento Valley are generally not optimal for rice blast development. Fungal spores do not usually complete the infection process due to normally insufficient leaf wetness periods. Environmental conditions in California rice producing areas are thus characterized as permissive for rice blast but generally not optimal for epidemic development.
Long periods of leaf wetness and high relative humidity that are conducive to disease infection were present in 1998 and 1999. However, conditions for peak spore release were generally insufficient for efficient infection.
When blast was first found in California, there was considerable speculation that it was due to unusual weather during the 1996 season. After continued examination of the weather data, however, pathologists say that the unusual conditions at the time of the initial blast outbreak in 1996 were not solely responsible for its occurrence. Microclimate data gathered by solar-powered, in-field weather stations included temperature, relative humidity, leaf wetness, precipitation, wind speed and wind direction from five rice fields in Colusa, Glenn and Butte counties. Spore trapping experiments suggest that monitoring inoculum pressure may be an effective way of predicting the risk of neck blast under conducive climatic conditions.
Sources of Resistance
Although there are no known major blast resistance genes in widely grown commercial California cultivars, there have been observed differences in susceptibility to the disease. In 1996, M-201 was the most severely affected cultivar and confirmed in variety trials during 1997 and 1998. Because of this susceptibility, very little M-201 has been grown since 1996. Ironically, this may have been a significant factor in the less severe infections in subsequent years. In 1999, each case of severe leaf blast was associated with M-204.
The organism that causes blast disease, P. grisea, has been recovered from rice crop residue and commercial seedlots in California but not from weed species in and around rice fields. The blast pathogen is easily recovered from residue during the fall after harvest. As the overwintering season progresses, the presence of other microorganisms complicates isolation of P. grisea. Nonetheless, it is still identifiable right up to planting time in the spring.
Pathologists examined leaf spots on many weed and grass species in and around rice fields for the presence of P. grisea but determined the leaf spots or lesions were caused by something other than the blast fungus.
Extensive DNA fingerprinting of P. grisea isolates reveal a remarkably homogenous population. Since the infestation began in 1996, researchers have looked at hundreds of isolates from dozens of grower fields and determined there is only one known race of the disease in California IG-1. This suggests that the current infestation is the result of a single source. Knowledge of the race or races of the disease present in California is essential for rice breeders in selecting parents for crosses and screening in attempts to produce cultivars with improved resistance to blast.
Trials to determine the efficacy of the fungicide Quadris® for control of the neck blast phase of the disease have shown promise when the product is applied at the appropriate time and when disease pressure is significant. Treated portions of four M-202 fields yielded 300-900 pounds/acre more than untreated portions.
One application of Quadris® at 50 percent boot-split to 50 percent heading may be effective in reducing the risk to both rice blast and aggregate sheath spot when conditions are favorable for disease development. Additional study is needed to assure that this expensive control option is used only when warranted.