Bipolaris sorokiniana (Common Root Rot and Spot Blotch of Barley and Wheat)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Dothideomycetes
Subclass	Pleosporomycetidae
Order	Pleosporales
Family	Pleosporaceae
Genus	Bipolaris

Scientific Name

Bipolaris sorokiniana

Scientific Name Synonyms

Helminthosporium sativum

Cochliobolus sativus

Common Name

Cochliobolus leaf spot

Author:Tom Creswell, Purdue University

Pathogen

Bipolaris sorokiniana (Sacc.) Shoemaker is an ascomycete fungus with a wide host range in the Poaceae family. It is of greatest economic importance as the cause of seedling diseases, common root rot and spot blotch of barley and wheat. Mycelium: deep olive brown on plant material. Conidiophores: single or clustered, simple, septate, erect, 6-8 x 110-150 μm Conidia: Produced terminally, olive brown, eliptical with tapered ends and a prominent hilum (basal scar). 15-20 x 60-120 μm, 3-10 septate, slightly curved, conidial wall smooth with thickenings at the septa.

The sexual reproductive stage [Cochliobolus sativus (S. Ito & Kurib.) Drechsler ex Dastur] is rarely seen in nature but may be induced to form in pure culture with compatible mating types. Pseudothecia: Dark brown to black, globose, 300-440 μm diameter Asci: Clavate, one to eight spored; bitunicate; 20-45 x 120-250 μm Ascospores: Hyaline to light brown, with 4-14 septa, filamentous and spirally arranged in the ascus, 5-10 x 200-450 μm.

Synonyms include: Helminthosporium sorokinianum Sacc. 1891, Helminthosporium sativum Pammel, C.M. King & Bakke 1910, Drechslera sorokiniana (Sacc.) Subram. & B.L. Jain 1966, Cochliobolus sativus (S. Ito & Kurib.) Drechsler ex Dastur 1942 and Ophiobolus sativus S. Ito & Kurib. 1929

Symptoms and Signs

B. sorokiniana causes seedling blight, leaf spotting (spot blotch), root rot and black point of wheat and barley. Infected seedlings have brown lesions on roots, coleoptiles and subcrown internodes. Brown lesions are also a feature of the crown and root rot infections pm older plants. Leaf spotting (spot blotch) shows up as small brown lesions less than 1 cm in diameter, which may coalesce into large elongated blotches of necrotic tissue. Spot blotch appears more frequently in wet weather, on lower leaves and following head development.

Ecology and Spread

Although the fungus may persist for months in host debris or as conidia in the soil it is considered a poor saprophyte. Seedlings or young plants become infected at the roots, crown or other below ground parts. As infections spread to above ground stems conidia develop and secondary spread of conidia occurs with the help of wind or splashing rain. Seedling disease is favored by stress factors such as planting in hot, dry soils. Warm humid conditions favor development of the spot blotch phase.

Geographic Distribution

B. sorokiniana is ubiquitous wherever wheat and barley are grown.

Management

Rotation with non-host crops can reduce inoculum but may not be practical due to the wide host range. Partially resistant or tolerant varieties have been developed for both barley and wheat but can not be used as the sole management tool. Recommendations include planting healthy seed, shallow planting in cool soil and avoiding nutrient, moisture and other stress factors. Seed treatment with fungicides may also be useful. Foliar fungicides may have limited utility due to the need for multiple applications. Consult your local extension specialist or agent for recommendations relevant to your particular host and state. Remember: the label is the law.

Diagnostic procedures

Incubation of affected leaves should produce abundant sporulation of the conidial stage. The fungus is readily isolated from infected seedlings and roots using non-selective media. Selective media are required for recovery from soil. In pure culture the fungus produces a fluffy cottony mass with colors ranging from white to gray or dark gray, depending on the isolate.

Resources and References

• W. W. Bockus, R. L. Bowden, R. M. Hunger, W. L. Morrill, T. D. Murray and R. W. Smiley. 2010. Compendium of wheat diseases, third edition. The American Phytopathological Society, St. Paul, MN.
• Kumar, J., P. Schäfer, R. Hückelhoven, G. Langen, H. Baltruschat, E. Stein, S. Nagarajan and K. Kogel. 2002. Bipolaris sorokiniana, a cereal pathogen of global concern: cytological and molecular approaches towards better control. Molecular Plant Pathology 3(4): 185-195.
• D.E. Mathre. 1997. Compendium of barley diseases, second edition. The American Phytopathological Society, St. Paul, MN.
• Sivanesan, A. and P. Holliday 1971. Cochliobolus sativus. CMI Descriptions of Pathogenic Fungi and Bacteria. Set 71, Sheet 701.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30818 as part of "Diagnostic Image Series Development for Supporting IPM in the North Central Region" (USDA-NIFA-RIPM-003349)