Is prochloraz about to make a comeback in fungicide programmes? Mike Abram examines the hype

Septoria resistance to strobilurin fungicides in wheat gave the impetus for a chlorothalonil comeback. Now, perhaps, greater understanding of what causes septoria insensitivity to some triazole fungicides could be the stimulus prochloraz needs to become a bigger part of growers’ plans.

The reason for the downward spiral of some azole fungicides in controlling septoria since the mid-1990s, so graphically illustrated by the Pesticides Safety Directorate-commissioned trials in 2006, is becoming clearer on the back of some detailed molecular analysis of the septoria pathogen.

Put simply – or as simply as detailed molecular analysis ever can be – there are several different strains of septoria. Investigations by Rothamsted Research and INRA in France, among others, have found that these strains can carry one or more mutations in their genetic code that make the fungus less sensitive to control by azole fungicides.

The complicating factor is that some azole fungicides are more affected by certain strains than others. While the market-leaders, epoxiconazole and prothioconazole, appear to be relatively unaffected by what mutation the pathogen is carrying, it makes a big difference to the activity of tebuconazole and prochloraz, for example.

The former struggles to control the septoria strain carrying the I381V mutation, while prochoraz has been found to be relatively more effective against that strain, but weaker on another mutation, V136A. It just so happens that tebuconazole is much more effective against that particular strain.

That discovery has prompted hope, not to mention hype, that prochloraz, in particular, might have a role to play in disease control programmes. The theory is that using prochloraz early in the programme should select for the more sensitive V136A strain of septoria, which most other azole fungicides find easier to control.

There is some initial practical evidence that this might work from, for example, a Velcourt trial in Kent within the HGCA-funded azole resistance LINK project. In that trial a two-spray programme of Poraz followed by Folicur yielded 0.5t/ha more than the three-spray programmes of either. A possible explanation was that the two sprays of Poraz selected for the V136A septoria population, which the last spray of Folicur was able to control more effectively, says Bill Clark of Broom’s Barn.

Further evidence in the trial was given by the yield of a plot treated with a mixture of lower rates of Opus and Poraz equalling that of higher rates of Opus alone, he suggests. “It shows prochloraz could improve performance of azole fungicides as a pre-treatment or in mixture.”

Crops opinion:
Case not proven (yet)
The case for prochloraz is nowhere near as strong, at least not yet, as it was for using chlorothalonil following septoria resistance to strobilurins. While there have been promising field trials results for using prochloraz, it must be remembered that these are relatively few in number and most came in a season when septoria took a back seat to brown rust as a threat in winter wheat.

Harper Adams University College trials, for prochloraz manufacturer Nufarm, were also encouraging. Septoria control from two sprays of a combination of tebuconazole and prochloraz (Grail) yielded 1.4t/ha more than the untreated, says Nufarm’s Tudor Dawkins, while yield responses from Grail’s components were 0.5t/ha from tebuconazole and 0.7t/ha from prochloraz when applied alone. “The combination gave more yield than you might expect from simply adding the components.”

Adding prochloraz to other triazoles, including epoxiconazole and prothioconazole also improved both disease control and yield of the partner triazole applied alone, he adds. “Prochloraz isn’t the silver bullet, but there is enough circumstantial evidence to suggest that it can be a useful component of the programme, and help maintain the activity of other triazoles for longer.”

Should growers use prochloraz this season?

Not surprisingly Dr Dawkins believes adding 0.75 litres/ha of prochloraz would prove prudent for many growers, particularly at T0 or T1. “There is a strong case for a triazole/prochloraz/chlorothalonil mix. The prochloraz adds another weapon against disease sensitivity shifts.”

Using the three-way combination would cover the spectre of brown rust, as well as septoria, and provide some eyespot control, he says.

But other experts are more cautious. Rosie Bryson of BASF says it is important to not get distracted by the resistance management issue. “You’ve got to bear in mind prochloraz’s disease control capabilities, and while there is some data to suggest we are seeing better control with it recently than historically, epoxiconazole should remain the mainstay for control.”

She views it as an extra tool for growers to consider – and only in mixture with a strong partner. “I also don’t see it replacing chlorothalonil as some have suggested, although there could be a case for swapping where there might be a problem with chlorothalonil antagonism.”

Patrick Stephenson, Farmers Weekly’s Arable Adviser of the year, is also sceptical of using prochloraz this year. “At the moment it is an academic exercise – I won’t be using it in any form this season. I just don’t see it adding enough to disease control to warrant its use yet, or seen any yield improvements in independent trials.”

Until these results have been proven further in trials it would probably be prudent to remain faithful to programmes based on applying the optimum amounts of the strongest triazoles available in the vast majority of cases, although there would be no harm in doing some small-scale tests on farm, particularly where septoria is expected to be the main disease present.

Agronomists around the country will have to decide whether to believe the prochloraz hype. FW Arable adviser of the year, Patrick Stephenson, remains to be convinced

More resistance in south 

Research by Steve Rossall (pictured), associate professor of plant pathology at Nottingham University, is suggesting that the strains of septoria most insensitive to triazole fungicides might be more common in the south of the country. “There is a suggestion there is a north/south divide. It is something we want to investigate further this year.”

His suspicion is based on results of tests using septoria isolates collected from across the country to inoculate wheat plants grown in a controlled environment one day after being sprayed with a variety of different azole fungicides.

Three main conclusions could be drawn from the results, he says. First, there has definitely been a shift in sensitivity based on a comparison with a reference 1995 septoria isolate. Second, in general it is older azole fungicides that have been worst affected, but no azole, not even epoxiconazole or prothioconazole, is immune and, finally, a tentative indication that the problem was worse in the south of the country.

The latter was based on the more affected samples – ones where older triazoles, such as tebuconazole or difenoconazole struggled to control the disease – mainly coming from either France, or places in the south of the country, he says.

“In contrast the [dose needed] for tebuconazole [to control the septoria strain] from Scotland last year was very low, as it was from the Northumberland isolate in 2006.”

It made performance of tebuconazole inconsistent in the experiments, and could explain why tebuconazole works well on some occasions and doesn’t on others, Nufarm’s Tudor Dawkins suggests.

According to the results there had been some shift in epoxiconazole and prothioconazole performance, although by no where near as much as tebuconazole, while prochloraz also gave promising results.

That suggests prochloraz is a promising alternative or addition to chlorothalonil for septoria resistance management, Dr Rossall says. “Its preventative control in the controlled environment plant trials was as good as epoxiconazole or prothioconazole.”

Complicating factor?
Joint TAG and INRA investigations add to an already complicated picture. Their research has found that there are at least three moderately resistance sections of the septoria population, says TAG’s Jim Orson.
Each of the three, which INRA has called TriMR6, TriMR7 and TriMR8 contains the I381V mutation, plus other mutations that vary between the three, he adds. In six TAG trials last year TriMR6 dominated, while the moderately resistant population as a whole made up 80% of the population.
“Unfortunately for prochloraz, of the three moderately resistant strains, TriMR6 is the one it works less well on.”
The trials showed no serious population shift unless prochloraz was used alone, which for control purposes is not commercially acceptable. “Once it was used with an effective triazole there was no major shift.”
It means prochloraz must add to economic disease control and yield if it is to be used, he says. “There appears to be no other benefit in economic programmes.”