Banded leaf and sheath blight (BLSB) is caused by the basidiomycete fungi Rhizoctonia solani. BLSB is a significant impediment to maize production in many hot and humid environments in the tropics and subtropics. In particular, BLSB is recognized as a serious constraint to maize production in China, South Asia and Southeast Asia.  In China, yield losses close to 100% have been attributed to BLSB.

Pathogen

• Rhizoctonia solani
• Teliomorph: Thanatephorus cucumeris

Symptoms

The disease develops on leaves and sheaths and can spread to the ears. Characteristic symptoms include concentric bands and rings on infected leaves and sheaths that are discolored, brown, tan or grey in color. Typically, disease develops on the first and second leaf sheath above the ground and eventually spreads to the ear causing ear rot. Ear rot is characterized by light brown, cottony mycelium on the ear and the presence of small, round, black sclerotia (compact mass of hyphae that can survive in unfavorable conditions). Ears dry prematurely and caking of the ear sheaths is common.

Confirmation

R. solani does not produce spores and is generally identified by characteristics of the mycelium and sclerotia. Mycelium is colorless when young, but assumes a light brown color as it matures. Under microscopic examination hyphae are multinucleate, hyaline, septate, and branch at right angles. Hyphae are typically constricted and septate at the point of branching. Septae contain a doughnut-shaped pore that enables nuclei, mitochondira and septae to migrate between cells. Hyphae are 4 to 15µm wide. On agar media, R. solani produces white to deep brown, cottony mycelium. Sclerotia are produced abundantly in culture and on infected plant parts. Sclerotia are typically 1 to 5mm in diameter, spherical, and dark brown to black.

R. solani is separated into 14 anastomosis groups based on hyphal fusion compatibility.  The anastamosis group AG1-IA is the causal agent of BLSB in much of Asia where the disease is a constraint to maize production.

Why and where it occurs

Disease severity is often closely associated with prevailing climatic conditions and farming practices.  Humid conditions and irrigated fields are highly favorable for the disease.  Additionally, high crop densities impact disease severity.

R. solani is widespread in tropical and subtropical regions of the world and once established in a field the fungus often remains indefinitely. Sclerotia of R. solani are known to survive for several years in the soil. As a result, crop rotations and tillage practice often have little effect on disease severity.

Host range

R. solani is also pathogenic to a wide range of cultivated crops. In addition to maize, anastomosis group AG1-IA is also pathogenic to rice, wheat, sorghum, bean (Phaseolus species), and soybean.

Life cycle

R. solani survives in the soil and on infected crop debris as sclerotia or mycelium. Sclerotia are known to survive for several years in the soil. The fungi spread by water (flooding), irrigation, movement of contaminated soil, and plant debris. At the onset of the growing season, in response to favorable humidity and temperatures (15 to 35°C), fungal growth is attracted to freshly planted host crops by chemical stimulants released by growing plant cells. The fungi infect plants, leading to characteristic symptoms on the stem, sheaths, leaves and ears. The fungi overwinter as sclerotia or in infected crop debris. 

Damage

• Mechanism of damage:Crop damage is caused by loss of photosynthetic leaf area due to foliar infection and stalk rot, leading to crop lodging.  Maximum damage is caused when ears are infected. In addition to ear rots, kernels are often wrinkled, dry, chaffy and light in weight.

• When damage is important:Damage is critical when infection occurs at early growth stages and spreads to the ear resulting in premature drying. Severe ear rot in China has resulted in yield losses close to 100%. Disease is most prevalent when maize is cultivated at high densities in very humid and warm regions. High irrigation levels also further disease severity.

• Economic importance: Yield losses close to 100% have been documented in southern China. In India, yield losses as high as 40% have been recorded.

Global distribution

Devastating epidemics of BSLB have been reported from Bhutan, China, India, Indonesia, Nepal, the Philippines, and Vietnam. Additionally, the disease has been reported in several African and Latin American countries. Generally the disease is most prevalent and severe in Asian countries where rice is intensively cultivated. R. solani (anastomosis group AG1-IA) is also the causal agent of Rice sheath blight.

Management principles

Host resistance

• Management of BSLB has been impeded by the lack of resistant commercial varieties.

• However, intensive efforts are being made to identify sources of resistance and where available tolerant germplasm should be cultivated.

Fungicides

• In the absence of host resistance, many farmers are reliant on the use of systemic fungicides for management of BSLB. Application of fungicide is economically viable when susceptible varieties are grown and climatic conditions favor disease severity.

• Treatment of the soil with fungicide prior to planting can reduce survival of the pathogen and reduce disease severity.

Biological control

• Several micro-organisms are known to parasitize Rhizoctonia species, including several fungi (Trichoderma, Gliocladium and Laetisaria species), bacteria, and nematodes (Aphelenchus avenae). Addition of these organisms to planting material or soils infested with Rhizoctonia may reduce disease pressure.  However, commercial biological control products remain at the experimental stages in most regions.

• Rhizoctonia decline is caused by various infections double-stranded RNAs that spread through anastomosis. Infection of Rhizoctonia with these RNAs can lead to reduction of disease severity and survival of Rhizoctonia in the soil.

Cultural control

• Composting of hardwood on Rhizoctonia-infested soil has been found to reduce disease severity, apparently by promoting the growth of Trichoderma and other antagonistic micro-organisms.

• Fields should be well drained prior to planting.

• Seeds should be planted on raised beds to avoid water contact and promote faster development.

• Soil in greenhouses can be sterilized prior to use.

References

Agrios, G.A. 1988. Plant Pathology. San Diego: Academic Press, Inc

Kumar, R. and I.S. Singh. 2002. Inheritance of resistance to banded leaf and sheath blight (Rhizoctonia solani f.sp. sasakii) of maize (Zea mays L.). In G. Srinivasan, P.H. Zaidi, B.M. Prasanna, F. Gonzalez, and K. Lesnick (eds.), Proceedings of the 8th Asian Regional Maize Workshop: New Technologies for the New Millennium, Bangkok, Thailand: August 5-8, 2002. Mexico, D.F.: CIMMYT. Pp. 356-65.

Saxena, S.C. 2002. Bio-intensive integrated disease management of banded leaf and sheath blight of maize. In G. Srinivasan, P.H. Zaidi, B.M. Prasanna, F. Gonzalez, and K. Lesnick (eds.), Proceedings of the 8th Asian Regional Maize Workshop: New Technologies for the New Millennium, Bangkok, Thailand: August 5-8, 2002. Mexico, D.F.: CIMMYT. Pp. 380-90.

Sharma, R.C., S.K. Vasal, F. Gonzalez, B.K. Batsa and N.N. Singh. 2002. Redressal of banded leaf and sheath blight of maize through breeding, chemical and biocontrol agents. In G. Srinivasan, P.H. Zaidi, B.M. Prasanna, F. Gonzalez, and K. Lesnick (eds.), Proceedings of the 8th Asian Regional Maize Workshop: New Technologies for the New Millennium, Bangkok, Thailand: August 5-8, 2002. Mexico, D.F.: CIMMYT. Pp. 391-97. 

Sneh, B., L. Burpee and A. Ogoshi 1991. Identification of Rhizoctonia species. St. Paul, MN: APS Press.

Zhao, M., Z. Zhang, S. Zhang, W. Li, D.P. Jeffers, T. Rong and G. Pan. 2006. Quantitative trait loci for resistance to banded leaf and sheath blight in maize. Crop Science 46:1039-1045.

Contributor: Biswanath Das