Rice Disease Control 89

Rice Disease Control-89

Project Leader and Principal UC Investigators

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

The major fungal diseases of California —ricestem rot, bordered sheath spot and aggregate sheath spot—survive year to year through stubble, straw and other residue. Burning, historically the most effective and economical method of control, has come under increasing public scrutiny and may no longer be an option in the not too distant future. Consequently, the focus of research into disease control lies in the following areas:

Microbial enhancement of rice residue degradation and destruction of overwintering pathogens.
Monitoring of non-conventional cultural practices on residue management and disease levels.
Isolation and testing of microorganisms that may provide in-season biocontrol for stem rot.
Evaluation of resistance of California commercial cultivars to stem rot, aggregate sheath spot and bordered sheath spot.

Microbial decomposition

" The researchers were particularly encouraged by one fungus, Sclerotium hydrophilum, which reduced stem rot in two of three cultivars."

Several fungi capable of accelerating the decomposition of rice stubble in the lab were identified from fields. The most promising of those identified are being studied in the field this season. The idea is to manipulate normal ecological processes by artificially moving late season fungal decomposers to the fall, which theoretically would cause residue to degrade more rapidly and reduce the inoculum of overwintering rice pathogens. Lab tests have varied, but general observations suggest that at least three different fungal species are partially effective against stem rot inoculum.

Nonconventional rice culture

Researchers also began monitoring cultural practices on one farm growing organic rice and another farm that is exploring the effectiveness of winter flooding and subsequent incorporation of rice straw.

Continuous years of monitoring will be required to determine how well these techniques work in enhancing straw degradation and to biological control of stem rot and other diseases.

Stem Rot Biocontrol

Researchers hope to manipulate microorganisms for better control of costly rice diseases such as stem rot.

Several fungi and bacteria antagonistic to the stem rot fungus have been isolated as potential in-season biocontrol agents. The researchers were particularly encouraged by one fungus, Sclerotium hydrophilum, which reduced stem rot in two of three cultivars tested in greenhouse experiments.

The fungus, which does not appear to be pathogenic to rice, grows along the water line of the rice plant and competes for the same ecological niche with the stem rot fungus. Field trials with S. hydrophilum will be conducted during the 1990 growing season.

Genetic resistance

Researchers examined the disease-resistance of 10 commercial rice cultivars. Their greenhouse study determined stem rot caused the greatest yield losses, followed by bordered sheath spot and aggregate sheath spot.

Stem rot causes yield loss by reducing the number of filled grains per panicle. Secondary tillers are more affected than primary tillers. One cultivar, M-102, showed the best resistance to all three diseases. These experiments were conducted in a greenhouse and may not reflect the same magnitude of difference that occurs under field conditions.

Seed treatments

Few growers experienced stand establishment problems during the 1989 growing season because of favorable temperatures during April and May. Consequently little new knowledge was gained from tests comparing various chemical seed treatments: