Cochliobolus sativus

Scientific Name Cochliobolus sativus (S. Ito & Kurib.) Drechsler ex Dastur [teleom.] Bipolaris sorokiniana (Sacc.) Shoemaker [anam.]
Synonyms Drechslera sorokiniana (Sacc.) Subram. & B.L. Jain, Helminthosporium sativum Pammel, C.M. King & Bakke [obs.], Ophiobolus sativus S. Ito Kurib. [obs.].
Common Names English: Common root rot, dryland root rot, root and foot rot, spot blotch, black point (kernel blight); German: Braunfleckenkrankheit, Wurzelhals- und Stängelfäule.
Description C. sativus is an ascomycetous fungus (order Pleosporales), however, the teleomorph is not involved in disease epidemiology. Pseudothecia are dark-brown to black, bottle-shaped, up to 530 µm wide, with a cylindrical to paraboloid pore 80-110 µm long. Asci are cylindrical to claviform, 1-8 spored, bitunicate, 110-230 x 30-45 µm. Ascospores are hyaline to light-brown, 6-13 septate, filiform, coiled in a helix in the ascus, often surrounded by a thin, mucilaginous sheath, and 160-360 x 6-9 µm. Pseudoparaphyses are hyaline, filiform and branched.

Biology

Damage

Depending on the cultivation area and climatic conditions, C. sativus may affect all parts of wheat plants causing various diseases:
Seedling blight with first symptoms as brown lesions on coleoptiles, sub-crown internodes, roots and culms of seedlings. Colonization of the basal stem causes highest damage. Early infections may not produce symptoms, but result in reduced leaf area at GS 12 when the number of tillers is determined, limiting plant density and yield.
Common root rot, caused by C. sativus or often in a complex with Fusarium spp., is associated with dark-brown or blackened sub-crown internodes and roots. Due to the incidence the disease is hard to diagnose. Infection may extend into the crown and reach the culm. Diseased plants occur randomly or in irregular patches and are stunted and often chlorotic. Leaf spots (spot blotch) due to C. sativus often coincide with root infections that advance to foot and basal stem, and are favored by rainy periods. Distinct, elongate, brown-black lesions rarely exceeding 10 mm in diameter are most noticeable after heading and most frequent on lower leaves.
Head blight (premature blight) due to C. sativus is associated with premature bleaching of one or more spikelets or the entire head on emerged and immature heads. The tissue above infected parts of the rachis is affected producing sterile spikelets or shrivelled, discolored seeds (black point).

Lifecycle

The asexual olive-brown conidia are straight to curved, spindle-shaped to broadly ellipsoidal (40-120 x 17-28 µm), 3-12 distoseptate. They are produced on up to 220 µm long and 6-10 µm wide, mid-brown conidiophores, solitary or in small groups, straight to flexuous, sometimes geniculate. Conidiogenous cells are terminal, sympodial and cylindrical. The mycelium is usually deep olive-brown, sometimes forming also chlamydospores.
The primary inoculum of C. sativus is thick-walled conidia surviving on crop debris and in the soil; conidia formation starts when leaves senescence. The pathogen is seed-borne on several crops inciting symptoms and damage on seedlings and primary leaves. Seed infection is often associated with reduced seedling emergence.
In the soil C. sativus forms an extracellular sheath on germ tubes and appressoria attached to the root surface of host plants. In the epidermis and outer cortex pathogen growth is predominantly intracellular, whereas hyphae in the inner cortex are mainly intercellular. Cell death in the root cortex is correlated with fungal biomass.
Germination of conidia is followed by mycelial development, penetration of tissues, and brown-to-black lesions on the lemma, palea and pericarp. Infected seeds typically exhibit light-brown to dark-brown discrete lesions and a dull-white spherical or elliptical area in the centre ('eye-spot').

Cause

Airborne secondary inoculum cause foliar disease and reduces grain yield. Infection of leaves depends on high relative humidity and is favored by warm weather. Early infections at heading cause higher losses than later ones. C. sativus produces various phytotoxins with prehelminthosporol being the main toxic compound; it interferes with enzyme activities in the host plasma membranes and affects synthesis of 1,3-beta-glucan, proton pumping and ATP dependent Ca2+ transport. Non-selective toxins produced by C. sativus can induce chlorophyll loss.

Occurrence

Additional Crop Information

C. sativus infects barley, wheat, oats, rye, triticale, corn and various grass species.

Agricultural Importance

Diseases caused by C. sativus are widespread and may be serious wherever wheat is grown. Economic impact is high in dry soils like the Great Plains of North America, Australia, Argentina and Brazil, India as well as in Russia. Yield losses result from reduced numbers of tillers and kernels per tiller; seed quality (i.e. reduced kernel weight and seed germination, seed discoloration).

Control

Integrated Crop Management

Although resistant cultivars are available, disease tolerance of some cultivars (i.e. cultivars able to maintain higher yields under severe disease pressure) may reduce the losses.
Disease intensity is related to the inoculum density of C. sativus in the soil. Burning wheat residues reduces the pathogen population in the soil. In general, severity of common root rot decreases with crop rotation and reduced tillage practices due to changes in seeding depth, soil temperature, soil moisture and microflora.

Chemical Control

Seed treatments with chemical and some microbial fungicides reduce root rot deriving from seed borne inoculum. However, as infections can occur during the whole growing season, efficacy is limited. Standard seed treatments with broad spectrum azoles such as triadimenol, tebuconazole, triticonazole, fluquinconazole or with the dicarboximide iprodione show some suppression of the disease.
Fungicide sprays against the later disease stages with azoles such as tebuconazole may provide effective control of spot blotch and result in higher test weight, kernel weight and yield of wheat. However, as productivity is often low in the affected regions it should be checked if a specific spray treatment against spot blotch and head blight is cost-effective.

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