A ProMED-mail post
ProMED-mail is a program of the
Date: Wed 25 Apr 2012
Source: Southeast Farm Press [edited]
University of Kentucky [UK] specialists are encouraging wheat producers and crop consultants to scout for a new disease that could have important implications. UK's Lloyd Murdock found wheat blast on a single wheat head in 2011 at a research plot in Princeton. No additional cases were found in the area. It is likely, however, that additional infections existed but at levels too low to detect.
Wheat blast is recognized as an emerging threat worldwide [ProMED-mail post 20100521.1684]. The Kentucky find is the 1st known occurrence of wheat blast outside of South America. However, a related pathogen causes a disease called gray leaf spot on ryegrass [in the US], and there is also a disease common in rice called rice blast.
UK's Mark Farman has sequenced the genome of the isolate found at the UK [research plot] and compared it to the genetic structure of isolates from ryegrasses and from South American wheat blast. He found the UK wheat blast pathogen to be genetically very similar to the ryegrass pathogen but most different from wheat blast from South America.
This led him to think that the ryegrass pathogen gained the ability to infect wheat. His results suggest that the fungus from the UK wheat has probably been around for a decade or longer. It almost certainly was not imported with grain originating from South America and is not an exotic pathogen. Farman said the pathogen is usually host-specific, and this is the 1st time he has seen it [being] able to "jump" hosts.
It's very unlikely this is an isolated case. UK researchers suspect it may have been misdiagnosed in the past as Fusarium head blight, a very common disease with very similar symptoms to wheat blast. Although they do not believe wheat blast represents a current economical threat, they believe it is important to [know] how common it is.
Farman added it's an early warning of a possible emerging issue and, hopefully, will give researchers time to develop resistant varieties before it becomes a major threat.
[Byline: K Pratt, University of Kentucky]
[Wheat blast (or "brusone" in South America) is caused by a fungus closely related to the rice blast pathogen. Rice blast is one of the most destructive diseases of rice worldwide. Initially, wheat blast was thought to be caused by a fungal strain which crossed from rice to wheat, but it is now considered more likely to be a different species originating from local wild grasses (as mentioned above). A re-classification of species has been suggested with the wheat pathogen (which also affects barley in South America) as _Magnaporthe grisea_ and a new name, _M. oryzae_, assigned to the rice blast fungus. Wheat blast is now considered a new emerging disease caused by a distinctly different pathogen, and sequencing studies are being carried out to determine exact genetic differences between the wheat and rice fungi.
Blast symptoms on wheat and barley include bleaching of ears, shriveled kernels, and no seed production at all for severe infections. Yield losses seem to average 40 to 50 percent, but cases of 100 percent losses have also been reported, and wheat production in some affected areas has ceased. Humid and warm conditions favour disease development, but the life cycle of the fungus is still unknown. Spread of the rice pathogen occurs with infected plant material (including seed), mechanical means (including insect activity), water and wind, and it is likely that the wheat pathogen is spread in similar ways. More information is needed to forecast epidemics and protect other regions and cropping systems from infection.
Wheat blast control with fungicides may be problematic. The International Maize and Wheat Improvement Centre (CIMMYT) has reported that in some places farmers are using 4 fungicide applications with no results, suggesting that current chemicals may not be very effective against the fungus. In any case, correct application is vital and is often too expensive in developing countries. Currently available wheat cultivars lack resistance to wheat blast, and only limited tolerance can be found. Resistance breeding programmes are being established in the Americas.
Since the US pathogen was found to be different from the South American one, this would suggest that genetic events leading to a change in host range from grasses to wheat have occurred separately in the 2 locations and, therefore, may potentially occur again in different regions. Over 50 species of grasses and sedges can be affected by these related fungi, which appear to be highly variable favouring the emergence of new strains, but more information is needed regarding genotypic differentiation related to host range. Potential epidemics of wheat blast, for example in subtropical regions of Southeast Asia and Africa, are considered a threat to global food security.
Fusarium head blight (FHB) of small grain cereal crops (wheat, barley, rye, oats, maize) is caused by several fungal species originally classed into the genus _Fusarium_. For more information on FHB, see link below and previous ProMED-mail posts in the archives.
South & Central America, overview:
Blast symptoms on wheat:
For comparison, fusarium head blight symptoms on wheat:
Barley head with blast symptoms:
Grey leaf spot on ryegrass:
Information on wheat blast:
Recent research on wheat blast:
_M. grisea_ taxonomy and synonyms:
Information on rice blast:
_M. oryzae_ taxonomy:
Information on fusarium head blight of cereals:
A HealthMap/ProMED-mail map can be accessed at:
Blast, wheat & barley - South America: emerging disease 20100521.1684 and additional items on rice blast in the archives]