Double-binding fungicide brings practical benefits to growers

Early understanding of the mode of action of the new-generation SDHI fungicides has enabled Syngenta to develop an active ingredient with increased potency which gives longer protection, the firm said at a press briefing held to give greater understanding of the molecule's chemical properties.

Succinate dehydrogenase inhibiting (SDHI) fungicides were first introduced in the 1970s but had only limited activity on key diseases, Tyler Harp, Syngenta's global fungicide technical manager, explained.

But the knowledge gained about their mode of action has allowed pesticide manufacturers to develop new molecules with greater activity, with the resulting wave of new-generation products being introduced in the next few seasons.

The new products include Syngenta's Isopyrazam, introduced last year for barley as Bontima. The firm is hopeful of a registration in wheat for the UK this spring.

All of the new generation have a pyrazole ring, but Isopyrazam has a second benzonorbene chemical ring.

The two rings give unique binding characteristics, allowing the product to bind strongly to both the target site in the disease pathogen and to the leaf surface, Dr Harp said.

Tests have shown that Isopyrazam has the strongest affinity for the target site within the mitochondria of the septoria fungus of all the new SDHI fungicides, because of its chemical structure.

"It is a measure of the active's intrinsic potency. It is up to 20 times more potent than boscalid and five times more than bixafen."

Increased potency didn't necessarily translate into better field performance, he admitted, as formulation could change potency, but field tests had confirmed Isopyrazam as being more active against septoria than epoxiconazole.

"And because of that high intrinsic activity, it also has some curative activity - not as much as triazoles - but it is a good complementary fit."

The benz-pyrazole structure meant it also bound strongly to waxes and oils within the leaf, Dr Harp said. That had two results - one, that a pool of active ingredient was available for longer and provided a preventative shield against disease penetration, and two, increased rainfastness.

Practically this resulted in an extended duration of disease control, longer green leaf retention and ultimately higher yields, he said.

Field experiments conducted by Harper Adam University College suggested adding Isopyrazam to a base programme of full rates of epoxiconazole plus chlorothalonil, followed by prothioconazole plus tebuconazole retained green leaf area for a further two days.

"Research has shown delayed senescence is a good indicator of increased yields."

Some of the green leaf retention could come from physiological responses to the fungicide, he believed. "There is some evidence to suggest that there are similar effects as with the strobilurins, which are another fungal respiration inhibitor. It is an area we are doing more research in."

No delays in harvesting had been seen in trials after using the new fungicides, he added.

Source: FWI