New Season, New Approach to disease control in wheat crops – Farmers Guide’s Guide to Spring Spraying – Dr David Ellerton

This year sees a double whammy of hurdles for wheat growers – the loss of previously relied-upon ag-chem, combined with delayed drilling due to the weather means that strategies need to change this year.

The wet autumn has left a legacy of extremely challenging conditions as we head into the spring and the start of the new growing season. Drilling opportunities have been limited-some wheat crops were drilled in good time, but many have gone in much later-and these two scenarios hold their own distinct set of challenges for disease control.

In addition, the revocation of established fungicide active ingredients and registration of a new exciting fungicide means that disease control will be very different in 2020.

We are already seeing infection in the lower leaves of some crops. However, we know things can change rapidly through the season – septoria infections are generally governed by rainfall in April and May, so it’s not possible at this stage to say that crops are at a low or high risk either way.

Whilst at lower risk of septoria, remember that later drilled crops are at higher risk of yellow rust. In later drilled crops, less time to develop along with the plants’ physiology, means less tillers and leaves. These fewer leaves and tillers need protecting in order to optimise yield potential-and there is more potential in a season like this to lose tillers.

Weather effects and drilling date are not the only factors to take into account when deciding fungicide strategies. Varietal disease susceptibility is a vital component and will dictate much of how to approach disease strategies for the season.

We now have in our toolbox much improved genetic resistance against septoria. On the latest AHDB Recommended List, the range of resistance ratings to septoria range from the highest rating of 8.1 for KWS Extase and LG Sundance at 7.9, to 4.8 for Leeds, Viscount and RGT Gravity and 4.3 for varieties such as Barrel and Elation.

Those varieties with the lower ratings than say a 5, are all particularly prone to septoria infection and will need to be prioritised during the season. However, varieties with a rating above 7 – such as KWS Extase and LG Sundance – are at significantly lower risk and fungicide programmes can be tailored accordingly.

All of these factors are important to take into account when planning disease control programmes－not just to ensure that the most cost-effective programme is chosen, but also to reduce selection pressure on fungicide active ingredients.

This season will see growers lose some of the most established fungicide actives such as chlorothalonil (CTL) and fenpropimorph, along with the triazole propiconazole. Many others, such as epoxiconazole, are under threat-not just from the risk of revocation, but also from increasing risk of fungicide resistance which is having a serious impact on triazole and SDHI efficacy.

To this end, it is vital that we do all that we can as an industry to ensure that stewardship programmes are in place, to protect the remaining ingredients in our armoury. This includes using appropriate doses of fungicides and using combinations of active ingredients with different modes of action.

Any available stocks of CTL on-farm need to be used up by 20th May this year, so where growers do have supplies of CTL they have this option. For those without CTL, and looking ahead, folpet would be the next best multi-site fungicide option used at a rate of 1-1.5-litres/ha – any higher than that and it becomes a much less cost-effective option. Mancozeb comes in as a third option if needed, but there are some doubts over its long-term future.

Looking more specifically at particular timings, despite the current Iow levels of disease in many crops, there will still be some benefit for a To spray on more susceptible varieties. Whilst the risk of septoria is low now, if the TO is dropped and the weather delays T1 applications, it’s a catch-up game from then on-so it’s well worth the effort as an insurance policy. It is also important to check crops for yellow rust infection and treat accordingly.

We may well see T1 coming a bit earlier this season; usually leaf 3 comes out at around GS32 but, with the mild conditions, we would anticipate crops racing through the growth stages and leaf 3 could be out at GS31 – so it will be worth dissecting the plant to see when leaf 3 is out to ensure that sprays go on at the right time.

What is important is to ensure that the timing for folpet is the same as for CTL – it needs to be applied once the third leaf is out, to ensure it is protected from the start.

Multi-site fungicides continue to play a very important protectant role in disease control and in slowing down the development of resistance, and to this end it’s important that they are not used alone, but in combination.

Resistance to triazoles seems to have levelled out and, although they are certainly less effective than they once were, we can still expect to get levels of 20-40 percent septoria control. Resistance to SDHIs however is still shifting, and control can be unreliable.

What is exciting for this season is the availability of Revysol (mefentrifluconazole) – a new fungicide from the Demethylation Inhibitor group of fungicides, that has shown excellent control of a range of diseases in wheat and barley, including septoria tritici. In our own and other independent trials, at T1 the product offers a major step forward in septoria control but would still benefit from a mix with a multi-site product.

It is vital that strategies are put in place to limit the risk of resistance developing to the new active ingredient Revysol, to ensure it remains effective for many seasons to come.”