Cause and Control of
Rice Diseases-94



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Project Leader and Principal UC Investigators

Robert K. Webster, professor, Dept. of Plant Pathology, UC Davis

T. Miller, staff research associate, Dept. of Plant Pathology, UC Davis

N. Cintas, staff research associate, Dept. of Plant Pathology, UC Davis

Webster.jpg (335005 bytes)The primary objective of this ongoing project is to gain an understanding of the biology of rice diseases that occur in California and to develop methods for their control. Open field burning as a tool for controlling the main fungal disease problems - stem rot and aggregate sheath spot - is being phased out, so researchers are studying the effects of different methods of straw management. Attention is also focusing on microorganisms that may be used to control stem rot. Researchers are also developing a method to predict disease severity to help identify fields most in need of burning. Specific research objectives for 1994 included:

• Study disease incidence under various residue management treatments flooding, incorporation, burning, soil contact and residue removal.

• Continue the study of parasites that may affect the viability of disease organisms and the "inverse relationship" between stem rot and aggregate sheath spot.

• Continue evaluations of germplasm sources for improved disease resistance.

• Continue collecting data to determine ability to predict which fields may justify continued burning.

Residue Management Trials

Plant pathologists are cooperating with the continuous year field trials that have been established to monitor the effects various residue management treatments have on rice disease epidemiology. This is essential to develop an optimal rice disease management system as open field burning is curtailed.

One site is at Canal Farms in Colusa County, where researchers are studying several treatments: straw burned, straw incorporated in the fall, straw rolled after harvest, straw baled and removed. Winter flooding is tested as a main plot treatment on each of the above also, with a non-flood comparison.

At another location, the Sills Ranch in southern Sutter County, comparisons include straw burned in fall and spring straw incorporated in fall and spring each with and without a vetch cover crop.

The first year's results from the Canal Farms site documented inoculum production and survival and provide the basis for determining the multi-year cumulative effects of the treatments being compared. They show that among residue treatments there is a difference in total number of sclerotia and percent viability (an indication of the inoculum level). Researchers observed lower inoculum and disease severity and higher yield in flooded treatments. The highest yield. however, was obtained when residue was burned, followed in descending order by baling, incorporating and rolling. These results are consistent with findings from previous studies. Conclusions on long-term effects of these treatments await further study.

A long-term experiment similar to that at Canal Farms was established in the fall of 1994 in Butte County. Researchers believe studies at this site should provide invaluable data regarding the effects of different residue treatments on a different soil from that at other sites.

Disease Severity Predictions

Results from consecutive sampling of disease severity and incidence at the Sills Ranch over the last three years indicate that it is possible to reliably determine fields that justify continued burning early enough in the season to allow timely action.

The researchers also documented how the incidence of both stem rot and aggregate sheath spot increased as the crop reaches maturity.

Disease Interactions

At the Sills Ranch, researchers studied the occurrence, severity and interaction of stem rot and aggregate sheath spot under various residue treatments - with and without a vetch cover crop. As in past years there appears to be an inverse relationship between the amount of stem rot and that of aggregate sheath spot.

Studies on the interaction of organisms at the infection site on the rice plant indicate that competition for the infection site may explain differences in the proportion of stem rot and aggregate sheath spot in a particular field. This also relates to differences in inoculum levels of the pathogens in a particular field.

Germplasm Resistance

Researchers report no promising sources of improved resistance to both stem rot and aggregate sheath spot in screenings of wild rice species. The search will continue on a large collection of Oryzae sativae.

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