Cause and Control of
Rice Diseases-97



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Project Leader and Principal UC Investigators

Robert K. Webster - Professor, Department of Plant Pathology UC Davis

Nicole Cintas - Research Associate Department of Plant Pathology, UC Davis

Chris Greer - Research Associate Department of Plant Pathology, UC Davis

Webster.jpg (51657 bytes)The major goal of this ongoing project is to further an understanding of the biology of rice diseases occurring in California and to develop methods for their control. Two factors are currently driving its research agenda. First, the legislatively mandated phase down in open field burning appears to increasing the severity of stem rot and aggregate sheath spot, thus underscoring the importance of alternative residue management strategies. Second, the appearance of rice blast in 1996 has compelled plant pathologists to expand their efforts into a whole new area of research to under- stand the nature of this potentially devastating disease and to develop methods for its control The narrative below summarizes progress in research into these diseases.

Developments On Blast Disease

Plant pathologists have devoted a substantial amount of time and energy attempting to learn more about the rice blast infestation. Here's a quick run-down of what they are finding:

  • The infestation was much more widespread in Colusa and Glenn counties in 1997 than in 1996. It also was confirmed last year in Sutter County, suggesting it may spread to new areas in the future.
  • Rice seed samples taken from 121 fields from 1996 were tested for the blast pathogen to determine whether this could be a source of inoculum. Only 15 of the fields tested positive and those that did had relatively low levels.
  • The blast pathogen is easily recovered from residue after harvest. Field observations suggest that residue is the primary source of initial inoculum for early occurrences of leaf blast.
  • Leaf spots from many weeds and grasses around rice fields were examined. The blast pathogen was not found to be the cause on any lesions on any material examined last year.
  • A seed treatment with sodium hypochlorite to eliminate blast from infested seed was not effective. The concentration and duration of the soaking necessary to eliminate the pathogen reduced both seed germination and seedling vigor.
  • Examination of weather data indicates that the initial occurrence of blast was not due solely to unusual conditions in 1996. Sacramento Valley weather is considered permissive for the occurrence of blast, with a few periods that become more favorable.
  • Fungicide trials determined that Quadris, applied at the neck blast phase, can effectively minimize losses in total yield and reductions in quality. Additional study of rates and timing are needed to ensure cost-effectiveness.
  • Evaluation of regional cultivar trials showed that among California cultivars, M-201 is most susceptible to blast, with others showing a wide range of susceptibility. These differences are encouraging because the genetic variability indicates breeding improvements are possible.

Residue Management Trials

Research is continuing on how various residue management practices affect aggregate sheath spot and stem rot at an on-farm site in Colusa County and at the Rice Experiment Station. Alternative practices in four-year-old study include incorporation, rolling, removal and burning under winter flooded and unflooded conditions.

At the Colusa site, winter flooding appears to be having a beneficial impact on stem rot incidence. Evidence of the stem rot-causing organism or "sclerotia" was lowest on winter flooded main plots, while they've been increasing each year on the unflooded plots. Not surprisingly, the fewest number of viable scierotia were observed on the burned treatments. Yields were also highest in the winter flooded main plots in the first three years but were considerably higher in all treatments in 1997. Researchers attribute this to lower overall disease levels, a change in harvest method and a generally favorable year for rice development.

Aggregate Sheath Spot has gradually increased at the Colusa site in all treatments, with the greatest increases in unflooded main plots. Overall, the lowest Aggregate Sheath Spot levels have been in the winter-flooded, burned subplot treatments.

Observations from the Butte County site reveal a pattern somewhat contra- dictory to those from the Colusa site. Aggregate Sheath Spot levels are considerably higher at the Butte site, suggesting that winter flooding favors the survival and occurrence of this disease. There is some evidence that yield differences in the various sub-plots may be due to potassium nutrition differences between the residue and treatments. Also, soil types differ between the two sites and the initial level of disease was much higher at the Butte site than at the Colusa site.

Fungicides Evaluated

Field tests were established at two sites in Glenn County and one in Sutter County to evaluate rates and timing of application for fungicides to control Stem Rot and Aggregate Sheath Spot. Quadris proved to be the most promising, with a 1,400 pound increase over the control in one trial. Another trial was severely infected with blast, which showed that two early applications of the fungicide did not reduce either leaf or neck blast. Plant pathologists emphasize that fungicides should be used as a protectorant of vulnerable neck nodes during heading and that attempts to control leaf blast with multiple, earlier applications are not only costly but futile.

Predicting Disuse Levels

Under the provisions of the burning phasedown, growers can be allowed to burn fields threatened by severe infestations of rice diseases. The challenge has been to find a reliable method to predict disease levels early enough to do something about it.

Based on two years worth of stem rot severity ratings taken during the month of August, plant pathologists have developed a reliable method of predicting current season yield loss and potential future stem rot damage in next year's crop. From these ratings, regulatory decisions could be made in time to allow burning in the fall.

Germplasm Monitored

Plant pathologists continue to evaluate rice from the National Small Grains germplasm collection for sources of improved resistance to Stem Pot and Aggregate Sheath Spot. Plants are scored for the presence of both diseases throughout the growing season. Of more than 6,000 entries, 29 have potential for use as future parents in plant breeding efforts.

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