Disease Control-91



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Project Leader and Principal UC Investigators

R.K. Webster, UC Davis

R.D. Cartwright, Research Associate

C.M. Wiccck, Cooperative Extension


As the rice industry begins to phase out burning rice straw to control stem rot and other rice diseases, research into alternative control methods will take on added significance.

The overall research objective of this ongoing project is to understand the biology of California rice diseases well enough to develop effective cultural and biological control methods for stem rot, as well as other fungal diseases. Specific objectives for 1991 included:

  • Evaluation of how rice residue management systems affect the organism that causes stem rot.
  • Identification of rice field fungi that may help decompose rice stubble and destroy the stem rot organism.
  • Screen residue decomposers and "mycoparasites."
  • Isolate and test potential agents for biocontrol of stem rot.
  • Establish a field trial to test promising fungi for biocontrol of stem rot

In one set of field tests, researchers examined several rice residue management systems for their effect on straw decomposition and survival of stubble-borne sclerotia of the stem rot pathogen, Sclerotium oryzae. They found little decomposition during winter months. Although a higher percentage of straw was decomposed in unburned systems than in burned systems, the difference was small.

Of much greater importance as far as growers are concerned, however, was the finding that regardless of the system, soil contact enhanced decomposition of rice residue. Researchers also determined that burned systems had the least viable stem rot sclerotic, while stubble and flooded systems had the the most viable sclerotic.

For the third season, researchers continued their examination of a wide array of saprophytic fungi isolated from decomposing stubble. Although initially encouraged by one organism identified in previous lab work, the decomposer did not perform as well on rice stubble as hoped. However, extreme environmental conditions (December freeze) in 1991 may have been responsible for the failure. Additional research is needed .

Fungi isolated from the sclerotic of S. oryzae were again tested in the lab for their ability to parasitize stem rot sclerotic. One such pycnidial fungus continued to be the most aggressive parasite of S. oryzae and other fungi in the lab. Although no significant effect on sclerotial viability was noted in a field trial, tests with the organism may also have been affected by the extraordinary freeze in December 1991. Researchers feel that additional research of this fungus is warranted. .

Of the many organisms tested in the laboratory and greenhouse as potential in-season biocontrol agents for stem rot, only two - Sclerotium hydrophilum and Rhizoctonia oryzae-sativae - appear to have much promise. R. oryzae-sativae is a minor rice pathogen and S. hydrophilum colonizes senescent rice tissue. Greenhouse studies showed no discernible symptoms nor yield reduction when rice was inoculated with S. hydrophilum. Researchers established a new three-year trial at the Rice Experiment Station with separate water systems to determine the true potential for these two organisms and the pycnidial fungus to control stem rot under field conditions.

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