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Karnal bunt on wheat: global risk assessment


A ProMED-mail post

ProMED-mail is a program of the
International Society for Infectious Diseases <http://www.isid.org>
 
Date: Fri 11 July 2014
From: David Jones <bananadoctor@bigpond.com> [edited]
 
 
Karnal bunt caused by the fungus _Tilletia indica_ has been considered an emerging disease that threatens wheat-growing nations worldwide [1]. But its importance lies chiefly in its status as an [European Plant Protection Organisation; EPPO] A1 quarantine pest [2]. This status has resulted in requirements for a zero tolerance of the fungus in grain exported from countries where the disease is present [3].
 
Karnal bunt global distribution has remained virtually unchanged since
2001 [4]. Infested lands are in the arid and semi-arid regions of the world, where there are long hot and dry periods. These environmental conditions would appear conducive for the pathogen's long-term survival [5, 6].
 
Arguments have been advanced for the vulnerability of wheat crops in many climatic regions, including Europe [7, 8]. Nevertheless, the fact remains that Karnal bunt has not been reported to have established anywhere outside the dry zones which circle the earth along latitudes
30 degrees north and 30 degrees south [3, 5] despite the movement of bulk seed lots from infested areas [e.g. Mexico] before quarantine risks were recognised [3, 6].
 
Research funded by the EU [European Union] has shown that environmental conditions conducive for infection can occur in Europe during a short period at heading when wheat is susceptible to infection [7, 8]. Although this is possible if teliospores capable of germination were dropped on the soil surface of fields when wheat was vulnerable and weather conditions were favourable, it is extremely doubtful whether establishment and spread would occur. Teliospores released from bunted heads onto the soil surface would most likely germinate during cool, wet periods that prevail in Europe in the spring, autumn and winter when wheat is not heading. The rapid depletion of soil surface inocula has in fact been shown to occur under wet and alternating wet and dry conditions by experimentation in growth cabinets adjusted to mimic the European environment during wheat growth [6].
 
Current wheat import restrictions in place for the EU and other countries outside the arid and semi-arid zones would appear to be justified only under the "precautionary principle" [see comment below].
 
References:
[1] Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers.
[2] Karnal Bunt - A Wimp of a Disease ... but an Irresistible Political Opportunity.
[3] A reappraisal of the current status of _Tilletia indica_ as an important quarantine pest for Europe.
[4] Karnal Bunt fact sheet
[5] Arguments for a low risk of establishment of Karnal bunt disease of wheat in Europe.
[6] Towards a more reasoned assessment of the threat to wheat crops from _Tilletia indica_, the cause of Karnal bunt disease.
[7] UK Pest Risk Analysis for _Tilletia indica_.
[8] The new Pest Risk Analysis for _Tilletia indica_, the cause of Karnal bunt of wheat, continues to support the quarantine status of the pathogen in Europe.
 
--
Dr David Jones
Consultant in Plant Health
Callala Beach, NSW, Australia
 
[We would like to thank Dr Jones for this informative contribution to the biosecurity debate and update of the global situation for karnal bunt.
 
A pathogen introduced into a new environment may profit from the absence of competing organisms or parasites, lack of genetic resistance in local hosts, new available hosts, or favourable environmental factors. It is sensible to err on the side of caution and take the long term view of protecting the importer's primary industries. Once a pathogen has been introduced into a new area, eradication attempts are very rarely successful and costs due to the incursion may be enormous.
 
In many cases, political decisions have been taken to accept a set limit of environmental risks for the benefit of commercial trade.
However, quarantine risk assessment for crop diseases is not a straightforward task due to the complexity of biological and environmental systems involved in pathogen epidemiology. Difficulties are often compounded by the absence of factual data, and levels of "acceptable" risk are largely a matter of opinion. Thus, biosecurity decisions frequently lead to disagreements between producers and importers of commodities (see, for example, ProMED-mail post 20040609.1549).
 
An important point in the ongoing global biosecurity debate is that the worldwide emergence of new pathogen strains with increased viability and agrochemical resistances has been documented in many cases (see ProMED-mail posts on emerging new strains in the archives).
Thus, even if a pathogen is considered not to be a threat under local environmental conditions (as stated above for karnal bunt) or is already present in one area, a new strain may have acquired abilities to thrive under different conditions.
 
Furthermore, movement of pathogens to previously unaffected areas due to global climate change has been reported in numerous cases. Previous examples in Europe of "warm climate diseases" moving northwards to cooler regions are apple diseases caused by _Diplodia_ species (ProMED-mail posts 20130927.1970792, 20110718.2172 and 20090914.3230), as well as soybean wilt caused by _Curtobacterium flaccumfaciens_ (ProMED-mail post 20120809.1235745). Therefore, from a plant pathology aspect, introduction of any pathogen must be prevented.
 
Karnal bunt
--------------
The fungus _Tilletia indica_ causes karnal bunt (also called Indian or partial bunt), a serious disease of bread wheat (_Triticum aestivum_), durum wheat (_T. turgidum_), and triticale. Disease development is favoured by cool, humid conditions at time of heading. Symptoms vary with environment and are difficult to detect in the field because of the irregular distribution of infected kernels. They may include reduction in the length of ears and number of spikelets, brown blotches on kernels, premature grain drop, and stunting of plants. All or part of the grain is replaced with a powdery mass of dark spores that emit a strong, fishy odour. Grain containing more than 3 percent bunted kernels is generally considered unfit for human consumption.
 
Harvesting shatters infected grains releasing the spores and leaving behind a broken, hollow shell. Spores can spread over long distances by wind and survive in the soil, on stored seed, or on contaminated equipment for up to 5 years. Disease management may include cultural and phytosanitary measures to prevent spread, use of clean seed, preventative foliar fungicides, and change to alternative crops not susceptible to karnal bunt (such as barley).
 
Economic losses are mainly due to lower grain quality, costs of quarantine measures and loss of international trade. Consequently, most countries regulate karnal bunt as a quarantine pest. The resulting restrictions cover import of grain, agricultural machinery, feed meals, seeds and agrochemicals that may have been handled in the same transportation system as infected wheat.
 
Related species cause common bunt (_T. tritici_) and dwarf bunt (_T.
contraversa_) of cereals, as well as black smut of rice (_T.
barclayana_).
 
Maps
Europe, overview:
 
Pictures
Karnal bunt affected wheat grain:
Karnal bunt symptoms on wheat ears:
Karnal bunt photo gallery:
 
Links
Karnal bunt information:
_Tilletia_ species, history and diagnosis:
_T. indica_ taxonomy:
Latest Karnal bunt risk assessment report, European Food Safety
Authority:
EPPO A1 quarantine list:
European Food Safety Authority (EFSA):
 - Mod.DHA
 
A HealthMap/ProMED-mail map can be accessed at:
 
[See Also:
2010
----
Karnal bunt, wheat - Europe (02): risk assessment 20100811.2744 Dwarf bunt, wheat - Sweden 20100805.2642 Karnal bunt, wheat - Europe: risk assessment 20100701.2190
2004
----
Karnal bunt, wheat - Australia: NOT 20040609.1549
2001
---
Karnal bunt, wheat - USA (Texas) 20010628.1227
2000
---
Karnal bunt, wheat - South Africa 20001223.2270 Tilletia indica - USA (Alabama, Tennessee) NOT 20000215.0203
1999
---


More news from: ISID (International Society for Infectious Diseases)


Website: http://www.isid.org

Published: July 15, 2014

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