Rice Diseases - 92




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Project Leader and Principal UC Investigators

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


The primary thrust of this ongoing project is to understand the biology of rice diseases in California and to develop methods for their control. Because of new restrictions on open field burning of rice stubble, the project's emphasis is now to find effective methods of biologically and culturally controlling diseases such as stem rot and aggregate sheath spot.

Toward that end, researchers:

  • Monitored fungi known to attack rice pathogens in several different management scenarios.
  • Conducted greenhouse experiments to determine optimal timing for microbiological control of diseases.
  • Continued a long-term trial to determine the effectiveness of promising parasitic fungi to control disease.
  • Established new trials to test differences in cultivar reaction to diseases where straw was incorporated in fields.
  • Established trials to see how nitrogen fertilization affects yield and disease under different residue management strategies.

Fungal ecology

Continuing studies of different residue management practices is revealing a change in the population dynamics of microorganisms of California rice fields. Researchers are seeing patterns emerge in similarly managed fields. Specifically, the population frequencies of the organisms that participated in rice straw degradation and parasitism of the stem rot pathogen, Sclerotium oryzae, are similar to those observed in previous years in those fields.

Most notably, beneficial mycoparasitic fungi are more frequent and others are more diverse from fields where residue is incorporated in the fall than from fields where residue is burned. In the fields where residue was incorporated and then flooded, the frequency of some of these organisms is lower. What the long term effects these shifts in the ecology of the "mycoflora" will mean for straw degradation, disease incidence and yield is unknown.

Stem rot monitoring

Researchers also monitored a number of growers fields to assess stem rot severity under different residue management practices. Stem rot was least severe in burned fields, intermediate in fall incorporated and flooded fields, and most severe in fall incorporated unflooded fields.

Yields obtained from these fields followed the same pattern, regardless of cultivar grown. Actual yields growers obtained from these fields varied widely. The high was 9,900 pounds per acre (M-201) from a fallburned field. The low was 7,100 pounds per acre (L-203) from a field fallow after residue was incorporated in 1990. M-201 is most resistant to stem rot yet quite susceptible to aggregate sheath spot.

Researchers caution, however, that additional analysis is necessary to determine whether factors other than stem rot incidence are affecting yield.

Yield studies

Researchers have also been attempting to determine how disease level under different residue management strategies affects rice yields.

The 1992 growing season produced some very high yields for some growers. There were, however, large differences observed among different cultivars and different cultural practices. As one might expect, the highest yields occurred at those study sites with the least amount of disease. Generally, aggregate sheath spot was more abundant in fields where residue was incorporated and then flooded in the fall than it was in other systems.

This work is still in progress but researchers hope to develop recommendations on how growers might manipulate their cultural practices for better management of rice diseases.

Biocontrol tests

Greenhouse experiments continue Sclerotium hydrophylum, a fungus that parasitizes the stem rot pathogen, with encouraging results. Stem rot was minimized to insignificant levels at all concentrations tested. However, continuing field trials at the Rice Experiment Station on S. hydrophylum and other fungi holding promise for potential biocontrol, has not resulted in a significant reduction of the stem rot pathogen. Research results also indicate that addition of mycoparasitic fungi to rice fields will have to be done annually in order to be effective if this practice proves feasible.

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