Aggregate sheath spot lesions first appear on the lower leaf sheaths at the water line 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. Later in the season, secondary infections frequently occur well above the water line.
Comments on the Disease
The disease cycles of aggregate sheath spot and stem rot are similar in a number of ways. Rhizoctonia oryzae-sativae produces sclerotia that are resistant to adverse conditions and allow the fungus to overwinter. In spring and early summer, these overwintering bodies float on the field water surface and are the source of initial infections that occur at the water line. Unlike the sclerotia of the stem rot fungus, these sclerotia are brown, rectangular to irregularly globose in shape and are much larger in size. Also, unlike stem rot, secondary leaf sheath infections progress up the stem and, under favorable conditions, may reach as high as the panicle. On the sheath, lesions often coalesce and may cover the entire leaf sheath. Leaves of diseased sheaths turn bright yellow and then die. Under favorable conditions (high humidity or rain), the disease can spread to the flag leaf and panicle rachises, killing entire tillers. Later in the season, the fungus begins to produce new sclerotia on or in diseased tissue. These sclerotia overwinter in crop residue or in soil.
Aggregate sheath spot of rice can also colonize the culm, where it may cause a culm rot, but this aspect of the disease is rare in California.
Aggregate sheath spot is similar to but distinct from sheath blight of rice caused by Rhizoctonia solani, a serious disease of rice in the southern U.S. and other parts of the world. Sheath blight has not been observed in California.
The most effective way to manage aggregate sheath spot is to limit the carryover inoculum from one year to the next by removing or destroying crop residues. Treatments may be necessary if monitoring indicates leaf lesions are approaching the flag leaf sheath.
Disease cycles of stem rot and aggregate sheath spot are similar and they are managed with similar methods, i.e., use of the most resistant 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 control for this disease. Complete removal of infected crop residues also minimizes carryover inoculum levels. Moldboard plowing, crop rotation, or fallowing should also minimize carryover inoculum. Avoid dense rice stands as they may enhance disease development.
Organically Acceptable Methods
All the cultural controls discussed above are organically acceptable.
Monitoring and Treatment Decisions
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 progress 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. Treat before lesions spread to the leaf sheath on the leaf below the flag leaf. When making a treatment decision, also consider the crop growth stage. For example, lesions that are approaching the flag leaf before boot are generally considered to be a greater risk than lesions approaching the flag leaf after flowering.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Quadris)||12.3–15.4 fl oz||4||28|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)|
|COMMENTS: A protectant fungicide. Follow label directions. Limited studies have shown a single application at the lowest labeled rate to be effective, but results may vary under different conditions. Under heavy disease pressure and conditions favorable for disease development, a second application may be applied. Water holding period is 14 days.|
|B.||AZOXYSTROBIN + PROPICONAZOLE|
|(Quilt Xcel)||14–27 fl oz||12||35|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Demethylation inhibitor (3)|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action Group number.|