Will stacking triazoles give better activity?

If Bayer CropScience’s azole fungicide Prosaro opened the door last year, this year’s launch of Cherokee by Syngenta has blown it off the hinges; triazole stacking is definitely the big “new” concept for this spring’s disease control programmes.

For the uninitiated, triazole stacking in simplistic terms means mixing or “stacking” two (or more) triazoles together to increase the total amount of triazole applied, the idea being either to boost performance and/or give a better spectrum of activity.

Each triazole fungicide intrinsically varies in its activity against disease.

For example, Proline is regarded as having the best protectant activity against septoria but is weaker against brown rust than Opus, while tebuconazole is strong against rusts, but is less powerful against septoria.

Part of the difference in activity is down to each triazole’s unique properties defining how quickly the fungicide gets into, and moves inside, both cereal leaves and fungi, and how long it lasts.

And it is those properties, together with a product’s protectant and curative profile, that need to be carefully targeted when building a successful triazole stack, says Bayer’s Alison Daniels.

“What you are looking for is azoles with unique and complementary disease strengths, which have different properties that work in synergy.”

But making a successful stack is not quite as simple as it may appear initially, she warns.

“You won’t always get what you expect just from looking at the active ingredients’ profiles.”

Formulation is as important as active ingredient.

“An active ingredient’s properties might suggest it’s slow moving, but in a formulation it might not be acting like that at all.”

Stacks that improve activity against one disease could potentially have a detrimental effect on a second disease, she suggests.

For example, increasing curative activity against brown rust, which tends to be deeper in the leaf and might need a triazole that is better at penetrating both the leaf and the fungus, could deplete azole reserves on the leaf surface and the ability to protect against septoria.

“It is not straightforward, and there are no watertight rules.”

Understanding how the azoles work does, however, offer some opportunities, she points out.

For example, both tebuconazole and cyproconazole could potentially improve on Proline performance.

“The combination of tebuconazole and prothioconazole makes the ideal T2 product.

The moderately fast penetrating tebuconazole helps pull the slower prothioconazole into the leaf improving rust activity and curative action against septoria without depleting the surface prothioconazole reserves needed for septoria protection.”

Formulating the two azoles into the one product Prosaro helps optimise its performance, she adds.

A second alternative “stack” for prothioconazole is a mix with cyproconazole.

Again the faster penetrating cyproconazole pulls prothioconazole through to enhance rust and septoria kickback.

“The drawback is because cyproconazole moves so fast, reserves of prothioconazole can be depleted reducing long-term persistence.

It makes it important to add in chlorothalonil to offset that.”

But it is a combination that has worked well in Agrovista trials, says the firm’s technical manager, Mark Hemmant.

“It has improved on prothioconazole’s already excellent septoria activity, and has been translated into yield responses.

The faster cyproconazole appears to give some vanguard activity before the Proline comes in.”

Agrovista will be using the mix, where appropriate, at the T1 timing, typically at 0.4 litres/ha of Proline and 0.12 litres/ha of Caddy.

“We’ll target septoria-susceptible varieties in particular.”

It is not a recommendation Dr Daniels believes is necessary, however.

“Our position is Proline doesn’t need another triazole partner at T1, although one might be mixed at flag leaf.

It is an extraordinarily good formulation, which gives growers everything they need at the T1 timing:

Long-term septoria protection, yellow rust activity and good stem-based disease control.”

Syngenta’s Cherokee, which has overcome the current compatibility agent problem, is another stack growers will consider this spring.

It too contains cyproconazole, along with propiconazole and chlorothalonil, and delivers a total dose of around 1.75 units of triazole if applied at the full 1.5 litres/ha rate.

The high triazole dose is part of the reason Syngenta believes it can compete with Opus and Proline.

“It allows more to be applied than may otherwise be possible, which might be appropriate in high disease situations or where less sensitive isolates are present,” the firm’s David Ranner says.

As with Prosaro, Mr Ranner suggests the differing speed of uptake and movement of the two triazoles is an important factor in its success as a triazole stack, along with an optimised formulation.

But not all triazoles are suitable for stacking. Both Bayer and BASF agree epoxiconazole and prothioconazole are not suitable mix partners.

In fact BASF do not make any recommendations to mix Opus with another triazole, says the firm’s Steve Dennis.

“The HGCA wheat disease management guide confirms Opus is the most complete triazole for foliar disease control, so why compromise by reducing the rate and adding a less complete product?”

Independent trials confirm that view, he claims.

“The data show a 50:50 mix of Opus and Proline is not as effective against septoria as Opus, and the mix would be weaker against rusts, to which a large area is at higher risk this year.”