GCTV Ep#12 Screening for Blackleg
Narrator: Darryl Anderson Silently gathering in every paddock of canola
stubble across this country is a highly destructive storm.
The threat comes from the fungal disease Blackleg.
That you can even hope to successfully grow a canola crop in Australia is thanks almost
entirely to being able to choose resistant cultivars but even then the resistance is
short lived.
Steve Marcroft: Canola Pathologist This work is very very significant, in Australia
if we don’t control Blackleg it will totally kill the crop so in the 1970s for instance
when we had no disease resistance every crop died and the canola industry was pretty much
destroyed right up until the 1990s when the breeders had got resistance back into the
cultivars.
Narrator: Steve Marcroft leads the canola disease-screening
project, which has been running for 10 years.
It trials cultivars, calculating their level of resistance and GRDC alone will fund this
vital research for the next five.
Steve Marcroft: Its like a co-evolution where the pathogen
and the plant are in this war of resistance and virulence fighting each other all the
time.
Narrator: The fungus survives on canola stubble and
this year there’ll be about 2.5 million hectares of it.
The Autumn break triggers masses of fungal fruiting bodies, each producing thousands
of spores.
They get splashed around by the rain and can travel on the wind, so a new crop emerging
in the same paddock or nearby is going to cop a massive disease load.
It’s why canola shouldn’t be sown into the previous year’s canola stubble.
Steve Marcroft: If you imagine there’s trillions and trillions
of spores out there from those two and a half million hectares of stubble from last year
and every one of those spores because they’re a result of sexual reproduction they’re
all genetically slightly different to each other.
So what happens is when you grow a brand new resistant cultivar there’s already a small
proportion of those spores which can attack that cultivar.
Narrator: Previously resistant cultivars can become
totally susceptible within a few years.
And grain growers need to know when their chosen cultivars are about to tip over the
edge of yield loss and be ready to select a replacement cultivar.
That’s where the disease screening trials come in.
On eight disease nursery sites like this one at Wagga Wagga in NSW, every commercial and
pre release cultivar is sown.
Steve Marcroft: We sown them in our high rainfall zones where
we expect to get high levels of disease and we sow them into canola stubble form the previous
year.
So we get very high levels of disease and very uniform disease across the paddock.
We sow them in rows of about 150 plants, we count how many germinate and then we count
how many survive to the end of the year.
And it’s the survival data that we’re interested in.
Narrator: As well as a ranking of the cultivars’ resistance
there’s a rating for seed that’s been treated with fungicide prior to sowing.
And cultivars are now grouped to further improve management of the blackleg disease.
Steve Marcroft: We’re now at the stage where we can identify
the individual resistance genes in each cultivar.
So that if we start to see particular cultivars dying in a particular region we can say its
because they contain resistance gene one.
Then we can inform farmers if their resistant gene one is being overcome they can switch
to resistance gene two.
Narrator: A Blackleg Management Guide on GRDC’s website
is updated as this new data becomes available.
And as the presence of Blackleg is highest in the previous season’s stubble there’s
a guide to indicate the severity of the disease based on how close the new crop is to the
last year’s stubble.
There are other crop management practices to help growers reduce their crop’s disease
risk, including the use of a foliar fungicide.
Steve Marcroft: The trick for that is we know that where yield
loss is occurring that the foliar fungicides are very very effective but most growers don’t
have yield loss from Blackleg if they are managing the seeds correctly.
So they’ve got to make the economic decision whether its worthwhile them spending the money
on the foliar fungicide or not.
Narrator: Being stubble borne and sexually reproduced
Blackleg will always threaten to be canola’s most devastating disease.
However following the Blackleg Management Guide and inspecting your crop for signs of
disease will give you the best chance of managing the risk.
Ends