A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Fri 12 Nov 2021
Source: The Hindu [summ. Mod.DHA, edited]
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]
[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.
India (with states):
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
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):
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/