Disease control options in crops this April

Crop Progress

Cereals look in good health as we approach the end of March, although they haven’t started to really motor yet after being held back by the odd spell of cold weather, which isn’t a bad thing.

Oilseed rape is all over the shop, but a fair few more crops have been ripped up since my last column two months ago. It’s been a combination of either one of the two pests and / or frost that’s done for a lot of crops. Some crops that only had flea beetle attack might have made it through, but the frost has got into the stems possibly off the back of flea beetle damage. All told in this region I estimate around half of all oilseed rape crops have been lost.

Ben’s agronomy tips for April

1. Net blotch is enemy number one

There’s some brown rust in hybrid barley varieties, which considering the cold is a bit of a surprise. Fortunately barley brown rust is much easier to control than wheat brown rust so shouldn’t be much of a problem.

Much more concerning is there’s some high levels of net blotch in some crops, which could be the result of one or more of infected seed, the weather or more barley in the rotation. So that’s something growers are going to need to consider when it comes to the first fungicide of the year.

Everyone is worried about Septoria resistance in wheat, but net blotch in barley is more of a problem. We are without doubt seeing a shift in prothioconazole – it’s a small change but it’s there. We know SDHI fungicides aren’t working as well as they used to on some of the net blotch isolates, and the F129L mutation to strobilurins is in around 80% of isolates collected across Europe – so there is no doubt net blotch is a problem.

The key in winter barley at T1, which is the most important timing in terms of tiller retention and therefore yield protection, is mixed modes of action. If you think net blotch is going to be a significant problem in your crop, I would go for 0.9 L/ha of Ascra  (bixafen + fluopyram + prothioconazole) as the fluopyram improves activity or 0.6 L/ha Siltra (bixafen + prothioconazole) + Comet (pyraclostrobin) to bring in three modes of action. An alternative to Comet could be trifloxystrobin if you have access to it through your supplier, as that seems to be probably the best strobilurin against net blotch.

Where net blotch is less of a problem then Siltra is probably enough as long as you keep the dose up at 0.6 L/ha to get 120g/ha of prothioconazole.

There are alternative competitor product co-packs that contain prothioconazole, strobilurin and SDHI which could be good options too, but you may have to drop rates to get them to a realistic price and that’s not what you need with barley at the moment.

2. Consider beefing up first spring barley fungicides

The same also applies in spring barley, where the first sprays are likely in April around the end of tillering, particularly with quite a lot of February drilled crops this year.

Usually in spring barley the second timing at awns emerging brings the biggest yield response, but with the threat from net blotch it might be better to do a 50:50 split. Without Raxil Star seed treatment to use, I’ve heard reports of seed with 20% infection, so if you’ve got higher levels on seed or it’s in the crop going light at T1 could be a problem.

Technically that might mean doses near what you would use on winter barley, albeit in spring barley it can be quite difficult to get the yield responses on occasion. I’d suggest 0.4-0.5 L/ha of Siltra or 0.7 L/ha of Ascra at T1 in spring barley.

3. Target yellow rust at T0 in wheat

I think most growers would agree that T0 in wheat is now a yellow rust timing. If you have a variety that you are worried about getting rust in, or already has rust, then a tebuconazole with or without a strobilurin fungicide will be worthwhile.

To be honest it’s probably quite a short list of varieties that aren’t at risk, as even some of those with good adult plant resistance scores are susceptible as juveniles, and if that might not kick in until flag leaf that’s a long time to hope the genetics get you out of a hole eventually.

But if you aren’t concerned about rust you can probably relax until T1 because there’s very little evidence that folpet, which is really all we have for Septoria at that timing is going to have much effect at T0 in this part of the country.

4. Monitor for Septoria during April

Some crops have got high levels of Septoria in the base, which doesn’t necessarily mean much in March as if it doesn’t rain in April when the crop starts stem extension, the pressure will be low – but there is inoculum about.

What your T1 should be depends very much on drilling date and variety choice, and whether you also consider yellow rust to be a problem coming into T1. Obviously I would say this, but I don’t think you can go too far wrong with 1.0 L/ha of Ascra, maybe up to 1.2 L/ha if the weather does carry on being iffy through April.

There are other options available – but while the relatively lower price of Univoq (fenpicoxamid + prothioconazole) makes it an interesting option at T2, I don’t think it is an option at T1.

If Septoria pressure is high at T1 then growers could turn to Revystar (mefentrifluconazole + fluxapyroxad) but you will need to be prepared to spend decent money on decent doses. By decent doses I mean 1.0 L/ha . Where growers are using 0.75 L/ha on weaker varieties like Skyscraper that’s not sensible. There’s no doubt mefentrifluconazole is not as strong in trials as it was a few years ago – Revystar is almost a straight azole product as the fluxapyroxad dose is low at field rates, while Ascra’s Septoria activity is driven predominantly by the SDHI component with prothioconazole offering only a supporting role. 

Where prothioconazole does stand apart, however, is on other diseases such as eyespot and mildew. I’m yet to be convinced from what I’ve seen that Revystar has eyespot activity and the same is true of yellow rust where Ascra is a class apart. Obviously on the latter Elatus Era (benzovindiflupyr + prothioconazole) fits in, but I feel it’s weakness on Septoria now outweighs its strength on yellow rust.

The last product consideration at T1 is whether to use folpet. From a purely resistance management point of view, you should use it at 1.5 L/ha with your T1, but from a farmer’s perspective I can’t guarantee the yield response is going to pay for itself.

As for timing, it’s a case of cutting plant’s open, finding final leaf three and ensuring that leaf three is as good as fully out on the main tiller by the time you apply T1.

5. Watch out for variable flowering in oilseed rape

If you’ve still got a crop worth combining, it’s quite likely there will be variable dates of flowering given some plants might have survived better than others. If flowering extends longer than three weeks, it might mean you require two sprays to protect against Sclerotinia.

Use the strongest option for the first spray, and a cheaper option for the second as late infections will be significantly less damaging than earlier ones.

Watch out for light leaf spot too – February results from SpotCheck showed a high percentage of crops with light leaf spot. That will push you towards a choice of prothioconazole-based product, if you’re finding that. While Aviator (bixafen + prothioconazole) is at a different price point compared with generic prothioconazole, what you can’t buy as easily is the second effective mode of action in Aviator which is important to have especially with light leaf spot. Add in the formulation advantages and it’s worth considering the trade up for the first spray.