Bacterial leaf blight: BLB of rice
BLB has been known as epidemic in Japan since 1881. Whereas in Nepal it was reported for the first time in 1965.
BLB was confined in plains but few years ago, disease has been reported from hilly region including Kathmandu.
Damage is due to the partial or total blighting of leaves or due to complete wilting of the affected tillers leading to unfilled grains.
Casual organism: Xanthomonas campestris pv. oryzae
Gram negative, rod shaped bacteria.
Symptoms of Bacterial leaf blight:
- In leaf, disease usually appears 4-6 week after transplanting.
- Linear yellow to straw colored stripes appears generally on both edge of the leaf in wavy fashion.
- The stripes usually start from tip and extend downward.
- Drying and twisting of leaf tip and rapid extension of margin blight lengthwise and crosswise and covers the larger areas.
- Blight may extend to leaf sheath and culm killing the tillers or the whole clump.
- In dry weather, opaque and turbid drops of bacterial ooze can be seen on the leaves which dry into yellowish beads.
- Glumes and even endosperm of seeds also get infected, but symptoms are not well defined.
- Microscopic examination of affected leaves show ooze at cut ends.
- Seed borne inoculums or early infection results ‘kresek’ or wilt phase which is much destructive.
- Leaves roll completely, droop, turn yellow or grey and ultimately tiller withers away.
- ‘Kresek’ affected symptoms may be confused with stem borer attack, however, in ‘kresek’ phase, tillers are not pulled easily.
- Another common symptom of the disease in the tropics is pale yellow leaf phase.
- Some of the youngest leaves in a clump become pale yellow or whitish, which later turn yellowish brown and wither.
- Yield loss varies from 6-60%
*kresek: It is the systemic infection which is characterized by wilting, desiccation and death of leaf.
Disease cycle of BLB in rice:
- Primary source of inoculum: seeds, straw, leaves, even stubbles, wild rice and other grasses eg. Cyperus rotundus, Panicum repens
- Secondary infection: Ooze which is disseminated by rain and other carriers like grasshopper, leafhopper externally/mechanically but not internally.
- Penetration takes place through wounds and stomata.
Epidemiology of BLB in Rice:
- High temperature i.e >25°c, high R.H., cloudy weather or with a little rain, poorly drained condition are favorable for disease development.
- No symptoms below 20°C
- High nitrogen fertilization and shade condition aggravate disease.
Management of BLB in Rice:
- Burning the stubbles, remove weed host.
- Balanced use of fertilizers
- Avoid clipping of tip of seedling at the time of transplantation.
- Avoid flooded conditions.
- Use resistant varieties: IR20, IR 26, CH 45, Basmati, Pokhreli masino.
- Seed treatment by soaking seeds in 0.025% Agrimycin+0.05% Cerasan for 12 hours and then transferring to hot water at 52-540c for 30 minutes.
- Five spray of Agrimycin (250 ppm)+ Copper oxychloride (0.05%) at 12 days interval reduce the disease.