Steady progress continues to be made in development of alternative metbods of controlling stem rot. These diseased samples were on display at last fall's Rice Field Day.

The major rice disease in California is stem rot, followed by aggregate sheath spot. Severity of these diseases varies year to year and is affected by the effectiveness of rice residue disposal and destruction of overwintering pathogens. Since burning has become more restricted and may not be an option in the future, many growers are experimenting with alternative methods of residue disposal.

Although much information exists on what happens to rice when residue is not burned, much remains to be learned about whether any biological manipulation might improve straw degradation and disease control.

One biological method under investigation is directed at straw-decomposing fungi to enhance decay and promote destruction of overwintering pathogens. During 1990 a great deal of effort went into isolating and testing naturally occurring fungi from overwintering rice straw. One fungus, a binucleate Rhizoctonia, previously isolated from decomposing rice straw was able to colonize strew and slightly increase the final level of decomposition. This will continue to be a priority research area.

In addition to straw decomposers, researchers were for the first time able to isolate naturally occurring fungi capable of parasitizing the sclerotia of the stem rot fungus. These fungi will be further investigated. Considering the number of different fungi isolated from both straw and sclerotia, more work will be needed to select the most effective combination of useful organisms.

"Fungi capable of parasitizing stem rot were isolated for the first time."

Studies of different residue management systems showed little difference in the final decomposition levels of straw but did illustrate the importance of soil contact for effective degradation.

Samples of soil sclerotial populations and resulting disease levels in sampled fields from different management systems revealed little new information. The data supported earlier observations that high levels of aggregate sheath spot correlate with reduced stem rot severity. In addition, the importance of cultivar selection, stand density and nitrogen management in stem rot infested fields was repeatedly noted.

Sclerotium hydrophilum appears to be the most promising in-season biocontrol agent of stem rot in greenhouse tests. It was only slightly effective in a small field trial in 1990, probably due to unusually severe stem rot pressure within the trial rings. This fungus was found to be nonpathogenic to rice in greenhouse trials.

S-201, M-201 and M-103 are the cultivars exhibiting the greatest resistance to stem rot in greenhouse experiments. On the other hand, M-202 and L-202, while showing moderate to weak resistance in the greenhouse were consistently noted as having stem rot problems in the field. Another greenhouse experiment showed that stem rot severity is reduced when it competes with aggregate sheath spot, bordered sheath spot or other pathogens