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International Society for Infectious Diseases

Date: Thu 21 Jun 2018
Source: VietnamNet [edited]

Farmers in the Mekong Delta who sowed the summer - autumn rice crop
early are facing a threat of disease outbreaks. Nguyen Van Hien, Plant
Cultivation and Protection, said [that] unfavourable weather has
brought rice diseases which could hit yields. Diseases often occur
when rice is sowed early or late since brown-plant hoppers and other
pests from unharvested fields from the previous crop move to the newly
sowed fields, he said.

In Dong Thap Province some 560 ha sowed early have been affected by
brown plant hoppers. [A farmer] said local agricultural officials did
warn they had to have a gap of at least 3 weeks between crops to
eliminate diseases. "But since the price of rice was high, farmers
here sowed early."

In Kien Giang Province, more than 252 140 ha have been planted, with
15 500 ha being affected by diseases such as rice grassy stunt, rice
ragged stunt and rice blast. 

Plant Cultivation and Protection has instructed district bureaus to
regularly report about diseases and warned farmers to destroy rice
plants which are severely affected. This [2018] summer - autumn the
delta has planted more than 1.65 million ha, according to the Ministry
of Agriculture and Rural Development. 
Communicated by:

[Rice viruses
_Rice ragged stunt virus_ (RRSV; genus _Oryzavirus_) has been reported
to cause yield losses of up to 90 per cent in several Asian countries.
Symptoms include ragged appearance of plants, twisted leaves, vein
swelling, unfilled grains and severe stunting of plants. Its only way
of spread is vector transmission by brown plant hoppers (BPH;
_Nilaparvata lugens_). The insects transmit the virus in a persistent
manner and remain infectious for life. Infected hoppers can be
transported on air currents to spread the virus quickly over long

_Rice grassy stunt virus_ (RGSV; genus _Tenuivirus_) has been reported
to cause serious damage in limited areas during sporadic outbreaks of
its vector. It is spreading in large parts of the rice-growing areas
of Asia. RGSV is also transmitted exclusively by vectors (_N. lugens_
and other species in the genus) in the same way as RRSV. Symptoms
include severe stunting, excessive tillering ('grassy' appearance),
erect growth habit, leaf spots and reduced pale leaves. Infected
plants usually survive until maturity, but produce no panicles.

A number of other rice viruses (several also present in the region)
are spread by different species of plant - or leaf hoppers. In
tropical regions, levels of virus infection and vector density may be
very high. High vector populations are often associated with virus
outbreaks with crop losses of up to 100 per cent reported. In its own
right, BPH causes yellowing, browning and drying of plants ('hopper
burn') and leaves small injuries exposing the plant to fungal (such as
rice blast) and bacterial infections.

Disease management for these viruses includes cultural practices (such
as short term draining of rice fields, crop rotation, or rice-free
periods), vector control and use of cultivars with reduced sensitivity
or resistance to the virus, the vector, or both. Crop cultivars
resistant to BPH have low disease incidence, but insect adaptation
overcoming plant resistance has been a problem. The application of
insecticides to migratory plant hoppers is being used in some areas to
reduce disease incidence.

Rice blast
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 per
cent. 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-mail posts and 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 fertilizers). 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. However, the fungus is highly variable and this favours the
emergence of new strains with increased virulence. Use of certified
clean seed is essential, and farm saved seed poses a high risk of
carry-over of the fungus to subsequent crops.

Viet Nam and neighbours:
Viet Nam provinces:

RRSV symptoms:
<> and
RGSV symptoms:
Rice blast symptoms:
(different symptomatic forms) and
Rice fields affected by blast:
<> and
Brown plant hoppers:

Information on RRSV:
<> and
Information on RGSV:
Virus taxonomy via:
Brown plant hopper information via:
(with pictures)
Information on rice blast:
(with pictures),
Rice blast disease cycle:
Impact of rice blast (and other fungal crop diseases):
_P. oryzae_ taxonomy and synonyms:
- Mod.DHA

HealthMap/ProMED map available at:
Mekong Delta, Vietnam: <>]

[See Also:
Blast disease, rice - Bangladesh: (RJ)
False smut & blast, rice - Bhutan, India
Black streaked dwarf disease, rice - Viet Nam: (TB, NA)
Blast disease, rice - India: (JK)
Blast disease, wheat & rice - Bangladesh
Blast disease, rice - India (02): (JK) origin
Blast disease, rice - India: (TN)
Rice diseases - Malaysia: (western) emerging strains
Blast disease & bacterial blight, rice - Viet Nam: Mekong Delta, alert
Black streaked dwarf, rice - Viet Nam: (TT, QT)
Rice yellow stunt virus - Viet Nam: (BG)
Multiple disease, rice - Viet Nam (03)
Ragged & grassy stunt viruses, rice - Thailand
Black streaked dwarf, rice - Viet Nam (02): spread
Ragged & grassy stunt viruses, rice - Indonesia: (JB)
Black streaked dwarf, rice - Viet Nam: alert
Multiple disease, rice - Viet Nam (02)
Multiple disease, rice - Viet Nam
Undiagnosed diseases, rice - Viet Nam: (northern)
Ragged stunt, rice - Viet Nam
Rice viruses - Viet Nam: alert
and additional items in the archives]
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