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Date: August 7, 2007
Source: Indian Express News Service [edited] <http://cities.expressindia.com/fullstory.php?newsid=249898>

The Plant Clinic of Punjab Agricultural University (PAU) is witnessing many cases of botanical [bacterial] leaf blight in paddy, as shown by the farmers.

In a statement issued here [Ludhiana], PAU Director Extension Education, Dr NS Malhi, said once this disease starts in paddy, there is no way to cure it in the crop. Therefore, the farmers should refrain from using chemicals being sold for vested interests by unscrupulous agents.

Dr Malhi suggested some ways to mitigate the losses. He warned against ponding of water in the fields, and application of excessive doses of nitrogen. He said treating the seed before sowing to kill primary inoculum and growing resistant varieties can help curb the problem.

--
Communicated by:
J. Allan Dodds
Former ProMED-mail plant disease moderator
<dodds@ucr.edu>

[Bacterial leaf blight (BLB) of rice is caused by _Xanthomonas oryzae_ pv. _oryzae_ (previously _X. campestris_ pv. _Oryza_). It was 1st noticed in Japan in 1884 and has since been reported from Asia, northern Australia, Africa, and the USA. The pathogen causes yellowing and drying of leaves, wilting of seedlings, and reduces yield. In Asia, millions of hectares of rice paddies are severely affected every year with reported yield losses of up to 60 percent.
Mild strains of the bacterium are also known, for example in the USA, which do not cause any detectable yield loss.

Blight lesions caused by severe strains of the bacterium elongate over the entire length of the flag leaf, giving a striped appearance to leaves. Lesions caused by mild strains are usually only a few centimeters long. Various saprophytic fungi may invade the lesions, contributing to the damage. Panicles may be infected by the severe strains. Field patches infested with bacterial blight have a whitish, ragged appearance.

The bacterium is short-lived in soil and suspected to be seedborne, but also to be short-lived in seeds. Grassy weeds, infected plant material, such as rice stubble or ratoons, and contaminated irrigation systems are thought to be the primary pathogen reservoirs. The disease spreads by windblown rain and mechanical means (for example when transplanting seedlings) and is favored by rain, high levels of fertilizer, high humidity, standing pools of water, and temperatures above 25 deg C (77 deg F).

A different pathovar (strain) of the same species, _X. oryzae_ pv. _oryzicola_ causes bacterial leaf streak of rice. It occurs in Asia and West Africa and yield losses of up to 30 percent have been recorded. Symptoms vary early on, but in the later stages of the disease look similar to BLB.

Maps
India: <http://www.globalsecurity.org/military/world/india/images/IndiaMap_tourism.gif>
Punjab: <http://www.mapsofindia.com/maps/punjab/punjabroads.htm>

Pictures
BLB symptoms on leaves and whole plants via:
<http://www.invasive.org/browse/subject.cfm?sub=11182>
Comparison of BLB effect on resistant and susceptible cultivars:
<http://www.tari.gov.tw/tarie/photos/introduction/introduction_PPD_02.jpg>

Links
BLB disease information:
<http://www.knowledgebank.irri.org/riceDoctor_MX/Fact_Sheets/Diseases/Bacterial_Leaf_Blight.htm> 
and
<http://www.oisat.org/pests/diseases/bacterial/bacterial_leaf_blight.html>
Comprehensive BLB review:
<http://www.ias.ac.in/currsci/dec101999/REVIEWARTICLE.PDF>
Information on rice diseases, including BLB:
<http://msucares.com/pubs/publications/p1840.htm>
Information on rice bacterial leaf streak:
<http://www.knowledgebank.irri.org/riceDoctor_MX/Fact_Sheets/Diseases/Bacterial_Leaf_Streak.htm>
Genus _Xanthomonas_ taxonomy and species list:
<http://www.bacterio.cict.fr/xz/xanthomonas.html>
Punjab Agricultural University:
<http://www.ludhiana.nic.in/html/punjab_agri_univ.html>.  - Mod.DHA]

[see also in the archive:
Flag smut, bacterial leaf blight, rice - India (Punjab & Haryana)
20050927.2841]

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