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

Nicole Cintas, research associate, Department of Plant Pathology, UC Davis

Tom Miller, staff research associate, Department of Plant Pathology, UC Davis

• Continue studying disease incidence under various residue management treatments - flooding, incorporation, burning, soil contact and residue removal. Refine methods of studying soil inoculum levels and their relationship to disease severity.
• Continue the study of mycoparasites that may affect the viability of fungal sclerotia and the 'inverse relationship' between stem rot and aggregate sheath spot.
• Continue evaluations of germplasm sources for improved disease resistance.
• Continue collecting data to develop ability to predict fields justifying continued burning.
• Greenhouse and field testing of environmentally acceptable fungicides against stem rot and aggregate sheath spot.

Residue Management Trials

Continuing field trials have been established to monitor the effects of various residue management treatments on rice disease. Information gained from these studies will be essential to determine which straw management alternative offers the best protection from rice disease.

This study involves two locations - Canal Farms in Colusa County (second year) and the Rice Experiment Station in Butte County (first year). Soil samples collected from finished seedbeds at each site showed differences among the various residue treatments in both the number of sclerotia (disease-causing organism) and stem rot severity. Researchers say further study to determine whether any of the treatments results in lower initial levels of the inoculum is critical.

Data from the Butte site showed that viable sclerotia were lowest in the non-flooded plots, while mean yield was highest. However, the reverse was observed at the Colusa location, where for the second year in a row sclerotia were lowest in the winter-flooded plots and yield was significantly higher.

Researchers note that the initial disease levels at the Butte site were relatively high. Coupled with the unusual amount of rain during the winter of 1994-95, the results comparing the winter-flooded and non-flooded plots may have been unduly influenced. There were significant differences in viable sclerotia surviving between treatments at the Colusa site with the bum treatments being the lowest after two years.

Researchers also observed plant conditions at both trial sites that appeared to be abnormal and were probably related to nutritional factors, possible plant toxicities or both. Thus the differences in yield cannot be attributed to disease alone.

Where aggregate sheath spot is concerned, detectable inoculum levels and disease incidence was significantly higher for the first year at the Butte site than either of the two years at the Colusa site. There was also an increase in aggregate sheath spot at the Colusa site over the previous year, with the largest increases occurring in the incorporation treatments. The differences in disease levels between the two sites provide researchers with 'an ideal situation' to study the cumulative effects of various residue treatments on disease. Work at these locations will continue.

Germplasm Screening

Researchers began screening collections of Oryz sativae from the National Germplasm Collection, concentrating on rice from countries with climates similar to that of California. Evaluations of more than 550 entries revealed only six with resistance to stem rot as good as M-201 and none was superior. None of the six was considered promising as a source of resistance to aggregate sheath spot. Evaluations of additional collections will continue.

Fungicide Testing

Researchers also carried out greenhouse tests on three compounds for control of stem rot and aggregate sheath spot. ICIA5504, a wetable granular formulation, reduced the incidence and severity of both fungal diseases. The compound is planned for field testing during 1996.

Benomyl has been reported to control sheath blight and stem rot in other states. However, plant pathologists report their test showed no control of stem rot in either greenhouse or field tests. Two applications of Benomyl did reduce the incidence of aggregate sheath spot by 40 percent. Researchers will give Benomyl another look this year.

Disease Severity Predictions

Plant pathologists continued their efforts to predict which fields are most susceptible to fungal damage and, therefore, may be considered eligible for the 'safe harbor' provision of the legislation reducing open field burning.

Last year evaluations made within a month of harvest gave reliable estimates of the impact stem rot would have on the current crop (see graph). It seems that even though the percentage of infected plants remained relatively constant in the last month of the season, the severity of the stem rot disease on infected plants continued to escalate. Researchers attribute this to the fact that stem rot causes most of its damage during the time the pathogen is penetrating and rotting the culmes of infected rice tillers. These results are similar to those obtained in the previous year. The upshot: stem rot severity ratings are a more reliable indicator of potential loss than stem rot incidence.

Researchers also observed a strong correlation between stem rot severity and its impact to panicle weight (see graph). There is also a strong correlation between disease severity in the present crop and expected carryover to the following season. This is probably the best basis for predicting future disease incidence. The drawback to this approach is that it is very labor intensive and dependent on the validity of the sampling techniques used. Plant pathologists will continue their work in this area.