Sheath Blight

Sheath blight disease has been known in California since the late 1960s. Its incidence and severity has increased significantly in the past few years. The increase coincides with the introduction and widespread use of the high yielding short-statured varieties. The disease occurs throughout California's rice growing area. The causal organism has been identified as Rhizoctonia oryzae sativae. Early varieties of rice appear to be affected more than late varieties, and the disease is more severe where excess nitrogen fertilizer is used.

Tilt®, a newly developed fungicide, shows considerable potential for control of sheath blight. Registration is pending, but the FDA permitted its use on rice in the southern states during the 1982 season. Plant resistance to sheath blight has been identified in wild species of Oryzae, and the prospect of transferring it to cultivated rice is promising. Improvements are being made in methods used to determine disease severity. These methods are important for use in determining loss, control measures, and for evaluating breeding lines.

chemically treated plots. Nine lines selected for greatest tolerance averaged only 7.4 percent reduction in yield from water weevil.

Effects of Various Straw Removal and Rice Harvesting Methods

A three-year study has been completed on disease levels of sheath blight and stem rot following various harvesting and straw removal procedures. Treatments were burning, soil incorporation of the straw, and harvesting and removal of the straw from the field.

Conclusions from these studies are as follows:

• Burning the rice residue minimizes carryover of inoculum levels of Sclerotium oryzae (causal organism for stem rot) and disease severity.
• Removal of rice stubble and straw by cutting at ground level and baling also minimized overwintering inoculum levels. S. oryzae infects rice at the water level, and infections are mostly confined to these parts of the plant. Infected parts of the plant were removed from the field both by burning and straw removal.
• Rice should be cut below the infection site (water level) for either burning or for straw removal by baling. Short-statured varieties do not lodge significantly and burn poorly where there is not enough loose straw to carry the fire.
• Stem rot disease epidemiology appears to be generally the same for tall and short varieties, although there may be some exceptions.
•  Incorporating rice residue and stubble for consecutive years resulted in a build-up of S. oryzae sclerotia and an increase in stem rot disease.