गेहूं का झोंका (बलास्‍ट): एक विनाशकारी बीमारी

Wheat is the second most important food crop in the world and a staple food crop providing 20% of protein to the human population.  The production of wheat is over 700 million tonnes in the world and is cultivated over large area under varied climatic conditions.

Wheat production is affected by a number of diseases like rusts, karnal bunt, powdery mildew, loose smut, flag smut, leaf blight, head scab etc. Among these diseases affecting wheat production in the world, wheat blast is one of the devastating disease affecting wheat. Leaf blast, spike blast,collar rot node blast, or rotten neck blast are other names of this dreaded disease.

Blast disease was first identified in the state of Parana, Brazil in 1985 and since then spreading to an area of about 3.0 mha causing losses of 10-100% depending on years, genotypes, planting date and environment. It is reported in central and southern Brazil, low lying areas of Santa Cruz region of Bolivia, Southern and south eastern Paraguay, and North eastern Argentina.

It appeared in Bangladesh for the first time in February 2016 covering 15% of the total wheat area in Bangladesh.This disease can result in 100% yield losses. The pathogen can attack all above ground parts like leaves, stems, and panicles. Losses will be highest when fungus infect base of the rachis and killing entire spike.

The epidemic outbreaks in Paraguay and Santa Cruz region of Bolivia resulted in more than 70% and 80% yield losses. A recent outbreak in Bangladesh has affected wheat in 15000 hectares with reported losses in production by 20%. Countries surrounding Bangladesh should be vigilante to prevent this disease entry.

Etiology of the disease

This disease is caused by the fungus Magnoportheoryzae Patho type Triticum. This pathogen also attacks Triticale, barley and black oats. Host-specific sub-groups or patho types specialized for infecting rice (Oryzapathotype), wheat (Triticumpathotype), Ryegrass (Loliumpathotype), Foxtail millet (Setariapathotype), and many other plant species. These crop-specific isolates may occasionally infect plants from other genera.

Symptoms :

The disease affect all above ground parts of wheat plants. Infection on ear head is the most important symptom of this disease. Fungus can infect glumes and rachis. Elliptical lessions with reddish brown to gray margins with white to light brown margins will appear on glumes. The infection on rachis leads to blackening of rachis leading to death of entire ear head which leads to bleached appearance of ears, shriveled or no grains.

Since the fungal infection attacks and blocks the translocation of nutrients to spike above the point of infection resulting in complete or partial sterility of the earheads. Depending upon the point of the infection on the rachis, the disease can kill the spike partially or fully.

However in late infections results in formation of grains which carry the pathogen for next crop season.Under warm temperatures and high humidity conditions, if the Infection occurs in the seedlingstage, could lead to100% death of plants.

The pathogen producespyriform three celled conidia with two septae which is the most characteristic feature of this pathogen.  The each conidium cell has single nucleus. Melanin content in the conidial appresoria helps in building high pressure which enables the fungus to rupture the plant surface in order to enterplant tissue.

Within 4 days fungal colonization will occur without any external symptoms.So the outbreak of the disease is difficult to predict.  Fungus spreads to the new distant areas mainly through seeds.Seed is the primary source of inoculum.

However, infection of spike occurs through air borne conidia from various secondary hosts.Certain weed species such as Brachiariaplantaginea, Digitariainsularis, D. sanguinalis, Eleusineindica, Pennisetumsetaseum, Rhynchelytrumroseum, and Setaria geniculate were reported to be collateral hosts ofPyricularia(=Magnoporthe) species.

The wheat blast pathogen prefers high humid conditions resulting due to continuous rainfallalong with temperature ranging 18-20⁰ C.  The minimum temperature for infection was reported to be is 100⁰ C and the maximum is 32⁰ C, with optimum between 25⁰ and 30⁰ C and sporulation is favoured by high relative humidity.

It was also reported that blast intensity was highest at 30⁰ C under controlled conditions with a wetting period of less than 10h. Sprinkler irrigation after flowering may predispose wheat to blast.

Management of wheat blast

• Very strict quarantine measures should be in place to prevent flow of wheat seeds from areas with blast incidence. Seeds harvested from infected crop should never be used as seed material in the next cropping season.
• Weed hosts which harbor pathogens should be destroyed. Following crop rotation with non-host species like pulses and other crops will helps to reduce the inoculum.
• Sprinkler irrigation which predisposes the wheat plants should be avoided especially during and after flowering.
• Host resistance Complete to date genetic bases of resistance to blast is not very clear due to tricky and variable pathogen. Till now no varieties or germplasm found to be completely resistant to blast disease. However some of the varieties like BR18, IPR85, CD113 and a CIMMYT line Milan are reported to be carrying some degree of resistance.
• Seed treatment with tricyclazole 75WP @2g/kg or carbendazim 50WP 1g/kg seed will help in controlling seed borne inoculum.
• Spray with tricyclazole 75WP 0.6g/L or Propiconazole 25EC/ carpropamid 30SC 1ml/L or Isoprothiolane 40EC 2g/L can provide protection against this disease.
• Reports also suggest that spray with epoxiconazole+ pyraclostrobin and tebuconazole+ trifloxystrobinalso reduce the disease.

Authors:

Prakasha T.L. and J.B. singh

ICAR-Indian Agricultural Research Institute, Regional Station, Indore-452001

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