Rhizoctonia cerealis

Scientific Name Ceratobasidium cereale D.I. Murray & Burpee [teleomorph=sexual form], Rhizoctonia cerealis E.P. van der Hoeven [anamorph=asexual form]
Synonyms Ceratobasidium gramineum [syn.], Ceratorhiza cerealis (E.P. Hoeven) R.T. Moore [syn.]
Common Names English: Sharp eyespot; German: Scharfer Augenfleck; French: Rhizoctone des Céréales; Spanish: Mancha ocular
Description Rhizoctonia cerealis (Deuteromycetes, Agonomycetales) survives as mycelium or sclerotia in plant debris or sclerotia in the soil.

Biology

Damage

Sharp eyespot causes sometimes elliptical, often irregular light brown lesions circumscribed by a thin necrotic, darker margin (name!), on the outer leaf sheaths. This symptom is hard to differentiate from eyespot (strawbreaker, foot rot) caused by Oculimacula spp., but the lesions tend to be more superficial. Affected tissues rot leaving a hole in the sheath. Several lesions (<10 mm in diameter) may occur at the base of the same plant. Severe infections cause seedling blight, but most infected plants survive to maturity.

Lesions on the stem are similar to those on sheaths. They are also similar to eyespot symptoms but are present up to 30 cm above the soil and do not contain the carbon-like structures found with infection by Oculimacula species. Light gray mycelium is often formed beneath lesions on mature stems.
Severe infection may result in premature ripening. Whiteheads are formed and stems may lodge at the second or third internode. Affected plants do not exhibit twisting and lodging at the first internode that is typical for eyespot.

Lifecycle

The fungus does not form asexual spores and the teleomorph Ceratobasidium cereale (Basidiomycota, Ceratobasidiales) is rare in nature. Ceratobasidium forms a hymenium with ellipsoid to oval basidia (7-19 µm) with ovoid basidiospores (6-12 x 3-6 µm). As it is not observed in association with disease symptoms the importance of the perfect stage to the epidemiology of sharp eyespot is unknown.

Cause

Sclerotia are dark and irregular in shape and lack a distinct rind. They often occur within the stem or between the leaf sheaths. The mycelium is characterized by rectangular branching (T-shaped cells) of binucleate hyphae with dolipore septa. Mycelia from infected debris or from germinating sclerotia are the primary inoculum. Plants may be infected anytime during the growth season. Infection occurs when root or outer leaf sheath tissue is penetrated. The pathogen grows into the stem and sclerotia are formed in this structure or between the lower leaf sheath and stem. Infection is favored by cool, wet soils while subsequent disease development is optimum in cool dry soils.

Occurrence

Additional Crop Information

Sharp eyespot occurs on wheat, barley, and rye; oats are less susceptible. Strains of C. cereale also cause yellow patch in turfgrass.

Agricultural Importance

As sharp eyespot is generally not considered a serious disease on wheat control is not necessary in most cases. Disease incidence tends to be greater in continuously cropped cereals.

Control

Integrated Crop Management

Incidence of sharp eyespot is increasing in areas where farmers use fungicides for eyespot (Oculimacula spp.). Crop rotation and late seeding of wheat contribute to avoid effects of the disease.
PCR-based diagnostic kits are available for the differentiation of fungal pathogens involved in the foot rot complex.

Chemical Control

Specific fungicide applications against sharp eyespot are rarely necessary due to the limited importance of the disease in most regions. However, broad spectrum azole fungicides such as tebuconazole, prochloraz and prothioconazole will reduce sharp eyespot when applied against other fungal diseases.

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