Friday, December 15, 2017

Mild weather could prolong pest and disease challenge

December 1, 2017 by  
Filed under Crops

With winter crops now in the ground, growers should keep an eye on any emerging problems, says Richard Overthrow.

It is always dangerous to generalise but for most of us the autumn was not too bad in the end. The later drilling went in better than the earlier stuff and the feared increase in spring cropping as a result of failed autumn drilling didn’t materialise.

So far, the residual herbicides on wheat and barley look to have worked well and even where crops have been tickled by the herbicides where seedbeds and seed depth have been inadequate, crops are recovering.

Apologies to growers who are the exception, but most of us are up to date with this season’s crops. A few issues may cause us to break our hibernation over the next couple of months, however. At the time of writing cereal aphid numbers are starting cease flying into crops.

Mild spells

Warmer weather may extend this flight but it is driven by day length as well as temperature so doesn’t normally change much. What will be a of concern are any mild spells during winter which don’t necessarily encourage late flights into the crop, but help the very few aphids left to breed, proliferate and spread virus across large areas.

This is what happened during the 2015/16 winter, and we need to look out for similar conditions with a view to spraying a pyrethroid at the earliest opportunity in late winter or early spring, to crops thought to be at risk.

A warm wet winter will also encourage light leaf spot in rape crops. Many growers used a robust fungicide in mix with a propyzamide application last month, and this should protect the crop at least until stem extension.

Where single or final fungicides were applied early then protection may run out during winter, also if less-robust products were used, the disease may develop early in the New Year. That said, the risk models run by Rothamsted Research show the overall disease risk is much lower than it was in the recent peak years so we may find crops stay fairly clean and treatment unnecessary.

Serious issue

Phoma has been a serious issue for many this autumn and fresh infections were starting to appear in rape crops just as the main light leaf spot treatments were going on. These applications will also control phoma but if for any reason the crop is unprotected then any new infections appearing before the end of this month will need treating to avoid the possibility of stem canker. Don’t worry about the disease after the New Year.

Rape crops that came through a cabbage stem flea beetle attack may yet be threatened by larvae of the same pest. These can occupy rape plants in great number and there are no effective control options, we just have to see if the crop can come through it.

Wheat bulb fly can only be controlled by seed treatment now so any measures against it will already have been taken. However AHDB reports suggest this is a very low risk year, the lowest risk since monitoring began, so many with ‘borderline’ crops won’t have needed to take measures.

The friendly late autumn allowed timely sowing of winter bean crops but if there are any still to plant these can produce adequate crops when sown in December or January, though they will mature later than earlier sowings. Similarly, spring beans can be sown from early January onwards if desired.

Best wishes for Christmas and New Year.

Richard Overthrow is a regional agronomist with NIAB TAG, the UK’s largest independent agronomy organisation with several research centres in East Anglia. For more details, call 01223 342495.

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