Cause & Control of Rice Diseases 99

Cause and Control of Rice Diseases-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	The major goal of this project is to investigate rice diseases that occur in California and to develop methods for their control. The relationship between alternative rice residue management and the epidemiology of rice diseases has been a major focus the last six years. Stem rot and aggregate sheath spot are the major diseases under study. Research on blast disease is reported separately. Residue Management Trials Studies on the effect of alternative residue management practices were completed at one site in Colusa County and a second at the Rice Experiment Station (Butte County). Practices under study included incorporation, rolling, removal and burning under winter flooded and unflooded conditions. The results obtained over six years suggest that winter flooding, although not as effective, is the best alternative to burning for minimizing stem rot inoculum levels. Incidence of the stem rot-causing organism or "sclerotia" have been consistently lowest in winter-flooded main plots. Total sclerotia per gram of seedbed soil has decreased each year of the study. Conversely, the total number of sclerotia and viable sclerotia per gram soil were significantly greater each year in the winter unflooded main plots. Lowest inoculum levels continued to be observed in the burned sub-plots. Rolled treatments had significantly more stem rot than other subplots. Viability of sclerotia on the burned plots was half that of other treatments. Average stem rot severity was lowest in the flood main plots, with the lowest severity observed in the burned, flooded subplots. At the Colusa site, yields were highest throughout the study on flooded main plots, averaging 9,006 pounds/acre versus 8,548 from the unflooded main plots. The burn subplot treatment had the highest yield of the subplot treatments, averaging 9,021 pounds/acre over the six years. Aggregate sheath spot (AGSS) has gradually increased in all treatments with the greatest increase in the flooded main plots at the Colusa site. The highest percent AGSS occurred in the incorporated flooded subplots at this site. The Butte County site produced inconsistent results throughout the study. Stem rot disease levels appeared lowest on burned treatments but not as much as anticipated. AGSS was higher at the Butte site throughout the study and was unaffected by treatments. All treatments had about the same level of AGSS. The smaller size of the Butte site subplots and the difference in soil types between the two sites may have had an influence on the differences noted. Fungicide Evaluations Field tests showed the fungicide Quadris® to be an effective treatment against AGSS with proper rate and timing. Applications at 70 and/or 95 days after planting resulted in reduced levels of AGSS severity with small yield increases. The greatest benefits were higher grain quality and significant increases in percent head rice. These treatments did not significantly minimize stem rot severity. Additional study on the benefits and timing of Quadris® is needed, particularly in relationship to control of both AGSS and blast disease.

Cause and Control of Rice
Diseases-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

The major goal of this project is to investigate rice diseases that occur in California and to develop methods for their control. The relationship between alternative rice residue management and the epidemiology of rice diseases has been a major focus the last six years. Stem rot and aggregate sheath spot are the major diseases under study. Research on blast disease is reported separately.

Residue Management Trials

Studies on the effect of alternative residue management practices were completed at one site in Colusa County and a second at the Rice Experiment Station (Butte County). Practices under study included incorporation, rolling, removal and burning under winter flooded and unflooded conditions.

The results obtained over six years suggest that winter flooding, although not as effective, is the best alternative to burning for minimizing stem rot inoculum levels. Incidence of the stem rot-causing organism or "sclerotia" have been consistently lowest in winter-flooded main plots. Total sclerotia per gram of seedbed soil has decreased each year of the study. Conversely, the total number of sclerotia and viable sclerotia per gram soil were significantly greater each year in the winter unflooded main plots.

diseases 22.jpg (22219 bytes) Lowest inoculum levels continued to be observed in the burned sub-plots. Rolled treatments had significantly more stem rot than other subplots. Viability of sclerotia on the burned plots was half that of other treatments. Average stem rot severity was lowest in the flood main plots, with the lowest severity observed in the burned, flooded subplots.

At the Colusa site, yields were highest throughout the study on flooded main plots, averaging 9,006 pounds/acre versus 8,548 from the unflooded main plots. The burn subplot treatment had the highest yield of the subplot treatments, averaging 9,021 pounds/acre over the six years.

Aggregate sheath spot (AGSS) has gradually increased in all treatments with the greatest increase in the flooded main plots at the Colusa site. The highest percent AGSS occurred in the incorporated flooded subplots at this site.

The Butte County site produced inconsistent results throughout the study. Stem rot disease levels appeared lowest on burned treatments but not as much as anticipated. AGSS was higher at the Butte site throughout the study and was unaffected by treatments. All treatments had about the same level of AGSS. The smaller size of the Butte site subplots and the difference in soil types between the two sites may have had an influence on the differences noted.

Fungicide Evaluations

Field tests showed the fungicide Quadris® to be an effective treatment against AGSS with proper rate and timing. Applications at 70 and/or 95 days after planting resulted in reduced levels of AGSS severity with small yield increases. The greatest benefits were higher grain quality and significant increases in percent head rice. These treatments did not significantly minimize stem rot severity. Additional study on the benefits and timing of Quadris® is needed, particularly in relationship to control of both AGSS and blast disease.