Bipolaris
sorokiniana (teleomorph Cochliobolus sativus)
Prepared
by Nikki D. Charlton
PP728 Soilborne Plant
Pathogens: a class
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).
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 Ð 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).
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| 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).
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| 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). |
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| 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 16-40¼C, with optimal soil temperatures of 28-32¼C. Moist soils during planting promote infection
and colonization by soilborne inoculum as well (3).