Triazole fungicides are less effective against this disease than they were seven years ago.
At the same time Septoria became resistant to existing fungicides, a new mode of action arrived on the market in the form of SDHI (succinate dehydrogenase inhibitor) fungicides. These have prevented significant production losses because of the presence of the disease both in New Zealand and Europe.
However, Poole cautioned against over-using SDHIs in order to maintain sensitivity to the chemical. He said they should not be used more than twice in one cereal crop. Neither should they be used curatively.
Septoria, which is a stubble-borne disease, has the potential to reduce harvest yields by as much as 50%. The spores are dispersed by both wind and rain-splash and require 48 hours of 98% humidity in order to infect the crop.
Poole said the jury was still out on whether irrigation had the same effect of rain in so far as creating the conditions that facilitate the spread of Septoria.
The disease has a long latent period – time between the spores landing in the crop and symptoms appearing – which has been calculated at 250C thermal time.
A rough guide to calculating thermal time is to add together the maximum and minimum temperatures and divide by two. With a maximum temperature of 15C and minimum temperature of 5C it would take 25 days to reach 250C thermal time.
Over the past two seasons damp springs, susceptible cultivars, and resistance to fungicides had meant Septoria had been a big problem for wheat growers.
Poole said there was potential for Septoria to be a problem again this season because there was a lot of innoculum in early sown crops. However, the wet autumn meant that more crops were sown later and this would reduce the risk of the disease.
How Septoria develops depends entirely on the season.
There were cultivars showing resistance to Septoria and while late-sown wheat crops are less susceptible to the disease, there were yield penalties associated with late-planted crops.
Resistance to strobilurins, which have been widely used since the 1990s, was first reported in Europe in 1998 so the chemistry only offered crops six years of protection before resistance began to occur.
To date strobilurin resistance has been recorded in 50 pathogens around the world with the first reported cases in NZ occurring in 2006.
Septoria tritici blotch has become the principle disease in wheat crops over the past two seasons.
Strobilurins are still effective against rust but they were no longer effective against Septoria.
Triazoles have become gradually less effective over the years with laboratory tests showing Septoria had become 10-100 times less sensitive to triazoles since the early 2000s. They needed to be used at a high dose rate in order to be effective.
In the United Kingdom, the efficacy of triazole fungicides against Septoria had continued to decline and the field performance at low doses has deteriorated further.
Rob Craigie from FAR explained that resistance to triazoles was complex and not all triazoles were affected in the same way.
He cautioned against over-using triazoles and suggested using them with a partner chemical which had a different mode of action to reduce the risk of resistance.
This may improve disease control while increasing efficacy.
While SDHI’s will control Septoria, FAR’s Nick Poole stressed the need to use them judiciously because there were not any new modes of action on the horizon. Their efficacy needed to be maintained.
There is already some pathogen resistance to SDHIs in Europe. Poole said SDHIs should always be used in combination with another mode of action.
“The new SDHI fungicides used in combination with triazoles are effective at controlling the disease Septoria.”
