A ProMED-mail post
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International Society for Infectious Diseases
http://www.isid.org

Date: Fri 12 Nov 2021
Source: The Hindu [summ. Mod.DHA, edited]
https://www.thehindu.com/news/national/kerala/new-paddy-variety-on-250-acres-hit-by-rice-blast-disease-in-wayanad/article37462439.ece


The new rice variety 'Manuvarna' has been hit by rice blast disease on 250 acres in Wayanad district. The variety, intended for cultivation in wetland ecosystems, was released by the Kerala Agricultural University (KAU) only recently. Some farmers reported earlier that it had given higher yields than traditional varieties. Around 60 farmers who had bought 4600 kg of seeds from KAU noticed a few weeks ago that the crop was damaged by blast disease.

KAU's Regional Agricultural Research Station (RARS) said that the spores of the fungus could have been released from sedges and grasses, such as _Cyperus rotundus_ and _Echinochloa crusgalli_, which are abundantly present in the affected area. They said that appropriate prophylactic measures were not taken which led to severe damage due to neck blast.

[Byline: E.M. Manoj]

--
Communicated by:
ProMED
<promed@promedmail.org>

[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 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 and 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 only infect 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 defense-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.

From the story above, it seems utterly baffling that in a region with very high risk of rice blast a new crop variety was released for general cultivation, apparently without having been assessed for its susceptibility to the disease. At least a warning that its level of susceptibility is still unknown and that it may require strict management procedures to prevent crop damage should have been issued to farmers.


Maps
India (with states):
https://www.worldometers.info/img/maps/india_physical_map.gif and
http://healthmap.org/promed/p/308
Kerala districts:
https://www.mapsofindia.com/maps/kerala/kerala.htm

Pictures
Rice blast symptoms:
http://www.knowledgebank.irri.org/ricebreedingcourse/blast.jpg (different symptomatic forms) and https://www.dpi.nsw.gov.au/__data/assets/image/0006/798765/RiceBlast5.jpg

Rice fields affected by blast:
http://ucanr.edu/blogs/riceblog/blogfiles/22977_original.jpg, https://guardian.ng/wp-content/uploads/2019/06/Rice-Blast.jpg and https://previews.123rf.com/images/imagethink/imagethink1411/imagethink141100067/33260576-rice-blast-Stock-Photo.jpg

Links
Information on rice blast:
http://www.knowledgebank.irri.org/training/fact-sheets/pest-management/diseases/item/blast-leaf-collar (with pictures), http://www.oisat.org/pests/diseases/fungal/rice_blast.html, http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=46103, and
https://www.dpi.nsw.gov.au/biosecurity/plant/insect-pests-and-plant-diseases/rice-blast

Rice blast disease cycle:
https://ars.els-cdn.com/content/image/3-s2.0-B9780123820341000086-f08-05-9780123820341.jpg and https://www.researchgate.net/profile/Darren_Soanes/publication/7891924/figure/fig2/AS:271700617068555@1441789883887/Life-history-of-Magnaporthe-griseaa-Asexual-spores-called-conidia-germinate-and-develop.png

Research on rice blast host resistance:
https://www.sciencedirect.com/science/article/pii/S1369526618301808 (review), https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0211061 and https://www.researchgate.net/publication/330889038_Molecular_and_field_level_screening_for_blast_resistance_gene_donors_among_traditional_rice_varieties_of_Kerala

Impact of rice blast (and other fungal crop diseases):
https://doi.org/10.1111/j.1364-3703.2011.00783.x

Information on wheat blast:
http://wheatblast.org/ and http://wheat.org/wp-content/uploads/sites/4/2016/04/Wheat-Blast-Priority-Brief-web-07Apr2016.pdf

_P. oryzae_ taxonomy and synonyms:
http://www.indexfungorum.org/names/NamesRecord.asp?RecordID=224486 and http://www.speciesfungorum.org/Names/SynSpecies.asp?RecordID=224486

Kerala Agricultural University & RARS:
http://www.kau.in/ and http://rarsamb.kau.in/
- Mod.DHA
]

See Also

Blast disease, rice - India: (NL) 20210728.8549976
Blast disease, rice - Bangladesh: (KH) 20210422.8321022
Blast disease, wheat - global epidemiology 20210324.8267471
2020
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Neck blast, rice - Nepal: (CH) 20200611.7459995
Blast disease, rice - Viet Nam: alert 20200409.7202041
Blast & undiagnosed rust, wheat - Bangladesh: (RP) 20200323.7132348
Blast disease, rice - India: (TG) 20200303.7042063
2019
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Blast & bacterial leaf blight, rice - India: (TN) 20191110.6771677
Blast disease, rice - Bangladesh: (DH) 20190506.6457423
2018
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Blast & bacterial leaf blight, rice - India 20180914.6028737
Virus & blast diseases, rice - Viet Nam: (Mekong Delta) 20180712.5901134
Blast disease, rice - Bangladesh: (RJ) 20180419.5756531
2017
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False smut & blast, rice - Bhutan, India 20171102.5419405
Blast disease, rice - India: (JK) 20170725.5202371
Blast disease, rice - Bangladesh 20170428.5000484
Blast disease, wheat - India: 1st rep (WB) 20170306.4883233
Blast disease, wheat - Bangladesh: (KH) 20170123.4784298
and additional items in the archives

Website: http://www.isid.org

Published: November 18, 2021