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International Society for Infectious Diseases <http://www.isid.org>
Date: 31 Mar 2008
Source: Business Daily (Nairobi) [edited] <http://allafrica.com/stories/200803311661.html>
Farmers count losses after disease destroys rice
Rice farmers in Mwea, Kirinyaga District [Central Province], are
counting their losses after their crop was attacked by the rice
blast disease. The farmers have been complaining about the
disease, which has wiped out almost half of their crop. "It
started last year [2007] and since then it has been spreading
very fast," said Mr Francis Gichovi, a rice farmer in Mwea. The
disease is now threatening to drastically reduce harvests this
season. An acre of land under rice usually produces on average
25 bags of rice, but this may reduce to 10 bags, Mr Gichovi
said.
Many rice scientists consider it to be the most devastating rice
disease worldwide. It has been detected in 85 countries. Besides
attacking the leaves, the fungus may also attack the stem at the
nodes, causing the neck to rot, or at the panicle, causing
panicle blast. When this occurs, yield losses may be large
because few seeds in the panicle develop. "The rice plants have
turned white and there is no seed in them," explains Gichovi.
According to the district agricultural office in Mwea, the
disease has been spurred by the rains, which have been unusually
high this season. The disease thrives in long periods of
moisture, high humidity, little or no night wind and night
temperatures of around 20 degrees Celsius [68 degrees
Fahrenheit]. Poor crop rotation practices have also allowed the
disease to spread.
According to experts, successful control of the disease usually
requires an integrated management programme, including the use
of resistant varieties, cultural practices and chemical control.
Farmers have been advised to burn diseased rice straw and
stubble. This is an important control measure that reduces the
disease prevalence, but will not provide complete control by
itself. Use of clean seeds whenever possible is also being
encouraged to help control the disease, seed treatment to
eliminate blast is under study.
Continuous flooding is also recommended to limit its
development.
Fungicides used in the past have not been effective in tackling
the disease, but there are some fungicides that can control neck
rot or panicle blast. Even though they are expensive, they may
be economical to use at this stage.
Plans to screen and breed resistant varieties are in progress.
Varietal resistance is the most economical way to control the
disease. Resistance to the disease in the plant is effective
against attack at all stages of growth. However, the fungus is
highly variable -- new forms can appear which attack resistant
varieties.
[byline: Solomon Mburu]
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communicated by:
ProMED-mail
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[Rice blast is caused by the fungus _Magnaporthe grisea_
(synonyms _Pyricularia oryzae_, _P. grisea_). It is one of the
most destructive diseases of rice worldwide with reported losses
of more than 50 per cent in infected fields. Symptoms include
lesions on all parts of the shoot, 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. More than 50 species of grasses and sedges can be
affected by different strains of the fungus, but most strains
isolated from rice can only infect a limited number of
cultivars.
Symptom severity and spread of the blast fungus are influenced
by climatic conditions. The disease is spread by infected plant
debris, mechanical means, water, and wind. Disease management
may include fungicides and cultural practices, but relies mainly
on resistant varieties. The fungus is highly variable, however,
and new strains are emerging, breaking down host resistance.
Maps
Kenya:
<http://www.ogiek.org/photo-gallery/kenya-map-big.jpg>
and
<http://healthmap.org/promed?v=0.5,37.9,6>
Kenya provinces:
<http://www.lib.utexas.edu/maps/africa/kenya_pol88.jpg>
Pictures of rice blast symptoms:
<http://www.imtech.res.in/raghava/rbpred/lesion.jpg>,
<http://www.msstate.edu/dept/drec/rice/rice_blast/blast2.JPG>
and
<http://www.msstate.edu/dept/drec/rice/rice_blast/blast3.JPG>
Links
Rice blast factsheet (with pictures):
<http://www.knowledgebank.irri.org/ricedoctor/default.htm#Fact_Sheets/Diseases/Rice_Blast.htm>
Information on rice blast:
<http://www.imtech.res.in/raghava/rbpred/home.html>,
<http://ascus.plbr.cornell.edu/blastdb/about.html>,
and
<http://www.msstate.edu/dept/drec/rice/rice_blast/rice_blast.htm>
_M. grisea_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=317113>
- Mod.DHA]
[see also in
the
archive:
Virus & blast diseases, rice - Viet Nam (Mekong Delta)
20080204.0454
2005
---
Rice blast fungus, genome sequence - China 20050424.1147
2001
---
Magnaporthe grisea, rice blast - Vietnam 20010214.0298
2000
---
Rice blast: new method of control (02) 20000928.1678
Rice blast: new method of control 20000825.1419
1998
---
Rice blast - USA (California) 19980224.0361]
