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

C.M. Wick, farm advisor, Butte County Cooperative Extension

T. Miller, staff research associate

N. Cintas, staff research associate

• Continue monitoring populations of fungi known to attack rice pathogens in various management systems to assess any shifts in microorganism populations and to monitor disease.
• Conduct screening and greenhouse experiments to determine optimal formulations, rates and timing for microbiological control of diseases.
• Evaluate disease reactions of different cultivars in fields where residue has been incorporated.
• Evaluate germplasm collections in an attempt to identify new, more disease-resistant germplasm for use by plant breeders.

Fungal Ecology

Studies continue on different residue management practices - fall burned, fall chopped and incorporated, and fall incorporated and flooded. Researchers noted similar population frequencies of organisms as seen in previous years. They also note a succession of different fungi from early fall though spring.

Fungi isolated from fields where residue is incorporated are more frequent and more diverse than in burned fields. Researchers also note reduced frequency of most of the species monitored in fields where residue is incorporated and then flooded.

This year's results confirm earlier observations that residue incorporation is affecting the populations of mycoflora in rice fields. What the long-term effects of these shifts will be on rice culture and rice disease severity is unknown.

Nutrient Analysis

Researchers took soil samples last year to examine whether a relationship exists between fungal diseases and the nutrient levels for major and minor elements, as well as pH and other soil factors. Preliminary observations for possible differences in nutritional factors between fields where residue is incorporated or burned revealed no consistent pattern with the possible exception of potassium, iron and sulfur. Further study is needed.

Disease Interactions

Researchers had previously noted an inverse relationship between stem rot severity and aggregate sheath spot incidence. The pattern observed in field trials during 1993 is similar to previous greenhouse studies. In field studies the magnitude of this interaction varies with different cultivars. This interaction probably contributes to the relative differences in disease susceptibility measured between cultivars in the field under heavy disease pressure.

Biocontrol Work

Greenhouse experiments were carried out to determine the optimum inoculum levels and methods for applying Sclerotium hydrophylum, a fungus that attacks the stem rot pathogen. In all tests with S. hydrophylum, stem rot was minimized to insignificant levels by all concentrations tested.

Plant pathologists report, however, that results in both field and greenhouse tests suggest that field applications of these organisms may not be practical as an approach to controlling stem rot. Since all three occur naturally in rice fields, methods to enhance their natural populations may be more productive.

Germplasm Monitoring

Attempts to identify new sources of disease-resistant germplasm for use by breeders is continuing. Researchers examined 43 wild rice accessions representing 10 species of Oryza. Sources studied during 1993 showing the highest level of AGSS resistance were highly susceptible to stem rot. No germplasm studied last year showed promising levels of resistance to both AGSS and stem rot.