Investigations on Rice
Blast Disease in CA - 98



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Project Leader and Principal UC Investigators

Robert K. Webster, professor, Department of Plant Pathology, University of California, Davis

Chris Greer, Research Associate, Department of Plant Pathology, University of California, Davis

Concern about rice blast in California prompted the Rice Research Board in 1998 to support a separately funded project to study this serious fungal disease. As with work on other rice diseases, its purpose is to gain an understanding of the biology of rice blast and to develop information for management strategies. Research concentrated in four areas:
  • To learn whether one or more strains of blast are present in California.
  • To identify possible overwintering sources of the disease.
  • To monitor field and weather conditions favorable to the occurrence of blast.
  • To determine the effect of cultural practices on severity of blast.

Progress toward these objectives is reported in the summary below.

1998 infestation

Blast.jpg (167383 bytes)The blast disease appeared to get off to a much later start in 1998 than in the previous two years. First observations of the disease were in mid-July, about a month later than in 1997. Distribution of the disease was confined to about the same areas of Glenn, Colusa and Sutter counties as in 1997.

The infestation was not as severe in 1998 as in the previous two years. There were no large areas seen within fields where plants had been killed by leaf blast early in the season. Consequently, it was more difficult to observe early in the season. Although the severity of the disease was less, the number of affected fields remained about the same. Pathologists believe the reduced intensity of the disease was related to the late planting time, cool spring temperatures, the effects of the wet winter on disease inoculum and/or the lack of plantings of the susceptible variety M-201. ˘The observations of blast disease in California over the last three years are giving us a demonstration of the unpredictable and temperamental nature of this disease,÷ the project leader stated.

One Race

Studies on blast disease to date show that there is only one known race of Pyricularia grisea in California -- IG-1. DNA fingerprinting of approximately 60 samples thus far show they are remarkably alike from a genetic standpoint. This suggests they all may be from the same source. IG-1 was common in Texas and Arkansas about 20 years ago, but the DNA fingerprinting patterns show that samples from the current California infestation differ somewhat from those in that era. IG-1 is also known to occur in Japan. 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.

Overwintering in residue

Field observations suggest that rice residue is the primary source of overwintering disease inoculum. The blast pathogen was easily recovered from residue during and after the fall harvest. Although the presence of other microorganisms makes recovery more difficult, blast can still be identified in rice residue in spring just prior to planting. An examination of nearby grasses and weeds did not reveal the presence of P. grisea, suggesting they are not a source of overwintering inoculum.

Here to stay?

When blast was first found in California, there was considerable speculation that it was due to unusual weather during the 1996 season. After living with rice blast in California for three years, it is apparent that this was not the case. Pathologists collected microclimate data, including temperature, relative humidity, leaf wetness, precipitation, wind speed and wind direction from six rice fields in Colusa, Glenn and Butte counties. Spore trapping equipment was set up at four of these sites to determine the presence and abundance of airborne blast inoculum. There were no great differences in weather data to explain why disease would occur in some areas and not in others. Nonetheless, the spore trapping experiments showed that determining the inoculum pressure may be an effective way of predicting the risk of blast under conducive climatic conditions.

Weather data indicate that conditions are for the most part permissive for blast disease development during the growing season. Most days had periods of high relative humidity and leaf wetness that would be more than adequate for disease development. Cool night temperatures may slow disease development. It is unclear how the cool wet spring of 1998 affected the disease. Nonetheless, there must be favorable climatic conditions, a susceptible host and the pathogen acting in concert to bring about the development of the disease.

Cultural practices

Trials to determine the efficacy of Quadris to control the neck blast phase of the disease indicate that this fungicide can effectively minimize losses in total yield. Additional study is needed to ensure that this expensive control option is used at the proper times and rates.

Nitrogen fertilization trials underscored the importance of using only the necessary amount of nitrogen for optimizing crop yield. Excess nitrogen results in a plant that is much more susceptible to blast and the probability of reduced yields.

Continuing evaluation of California cultivars in Colusa County confirm that M-201 is most susceptible to blast, which is consistent with growersĂ field observations. Other varieties showed a wide range of susceptibility.

These results are similar to 1997 and are encouraging because genetic differences in susceptibility to blast among California varieties can be tapped to improve disease resistance. These results also suggest that the lack of M-201 plantings in 1998 may be at least partially responsible for the less severe blast outbreaks last year.

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