Sunday, November 19, 2017

Keep on top of fieldwork to ease pressure on crops

November 1, 2017 by  
Filed under Crops

A poor summer has left disease and pest challenges as its legacy, says Richard Overthrow.

The non-summer of 2017 is behind us and with a few sparse dry windows harvest and drilling were slowly but eventually completed. Despite much delayed sowing most crops have established well and with more growers needing to apply residual herbicides soon after sowing, much of the autumn spraying will have been completed.

Growers who use SU herbicides for post-emergence treatment of blackgrass or bromes, for example, should look for opportunities to treat this side of Christmas. Our own trials have shown consistently that autumn treatment is better – the exception being soft or meadow bromes which are largely spring germinating. Similarly for broad leaved weed treatments, if growing and spraying conditions allow then autumn treatment should be considered.

Meadow grass

As stressed before, annual meadow grass should also be considered for autumn treatment. It is a weed that is often underestimated, as not being as serious as blackgrass or bromes, for example. But it is very difficult to control it in spring if it has started to tiller, in fact the most effective products against it are the pre-emergence residuals used for blackgrass, so campaigns against meadow grass need early planning.

Cereal aphid numbers were again high in the Rothamsted Insect Survey traps and whilst they normally cease flying, and hence colonising new fields, in early November, if late autumn is mild then aphid activity can be extended and insecticide protection also needs to be.

If aphids can be easily found in crops this month then further pyrethroid treatments to follow earlier ones or to top-up Deter seed treatments would be advisable.Those who have drilled late, either for grass weed management or through lack of choice due to weather, may not need any aphid treatment if the crop hasn’t emerged by the second week of November, but the caveats on mild autumns also apply here.

Flea beetle

In oilseed rape, cabbage stem flea beetle damage was widespread but was no worse than previous seasons. Some crops required re-drilling but experiences were incredibly varied so it’s impossible to generalise.

Whatever the extent of adult damage, we could still see a larval infestation starting this month. Insecticide resistance means that there is no effective means of controlling the larvae so if a problem arises we have to suffer it and see if the crop can recover.

For the first time in over a decade phoma infection was seen early and in the south there were cases of lesions on newly emerged cotyledons. Many crops reached threshold early last month and treatments applied: this was too early to effectively address light leaf spot so a second spray is needed, typically in the early part of this month, though many will hope to mix with a propyzamide application so precise timing will be governed by this.

Critical point

The general rule for propyzamide use is not to apply until soil temperatures have fallen sufficiently – although with recent mild autumns, this critical point has been reached quite late. Applying too early will still achieve weed control but persistence will be affected and with continuous emergence of grass weeds now common, some long term efficacy is important.

Weed control in winter oat crops is also becoming an early autumn affair. For grass weeds, Liberator is increasing used pre-emergence though products such as Absolute and Excalibur can be applied post-emergence,. Most autumn herbicide options in this crop are covered by EAMUs so be sure to follow the appropriate procedures.

Many winter bean crops will have been sown last month. Ideally weed control would have been done or at least started pre-emergence as post-emergence options are few.

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

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