Cause & Control of
Rice Diseases - 96



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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

The major thrust of this ongoing project is to develop a greater understanding of the biology of rice diseases that occur in California and to develop methods Disease1.jpg (150338 bytes)for their control. Control methods under study focus on cultural practices affecting stem rot and aggregate sheath spot; improved sources of disease resistance; and evaluation of fungicides for potential use in disease control. News that rice blast appeared for the first time in California late last summer compelled plant pathologists to marshal efforts against this potentially devastating disease. This section summarizes the progress of plant pathologists toward their research objectives.

Blast Invades California

In early September plant pathologists confirmed reports that "blast", a hitherto unknown disease in California, had appeared in Glenn and northern Colusa counties. The blast disease causes considerable damage and yield loss throughout the world. If it were to become established here, blast could easily have a significant impact on the future of rice production in the state.

Plant pathologists directed efforts to determine the extent of the blast infection and the susceptibility. of California cultivars. They also participated in the effort to obtain permission to burn as much of the infected area as possible.

Pathologists are continuing study of the blast in an effort to develop a logical industry approach to eliminate possible spread of the disease. An-initial step was to explore the feasibility of reintroducing the practice of soaking rice seed in a mixture of water and sodium hypochlorite. Thus far results indicate that concentrations required to eliminate the pathogen from seed are detrimental to germination and thus its use is not advisable.

Residue Management Trials

Research into cultural practices currently focus primarily on residue management. At on-farm sites in Colusa and Butte counties, alternative residue management practices are being tested and analyzed for their effect on disease inoculum levels, disease severity and crop yield. Researchers are also examining methods of predicting future disease severity - techniques that will become crucial for growers requiring special burn permits where disease loss threatens.

Residue management alternatives under study at Canal Farms in Colusa County include burning, fall incorporation, rolling after harvest, and baling and removal in both winter flooded and unflooded conditions. Identical treatments are under study at the Rice Experiment Station in Butte County.

Thus far the second and third years of burning residue at the Colusa site has decreased the overwintering organisms or "sclerotia" that cause disease. At the Butte site, however, inoculum levels of stem rot selerotia increased in the second year in all treatments. Researchers speculate that differing soil types and field histories may be affecting the experimental treatments. They stress the need for further inquiry into possible differential effects

Especially needed is information on how populations of microorganisms that compete with pathogens are affected by various residue treatments and how this interaction affects straw decomposition. Pathologists point to prior observations of the inverse relationship between aggregate sheath spot and stem rot as an indication of the type of microbiological competition under way in grower fields. Of particular interest are any increases or decreases in organisms that affect the viability of pathogens and the infection process in rice. The initial years of this study may have just began to influence changes; additional years of continuous treatment are needed to allow observation and study of the effects of these phenomena.

Initial levels of the aggregate sheath spot pathogen were very low at the Colusa site and relatively high at the Butte site. Levels have increased in all treatments at the Colusa site over the three years, with the largest increases in the unflooded plots. Observations from both sites indicate that flooding results in a lower buildup and carryover of this pathogen than for stem rot. Pathologists caution, however, that their analysis is still under way and definitive conclusions are premature at this point.

Disease2.jpg (245209 bytes)Disease Resistance "Promising"

Pathologists continue to evaluate rice germplasm for sources of improved resistance to stem rot and aggregate sheath spot for use by plant breeders. More dm 6000 entries from the USDA rice germplasm collection were planted at the Rice Experiment Station for evaluation last year. Evaluation continued at different stages of plant development throughout the season. Plants were scored for both stem rot and aggregate sheath spot reactions. From this initial pool researchers screened 14 promising sources of genetic resistance to both stem rot and aggregate sheath spot. Cooperation with plant breeders will continue in hopes of exploiting these sources of resistance for improved California cultivars.

Fungicide Testing

Fungicide trials at four sites to deter- mine candidate chemicals for use against stem rot and aggregate sheath spot were not encouraging. Some reductions in disease severity were noted but no significant yield increases were observed at the rates and time of application tested.

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