Aggregate Sheath Spot of Rice

Aggregate sheath spot lesions first appear on the lower leaf sheaths at the waterline during the tillering stage. Lesions are circular to elliptical with gray-green to straw-colored centers surrounded by distinct brown margins. Frequently, additional margins form around the initial lesion producing a series of concentric bands. A strip of light-colored necrotic cells runs down the lesion center. As the season progresses, aggregate sheath spot lesions may expand vertically, frequently reaching upper leaf sheaths of the plant and occasionally affecting the panicle.

The disease cycles of aggregate sheath spot and stem rot are similar in several ways. Rhizoctonia oryzae-sativae produces dormant bodies called sclerotia that are resistant to adverse conditions and allow the fungus to persist between rice crops in the soil or infected crop debris. Sclerotia are the initial inoculum source for aggregate sheath spot disease. In spring and early summer, they float to the surface of field flood water, germinate, and may infect rice leaf sheaths at the water line. Unlike the sclerotia of the stem rot fungus, the sclerotia of the aggregate sheath spot pathogen are brown, rectangular to irregularly globose, and are much larger in size. In addition, aggregate sheath spot lesions tend to expand vertically up the stem and may reach the panicle under favorable conditions whereas stem rot lesions tend to penetrate into the culm of the stem.

On the sheath, lesions often coalesce and may cover the entire leaf sheath. Leaves of diseased sheaths turn bright yellow and then die. Under conditions favorable for the fungus (high humidity or rain), the disease may spread to the flag leaf and panicle rachises, and on rare occasions kill entire tillers. Late in the season, the fungus begins producing new sclerotia on or in diseased tissue. These sclerotia overwinter in crop residue or in soil and serve as inoculum for initial infections of subsequent rice crops.

The aggregate sheath spot pathogen may also colonize the culm, where it may cause a culm rot of rice tillers, but this aspect of the disease is rare in California.

Aggregate sheath spot is similar to, but distinct from, sheath blight of rice which is caused by Rhizoctonia solani and is a serious disease of rice in the southern U.S. and other parts of the world. However, sheath blight has not been observed in California.

The most effective way to manage aggregate sheath spot is to limit the quantity of sclerotia carried over from one year to the next by removing or destroying crop residues. Mid-season fungicide treatments may be necessary if field monitoring indicates aggregate sheath spot lesions are approaching the flag leaf sheath.

Disease cycles of aggregate sheath spot and stem rot are similar, and the diseases are managed with similar methods, i.e., use of the most tolerant varieties available and cultural practices that reduce carryover inoculum. All public rice varieties currently grown in California are susceptible to aggregate sheath spot of rice to some degree.

Burning of crop residues after harvest provides the most effective management of this disease. Nearly complete removal of infected crop residues by cutting rice near ground level, bailing and removal of the straw from the field may also reduce the amount of carryover inoculum in fields with high disease incidence. Moldboard plowing, crop rotation, or fallowing may also reduce carryover inoculum but may require multiple years without rice cultivation, and results may be inconsistent.

Dense rice stands may favor more severe disease development and should be avoided. Lastly, maintaining appropriate soil potassium levels is helpful, as potassium deficiency results in rice plants that are more susceptible to aggregate sheath spot disease.

All cultural controls discussed above are acceptable in a certified organic crop.

Monitoring is essential in making treatment decisions for aggregate sheath spot. After tillering, examine tillers on a weekly basis in several locations throughout the field for the presence and expansion of aggregate sheath spot lesions. If lesions have begun to elongate and are approaching the flag leaf sheath or the leaf sheath below the flag leaf, a treatment may be justified. Apply fungicide at the early heading stage, when the first panicles start to emerge from the boot and are visible above the canopy. A well-timed fungicide treatment will reduce the severity of the disease and may prevent potential yield and quality losses.