Talking Agronomy with Luke Wheeler: Planning to protect oilseed rape

As we head into late autumn, most oilseed rape crops are finally growing away from the intense flea beetle pressure of earlier in the season. Later drilled crops, and those which suffered high infestation levels, remain small, but with temperatures falling, flea beetle activity is decreasing.

Recent rain has meant phoma lesions are starting to appear on susceptible oilseed rape varieties, while cleaner varieties remain symptom-free. The AHDB phoma forecast predicted the 10% plant infection threshold would have been reached by the second half of October. Therefore most crops will have received a first fungicide and in most cases, where application is made before or at 10% infection, difenconazole as a protectant is both cost-effective and efficient.

With air temperatures and soil temperatures starting to drop, I am starting to think about propyzamide applications. In areas where black-grass levels are high, keeping the gap between clethodim and propyzamide applications relatively short (but with attention to clethodim label sequencing restrictions) will mean any black-grass receives two hits in short succession. Application of propyzamide while the grass-weed roots remain within the top 2.5cm of the soil is vital to ensure good uptake of the herbicide and satisfactory control.

A second fungicide to continue phoma control and perhaps also target light leaf spot (LLS) will likely be included with the propyzamide application. For the last few years we have seen an increasing level of LLS. Some vulnerable crops last year showed high levels of disease on both stems and pods at harvest. A preventative approach in autumn and early spring is vital for good control.

Peach potato aphids (Myzus persicae) are easily found in some oilseed crops on the underside of leaves. It is estimated about 70% of winged peach potato aphids carry turnip yellows virus (TuYV). In many cases symptoms of TuYV are not noticeable, but yield losses have been recorded.

Most winter wheat and barley crops were drilled by the middle of October. However, where winter wheat is planned for fields liable to high levels of black-grass infestation and a spring crop is not an option some growers are still waiting to ensure maximum black-grass emergence prior to drilling. Such a tactic reduces the pressure on the agro-chemistry but increases the risk of poor seedbeds, crop establishment or even failure to drill the total winter cereal area.

Rains in October have caused a good flush of black-grass so, if the weather allows drilling to be completed into good seedbeds, those which delayed drilling should have less weed trying to emerge post-drilling and their residual chemistry working better and for longer in the wetter, cooler conditions as we head into winter. Early drillers in mid/late September found seedbeds rather dry and where pre-ems were applied early, there is a risk residual herbicides may have degraded before the germination of black-grass has slowed. In such cases, the application of a residual top-up, with or without contact sulfonylureas might prove very helpful.

Slug pressure is variable, with the highest pressure following oilseed rape. Recent rain may cause an increase in pressure, with slugs moving up the soil profile. Remaining vigilant and regular monitoring will be key during establishment. Stewardship schemes limit metaldehyde applications to a maximum total dose of 210g a.i/hectare between August 1 and December 31, after which ferric phosphate will need to be used where necessary.

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