Disease Control 81

Disease Control-81

Project Leader and Principal UC Investigators

Robert K. Webster, Dept. of Plant Pathology, UC Davis

Goal of the disease control project is to find control methods that are the most effective, economical, and have minimal environmental consequences. The two principal rice diseases are stem rot and sheath blight. In 1981 studies were conducted on: 1) factors affecting the overwintering of the stem rot organism and subsequent disease levels, 2) relationships between stem rot and sheath blight disease severity and various cultural practices, 3) methods for determining soil inoculum levels of the sheath blight pathogen and its relationship to disease severity, 4) wild species and early generation crosses for sources of disease resistance, 5) improved methods of chemical control, 6) the effect of residue disposal methods on disease severity, and 7) the effects of alternate cropping and fallow on stem rot and sheath blight severity.

Nitrogen fertilization levels above those needed for optimum yield enhance stem rot and sheath blight severity for all varieties currently grown.

Sclerotia of stem rot (S. oryzae) and sheath blight (S. oryzae sativa) are produced in residue in the field when temperatures exceed 52°F. Thus, it is likely that fields not burned in the fall will have higher disease levels the next year.

Nitrogen fertilization levels above those needed for optimum yield enhance stem rot and sheath blight severity for all varieties currently grown.

The disease cycles for sheath blight and stem rot are similar. Inoculum levels of both pathogens increase more in fields where residue is incorporated than where residue is burned. The sheath blight disease is widely distributed throughout the rice growing area. Sheath blight is more severe on short-stature varieties than on taller varieties. Early maturing varieties are affected more severely by sheath blight than are late-maturing varieties.

Several wild species of rice (Oryzae) are promising sources of germplasm resistant to sheath blight and stem rot.

No chemicals more effective than Duter for control of stem rot and sheath blight have been identified. Prospects for registration of Duter are not encouraging.