By Rick Cartwright and Yeshi Wasmishe
Gaeumannomyces graminis var. graminis (Sacc.) Arx and D.Oliver causes crown (black) sheath rot disease of rice. This disease is common in Arkansas rice fields (MP192_Arkansas Rice production Handbook) but usually does not cause severe damage. The disease has also been named “Arkansas foot rot” (LSU Diseases ID and Management Series). High seeding and nitrogen fertilizer rates increase disease severity, but this disease is often masked by more important sheath blight or stem rot diseases. Historically, crown sheath rot was mostly observed in long grain rice in fields less frequently planted with rice or in new rice fields (MP192_Arkansas Rice production Handbook), probably because the fungus survives on weedy grasses. In 2014, a field planted to CL271, a new Clearfield© medium grain cultivar, was observed to have severe damage in Northeast Arkansas (Figs. 1 and 2) causing an estimated grain loss up to 20 percent.
The pathogen survives between crops in infected grass or rice residues and on other grass hosts. Infected rice plants of CL 271 showed mycelial mats running inside leaf sheaths (Fig. 5) and black perithecia (Fig. 6) (these contain spores (Figs. 3-4) protrude through the sheath tissues – both diagnostic features of the disease. Perithecia were easily seen with the unaided eye (Fig. 6).
Spores produced in perithicium (Fig. 3) known as ascospores (Figs. 3-4) were seen magnified using a compound microscope. Ascospores are long and cylindrical and are windborne or splashed in rain, easily moved. The disease probably starts early in the growing season, but symptoms become much more evident late in the season with damage noted late in grain fill or when the field starts to lodge at harvest. Damage can mean blanking or reduced grain fill, especially at the base of panicles on affected tillers. Rotting of the sheath and stem leads the crop to sink down and lodge similar to sheath blight (Figs. 1 – 2). Lesions inside the sheath are irregular-shaped and lighter in the center (Fig. 7).
Crown sheath rot disease Management:
Fungicides are not normally recommended for this disease. Historically, strobilurin fungicides have not been effective, while higher rates of propiconazole suppressed the disease if applied at early booting or before. Economic returns from fungicide application to control this disease are doubtful. Cultivar selection, optimal seeding and nitrogen rate should help manage the disease. The role of potassium in disease severity is not understood, but it is likely that rice plants growing in low potassium conditions will be more susceptible to this and many other diseases.
Acknowledgement: Pictures courtesy of Dr. Temesgen Mulaw, Scott Belmar and Tibebu Gebremariam .