A new race of a disease called black stem rust, caused by the fungus
Puccinia graminis, is on the move and posing a threat to many of
the world’s wheat growing regions. Although not a threat to
UK wheat crops today, scientists at SAC are looking to the future
and assessing what the scene could be by 2050.
The first map, with least red, shows the current
situation and the second map shows the predicted situation for
the year 2050.
Using a specially developed computer model the researchers are predicting
how certain crop diseases, not previously seen in the UK, may spread
in the future as a result of climate change. This surveillance of
threats from new diseases provides an opportunity for crop scientists
to take action before new diseases strike - thereby minimising the
threat to our food crops.
The new fungal race of black stem rust was first found in Uganda
in 1999 and has spread through wind and air currents to other regions
in East Africa, including South Yemen. It is likely to spread further
threatening large wheat regions in India, Australia and the United
States. Known as Ug99 this new race of the disease is a threat to
wheat as it has broken down the natural plant resistance defences
bred into current wheat varieties. The consequence of an epidemic
will be a reduction in wheat yields and this could impact on world
SAC’s Dr Andy Evans has investigated the current and future
risk to UK wheat (see maps).
The first map, with least red, shows the current situation and the
second map shows the predicted situation for the year 2050.
The climatic conditions in Uganda where the new race was found were
fed into the pest and disease model and the results confirmed the
current thinking that there is no immediate risk to the UK. Coastal
regions of the Mediterranean, northern Spain and southern France
are however potentially at risk. Since wheat is grown on a more intensive
scale in Europe compared to other large wheat regions growers, if
stem rust attacked today and weather patterns favoured the disease,
current fungicide programmes used to manage more common disease threats
including septoria tritici, yellow and brown rust would protect crops
from black stem rust.
However, fast forwarding the disease risk model to 2050 using current
climate change thinking shows black stem rust risk will spread further
north in Europe, covering the main wheat growing regions in the UK
up to Inverness. By carrying out similar scenarios for other pests
and diseases which are of economic importance in other parts of the
world, it is possible to plan ahead to ensure we have the varieties
and technical skills to deal with the potential threats in the future.
In common with other related diseases, such as wheat yellow rust
(Puccinia striiformis) a disease which is currently present in the
UK, black stem rust is best managed by breeding resistant varieties
that are better able to fight the fungus. Plants with the right properties
will have the power to prevent the fungus from breaking through their
natural defences. If the fungus manages to enter a plant it can spread
rapidly. Where large areas of a susceptible crop are grown, particularly
if weather conditions suit the fungus, it will have a field day resulting
in a weakened or dead crop and poor yields.
Dr Simon Oxley who is leader of SAC’s Crop Epidemiology Research
Team who monitor and give advice on both current pest and disease
threats and also on potential future threats says,
“Black stem rust is not unheard of in the UK and it has been
known to attack wheat crops late in the season in the south and East
of England. On the rare occasion it occurs, it is more a curiosity
than an economic threat in the UK today. Should spores of this new
race reach Europe, the impact in the UK will be minimal and nothing
like as devastating as could occur in major wheat producing regions.
“In Uganda, the appearance of the new black stem rust race
meant existing varieties became susceptible overnight. It then becomes
a race against time for plant pathologists and plant breeders to
understand more about the new race, find out which wheat genes can
effectively stop it and breed these resistance genes into new varieties.
All this takes time, so the sooner plant pathologists and breeders
understand what is happening, the better.”
Wheat Yellow Rust
Wheat yellow rust, a relative to black stem rust, is a major disease
threat to UK crops in 2007. The disease spreads rapidly in cool and
humid weather conditions and the popular wheat variety Robigus, named
after the Roman god of rusts, is very susceptible to it. Disease
management has therefore switched from variety resistance to fungicides.
The risk from yellow rust is high especially where wheat is sown
early in August. A mild winter and a humid mild spring will also
have increased the risk and the disease is already established in
crops throughout the UK. Early sowing meant there was no gap between
cropping seasons, providing the fungus with a green bridge to survive
from one season to another. The mild winter also enabled the disease
to thrive with few frosts to kill out the fungus. Regular crop monitoring
funded by SEERAD in Scotland, tracks the progress of this and other
diseases, giving growers early warning to prepare for an outbreak.
UK growers have the luxury of reducing the impact of this disease
through the use of fungicides. These must be applied before the disease
takes hold, since eradication is next to impossible. Fortunately
we had early warnings about a yellow rust outbreak. There is an effective
monitoring system known as the United Kingdom Cereal Pathogen Virulence
Survey (UKCPVS). This survey collects rust samples every year and
tests for new races. It is an important service for plant breeders
and to growers, because it is an early warning system for new races
of fungi which cause disease and also provides breeders with an understanding
into which fungal races will attack their varieties. Funding for
the survey is currently through Defra and HGCA. It is however a challenge
to maintain this survey, but the issues surrounding black stem rust
should be a warning that such surveys are essential to combat future
disease threats in the UK.
Fungicides should be seen as the last resort to managing disease.
This is why understanding the disease and the resistance mechanisms
of the varieties is important. Market forces usually dictate variety
choice based on the quality of the grain, but where possible, variety
resistance alongside good agronomic practice should be used.
In extensive wheat growing areas, fungicides are unlikely to be
an economic option due to the cost of production, the vast wheat
areas involved and the amount of fungicide which would need to be
produced to deal with the threat. It will now be a race against time
for breeders to understand more about the new black stem rust races
and breed in the resistance genes required to combat the new threat
for the regions affected now. It should be a wake up call to humanity
that we do not take crop diseases for granted.
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