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Source: Dhaka Tribune [summ. Mod.DHA, edited]
Farmers in Khulna and Sylhet Divisions are facing an outbreak of rice blast, which has appeared recently in their paddy. The disease is widespread in BRI-28 and BRI-63 rice varieties, leading to significant losses. It has increased in BRI-28. Farmers said that their fields had turned white, but fungicides did not get any results. Currently the disease is spreading. Some farmers are cutting their paddy for cattle feed or to clear the land. In some areas, almost all the paddy has rotted from blast disease.

Agriculture officers say that rotting and drying up of rice plants could be due to either the current heat wave or the fungus. They recommend spraying tricyclazole fungicide when rotting occurs and ask for 2 rounds of spraying. They say BRI-28 is a very old variety that farmers were discouraged to cultivate. BRI-88 or BRI-89 were suggested instead. Crops were destroyed because farmers did not follow the guidelines, or some spray may have been washed away by rain.

Communicated by:

[Rice blast is caused by the fungus _Pyricularia oryzae_ (previously _Magnaporthe oryzae_). It is one of the most destructive diseases of the crop worldwide, with potential yield losses of more than 50%. Symptoms may include lesions on all parts of the shoot, as well as stem rot and panicle blight. When nodes are infected, all plant parts above the infection die; yield losses are severe. When infection occurs at the seedling or tillering stages, plants are often completely killed; infection late in the growth cycle generally leads to less severe damage. Depending on which plant parts are affected, the disease may manifest itself as leaf, collar, node, or neck blast. More than 50 species of grasses and sedges can be affected by related pathogens, but most strains isolated from rice can infect only a limited number of cultivars.

The fungus also causes wheat blast (for example, see ProMED post 20210324.8267471). Although the pathogens are currently classified as the same species, the wheat blast pathogen is a distinct population (referred to as _P. oryzae_ Triticum population) and does not cause disease in rice.

Symptom severity and spread of the blast fungus are influenced by climatic conditions, including high humidity. The disease is also favoured by high nitrogen levels (for example, from fertilisers). The fungus is spread by infected plant debris, mechanical means (including insect activity), water, and wind. Disease management may include fungicides and cultural practices but relies mainly on resistant varieties. Use of certified clean seed is essential; farm-saved seed poses a high risk of carry-over of the fungus to subsequent crops.

The fungus is highly variable; this favours the emergence of new strains with increased virulence, including host resistance breaking strains. Environmental factors may also affect plant resistance. Both resistance and defence-regulator genes have been found to be involved in host resistance against blast (see links below) and could potentially be combined ("pyramided") to develop rice varieties with broad-spectrum host resistance against blast that cannot be as easily overcome by the fungus as varietal resistance based on single genes.

Rice varieties have been developed by the Bangladesh Rice Research Institute (BRRI) for local conditions and have been classed according to their levels of blast resistance (see link below). However, a lower blast resistance level is considered acceptable by some farmers for the sake of higher potential yield and quality of produce (ProMED post 20210422.8321022).

Tricyclazole is a systemic triazole fungicide. Triazoles belong to the group of demethylation inhibitors (DMIs; see links below), which was introduced in the 1970s.

Bangladesh divisions and districts:

Rice blast symptoms:
(different symptomatic forms) and
Rice fields affected by blast:, and

Information on rice blast: (with pictures), and
Rice blast disease cycle:
Research on rice blast host resistance: (review) and
Impact of rice blast (and other fungal crop diseases):
Information on wheat blast: and
_P. oryzae_ taxonomy and synonyms: and
Information on some BRRI rice varieties via:
Information on triazole fungicides:
Fungicide groups & modes of action: and
- Mod.DHA]


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