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Only time and the weather will tell how bad, or even if we have a problem with alternaria this season – Arable Farming – Darryl Shailes

Where there is complacency, the problem may be greater ...

Nature has an amazing way of compensating and we’ve seen it happen with gusto this spring.

The cold, dry April followed by nice rains and some heat into early June has meant that everything has grown at an astounding rate over the last few weeks and rapidly made-up lost time.

The grass in the garden seems to want cutting every few days. I think some sheep or goats are beckoning and the asparagus seemed to grow in front of our eyes in early June after a very slow start.

Sugar beet has also had a very strong growth spurt and some of the crops are across the row long before the Norfolk Show, if there was one at any rate. Next year, I’m sure will be more normal.

The aphid pressure has been much lower and the crops that I’ve looked at only received their first treatment in the second week of June, whereas last season when looking back through records it was toward the end of April. This supports the Rothamsted / British Beet Research

Organisation (BBRO) prediction model and we must applaud this sound use of science.

So, what’s next for beet? Cercospora and other foliar disease control must grab our attention. Last season was the first that most of the UK beet crop had been badly affected by cercospora. Some crops on the Suffolk coast had experienced severe issues in previous seasons but for most it has been an incidental disease, with rust and mildew being the dominant diseases in the UK beet crop.

Last season many fields suffered very badly and this may have been partly due to the industry being a bit complacent until they saw the issue and by then it was too late.

Where crops were treated with a more continental-type cercospora programme then they suffered less.

Less persistent

The trouble with cercospora is the resistance to strobilurins, which are the mainstay of rust and mildew programmes, and then the aggressiveness of the disease against the DMI or triazole group of actives. This means that the DMI component is less persistent than against rust and mildew and the strobilurin part has no effect.

The only recourse I see is that the old way of a treatment, early July and then again a month later, needs adjusting and warnings that will be coming out of BBRO will need to be acted upon. If the risk is high, then reduced intervals with strong DMI-based products will be needed.

In the rest of the world cercospora is the dominant disease and we need to learn from them. They have the advantage that they have resistant varieties being developed and more contact protectant fungicide, but still DMIs are used widely. We only have sulphur available to us as a broad-spectrum fungicide, but trials indicate its addition will help.

Alternaria could be the same in potatoes. Normally there would be a significant amount of mancozeb applied to potatoes by now, but the supply issues will mean that this loading will be down on the previous year. Mancozeb, while not completely controlling alternaria will have some effect, so can we expect more issues this season?

We do need to remember, however, that while we have approval for mancozeb until 2025, any decision could be made before then based on CRD review.

Similar to cercospora, alternaria is resistant to strobilurins in many situations and control will be based upon the approved DMIs with the lack of contacts. There is a new product available this season and it will be a useful addition to the programme.

Only time and the weather will tell how bad or even if we have a problem with alternaria this season, but maybe it will be a bit like cercospora where the risk is recognised and acted upon accordingly then the threat will be managed. Where there is complacency, the problem may be greater.

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