Bipolaris sorokiniana (teleomorph Cochliobolus sativus)

Prepared by Nikki D. Charlton

PP728 Soilborne Plant Pathogens: a class project

 


Introduction

Bipolaris sorokiniana (commonly referred to the sexual stage Cochliobolus sativus) causes common root rot of small cereal grains and numerous grass hosts.  Plants with common root rot produce fewer tillers and fewer kernels per ear.  Grain yield losses due to common root rot and seedling blight for Canada, Scotland, and Brazil have been estimated at 15, 10, and 20%, respectively (3).

 


Host Range and Distribution

Common root rot occurs in cereal-growing areas, and is prevalent in the warmer cereal-growing regions.  Hosts include barley, spring wheat, rye, and weed and grass species (1).  Wheat and barley are the most economically important hosts (3).


Isolation

Cochliobolus sativus is easily recovered from host tissue.  Sporulation of the asexual stage, Bipolaris sorokiniana can be induced by moist chamber incubation.  For isolation from the host tissue, the tissue is washed under running water, which can range from 20 min. to 2 hr.  The tissue is then plated onto agar media.  The tissue also may be surface disinfested using 0.5% NaOCl for 3-5 min, rinsed in sterile water and then plated on agar medium.  Several isolation media can be used, which include water agar, PDA, half-strength PDA, V-8 agar, and glucose-mineral salts agar.  Adding 2-10 mg benomyl/L of standard media can enhance isolation from mixed infections (5).

 

C. sativus is more difficult to recover from soil without certain procedures.  D-R (Dodman and Reinke) medium is commonly used and contains the ingredient Rose Bengal.  The D-R selective media is used to make soil dilution plates using the Warcup plate method (5).  For the Warcup method, 0.0005 0.15 g of the soil sample and 1 drop of water is added to a petri dish and mixed together.  Approximately 8-10 ml of the agar medium is then added to the plate and swirled to distribute the soil into the medium and incubated (4).

 


Identification

Cochliobolus sativus is an ascomycete in the Class Loculoascomycetes, while Bipolaris sorokiniana is in the Hyphomycete class (Phylum Deuteromycota).  The mycelium of C. sativus is dark olive-brown.  Conidiophores are produced abundantly by B. sorokiniana, which may be single or clustered and measure 6-10 x 110-220 mm with septations.  Conidia are deep olive-brown in color, oval-shaped, with rounded ends and a basal scar.  Some conidia may have a slight curvature.  Conidia measure 15-28 x 40-120 mm with 3 to 10 septations.  The conidial walls are smooth and distinctly thickened at the septa (1,2).

 

 

AppleMark

Conidiophore and conidia of Bipolaris sorokiniana. Photo courtesy APS (M. Wiese).   Conidia of Bipolaris sorokiniana.  Photo courtesy APS (D. Mathre).

 

 


Symptoms

Diseased seedlings develop dark brown areas on the seed and/or subcrown internode stems below the soil line.  Crown rot begins to develop later in the season.  Common root rot is distinguished by dark-brown to black necrotic lesions on roots, subcrown internodes, and the base of stems.  Multiple lesions often coalesce to form large areas of necrosis in the crown.  If the plant is pulled, the roots may tear off at the crown due to brittle roots.  Aboveground symptoms of infected plants are stunting and reduced tillering. Severely diseased stems may lead to premature death, resulting in whiteheads.  Kernels become shriveled and roots become dark brown and rotted.  Yields may be reduced due to root rot even though symptoms are not well-developed (1,2,3).

 

Crown infection by Bipolaris sorokiniana.
Photo courtesy APS (L. Piening).
White heads resulting from plant stress associated with root disease. Photo courtesy APS (BASF). Seedling blight caused by Bipolaris sorokiniana. 
Photo courtesy APS (I.L. Stevenson).

 

 

 

 

 

Plants with common root rot (left); healthy plants (right).  Photo courtesy APS (L. Piening).   Subcrown internodes infected by Bipolaris sorokiniana (left); healthy plants (right). 
Photo courtesy APS (R. Tinline).

 


Ecology and Life Cycle

Bipolaris sorokiniana is a saprophyte and survives primarily as thick-walled conidia.  It can also survive as mycelium in soil or crop debris.  The sexual stage is not important in the disease cycle.  Primary inoculum includes mycelium from infected seed, conidia in the soil, and conidia on the kernel surface (3).  Conidia germinate in the presence of susceptible hosts and initiate primary infection on the coleoptile, or primary roots.  B. sorokinana penetrates the host tissue either directly through the epidermis, or through natural openings or wounds.  Appressoria and dome-shaped infection cushions are formed prior to penetration.  Infection pegs form beneath the appressoria and infection cushions.  Infection continues from the epidermis to the cortex and endodermis, resulting in disintegration of the tissue.  Colonization of infected plant parts progresses by the spread of conidia (2). 

 

Any movement of soil by wind, water, and implements can move inoculum of the pathogen.  Infested seed can also serve as a means of dissemination of the pathogen over long distances (2).  Dissemination of secondary inoculum is not important for continued disease development below ground, but provides inoculum for subsequent crops (3). 

 

Environmental factors play an important role in the severity of disease caused by B. sorokinana and other root rot fungi.  Warm soil temperatures favor growth of B. sorokinana, where disease occurs between 16-40C, with optimal soil temperatures of 28-32C.  Moist soils during planting promote infection and colonization by soilborne inoculum as well (3). 

 


References

1. Jones, D.G., and Clifford, B.C. 1983. Cereal diseases, their pathology and control. John Wiley & Sons Ltd.

2. Mathre, D.E. 1987. Compendium of Barley Diseases. American Phytopathological Society, St. Paul, MN.

3.  Murray, T.D., Parry, D.W., and Cattlin, N.D. 1998. A color handbook of diseases of small grain cereal crops. Iowa State University Press, Ames, Iowa.

4. Singleton, L.L., Mihail, J.D., and Rush, C.M. 1992. Methods for research on soilborne phytopathogenic fungi. American Phytopathological Society, St. Paul, MN

5. Stack, R.W. 1992. Bipolaris. Pages 94-99 in: Methods for research on soilborne phytopathogenic fungi. Singleton, L.L., Mihail, J.D., and Rush, C.M., eds. American Phytopathological Society, St. Paul, MN.


Links to other sites

 

Pest Management Plant Diseases, Common Root Rot Manitoba Agriculture and Food

Root and Crown Rots of Small Grains

Bipolaris sorokinana, a cereal pathogen of global concern: cytological and molecular approaches towards better control

Bipolaris sorokiniana


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