Rice blast

Asexual stage: pyricularia oryzae ( p. grisae )

Sexual stage:  Magnaporthe grisea

First time reported in Nepal in 1966 A.D.

The typical symptoms appear on leaves, leaf sheath, rachis, nodes and even the glumes are also attacked.

Leaf blast

• Small water soaked bluish green specks,soon enlarge diamond shaped spots with grey centre and dark brown margin.

Node blast:

• In infected nodes, irregular black areas that encircle the nodes can be noticed.
• The affected nodes may break up and all the plant parts above the infected nodes may die (Node blast).

Collar blast:

• The collar of a rice plant refers to the junction of the leaf blade and the sheath.
• Necrosis at the union of the two tissues.
• Collar infections can kill the entire leaf and may extend a few millimeters into and around the sheath.

Neck or Panicle blast:

• The neck of the rice plant refers to that portion of the stem that rises above the leaves and supports the seed head or panicle.
• At the flower emergence, the fungus attacks the peduncle which is engirdled, and the lesion turns to brownish-black.
• Referred to as rotten neck/neck rot/neck blast/panicle blast.
• In early neck infection, grain filling does not occur and the panicle remains erect like a dead heart caused by a stem borer. In the late infection, partial grain filling occurs.
• Lesions can be found on the panicle branches, spikes, and spikelets.
• The lesions are often gray brown discolorations of the branches of the panicle, and, over time, the branches may break at the lesion.

Rice seed

• Brown spots, blotches, and occasionally the classic diamond-shaped lesion often seen on leaves.

Etiology:

Gray conidiophores that bear terminal, pear-shaped, mostly two-septate conidia

Favorable condition:

• Application of excessive doses of nitrogenous fertilizers,
• Intermittent drizzles,
• cloudy weather,
• high relative humidity (93-99 per cent),
• low night temperature (between 15-20 0C or less than 26 0C)
• more number of rainy days, longer duration of dew, cloudy weather
• Slow wind movement and availability of collateral and alternate host.

Survive and spread:

• Survive: Mycelium and conidia on soil, debris, seed, and collateral host.
• Spread: air born conidia, transfer through wind, rain.
• Toxin: Pyricularin and alpha picolinic acid contribute to the rice blast.
• Alternate hosts: Digitaria marginata, Panicum repens, Dinebra retroflexa.

Collateral host: Sugarcane

Management:

• Resistant varieties:

Leaf blast: Barkhe 2014, Barkhe 2024, Super 004.

blast: Barkhe 1005, Barkhe 3004, Barkhe 1006

• Seed treatment with Pseudomonas fluorescens (Biocare-B) @10 gm/lit of water for 30 min.
• Seed treatment with Bavistin or carbendazim @2-3 gm/kg of seeds before bed preparation.
• Seedling root dip treatment with P. fluorescens @ 4gm/lit of water for 30 min.
• Spray hexaconazole 5%EC (Avon, Comfort, Ki Hexa, Hexa plus) @2 ml/lit or Kasugamycin 3% SL (Kesu-b, KI-mycin) @1.5 gm/lit of water.
• Spray with Hinosan 1.5 ml/lit water . Once on a seed bed, two times on tillering stage, two times on neck emergence stage.
• Systemic fungicides like triazoles and strobilurins can be used judiciously for control to control blast. A fungicide application at heading can be effective in controlling the disease.