Getting the best out of fungicides to avoid building resistance

It erodes the efficacy of fungicide programmes, adds to input costs and leaves crops open to disease risks and yield losses. With a diminishing set of options as pesticides are withdrawn for regulatory reasons, the pressure on our remaining chemistry is intense and protecting and getting the best out of the fungicides which are left is crucial.

What are our current issues?

In wheat, the major concern is septoria, which is the primary target of fungicide programmes to protect yield.

Septoria has evolved almost complete resistance to the strobilurin fungicides and a steady decline in the efficacy of azoles has long been a feature for wheat growers – older azoles, such as tebuconazole, provide very little septoria control now.

Prothiconazole still gives effective control at higher rates but has declined to about 30% control at a full rate. The newest azole, mefentrifluconazole, remains stable but will not be immune to the shifts we have seen for all other azoles so protection is key.

The SDHI fungicides are at higher risk of resistance than the azoles and fewer gene mutations are needed to affect performance. Last season, we saw a higher frequency of problem mutations in the septoria population and also more isolates with multiple stacked mutations.

The newest group to the market are QiI fungicides (previously known as Inatreq). It is at very high risk of resistance, having a single site mode of action, and so must be used in mixtures and can only be applied once in a programme.

Yellow rust is major drive of fungicide use but, fortunately, is a low risk of resistance development.

In barley, the issues are less severe as strobilurins still retain good efficacy against the main yield robber, rhynchosporium, and there is a bit more diversity to bring to programmes as cyprodinil and spiroxamine are still in play along with SDHIs and azoles.

But broadly similar issues pertain and ramularia is particularly challenging as it has evolved resistance to SDHIs, strobilurins and older azoles. Net blotch has also evolved partial resistance to strobilurins and SDHIs so mixes are key to good management.

What to do?

The key principals that reduce the risk of resistance risk arising are:

• Do everything you can (before resorting to fungicides) to reduce the risk of a disease, so for example using resistant varieties, rotations etc to minimise disease pressure.

• Minimise reliance on any one active i.e. avoiding extra timings, minimising the rates used and using mixtures and alternations that mean that no individual active is over used.

• Make it as complex as possible for the pathogen to adapt. Again mixtures and alternations help here and should be from completely different mode of action groups.

Using multi-site fungicides, like folpet, is also helpful here as they require many, many mutations for the pathogen to break them so they are at very low risk of resistance. In addition using them might allow you to do other helpful things such as reducing the rate of the more at risk fungicides in your mix.

Wheat good practice:

T0: Try and avoid, but if you have to treat for yellow rust then use an alternative azole to the ones you will use later in the programme and minimise the rate you need by mixing it with a strobilurin.

T1: Use a balanced mix of an azole plus an SDHI or QiI fungicide and minimise the rates you use. Add in the multisite fungicide folpet to reduce the pressure on the more at risk chemistry.

T2: Alternate from the SDHI or QiI plus azoles used earlier in the programme, and again use a balanced mixture. Adding folpet as a multisite is also helpful.

T3: Try and alternate to a different azole and although not as effective as mixing with chemistry from a completely different mode of action group a mix of azole can be of some help. If other diseases are present then folpet, SDHI or strobilurin chemistry might allow you reduce the dose of azole required but be careful to stick within maximum application numbers and latest application timings.

Barley best practice:

T0: Only use if required and seldom required on a spring crop. On winter barley, try and use actives like cyprodinil and spiroxamine which you will not use later in the programme. Always use a mixture and use the minimum rate you can to get the control you wish.

T1: Use a balanced mix of an azole plus an SDHI or strobulurin fungicide and minimise the rates you use. You could add in the multisite fungicide folpet to reduce the pressure on the more at risk chemistry.

T2: Alternate from the SDHI or QiI plus azoles used earlier in the programme, and again use a balanced mixture. Adding folpet as a multisite can also be helpful in reducing the risk of ramularia.

T3: Avoid – this is very seldom cost effective in barley anyway.

Potatoes' best practice:

Potato blight has a long history of resistance issues and phenylamine resistance was an issue from the early days of systemic fungicide use, and their use in blight programmes eventually ceased.

Increasingly complex and aggressive strains of rust have evolved in recent years. Strains associated with reduced efficacy of fluazinam first appeared in the Netherlands a decade ago and have occurred sporadically in the UK since 2017.

The most recent concern is around the CAA group of blight fungicides which includes dimethomorph, mandipropamid and benthiavalicarb-isopropyl.

These were detected in Denmark in 2021 and a higher frequencies in 2022. They have not yet been reported in the UK but past history suggest the are unlikely not to spread eventually.

Potato best practice:

With a good number of different mode of action groups amongst the blight fungicides, the basic principles of mixing and alternating can be applied well.

Most labels carry specific advice about the maximum number of sprays that can be applied of any individual active, or state the maximum percentage of a programme that they can make up.

With regards to the CAA fungicides there have been a few new revisions which include:

• Do not use CAA fungicides for more than 50% of the total number of intended applications

• Do not apply CAA fungicides for more than 2 consecutive applications.

• Apply CAA fungicides in mixtures with effective partners belonging to different modes of action.

• Alternate CAA fungicides with other modes of action fungicides.