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Talking Agronomy with Luke Wheeler: OSR showing signs of disease

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As I write, at the beginning of February, crops are just starting to get moving, albeit very slowly due to conditions turning rather wet, and at times quite cold. However, it’s only a matter of time until the weather starts changing and crops begin to grow at a serious rate of knots.


Winter wheat is growing very slowly, which is to be expected at this time of year and black-grass control remains generally good. The combination of good seedbeds, a strong pre-emergence spray and some cold weather has all contributed to the longevity of the residual herbicides and the low weed levels being seen.


Oilseed rape disease levels are looking average, although higher than last year, with it being relatively easy to find both phoma and light leaf spot. An early spring fungicide for oilseed rape will be applied in the next two to four weeks, depending on weather conditions, aiming at the first signs of spring growth. The constitution of this application will depend on a number of factors.


Firstly the variety in question. Cleaner varieties such as Elgar will be getting a reduced application, whereas dirtier varieties such as Charger will be getting a far stronger mix. In some cases, active phoma can be easily found which will mean a fungicide choice and rate to target this infestation.


This winter is also proving to be colder than the previous few, so light leaf spot is also more of a risk. In some varieties it is also easy to find. All applications will include micro nutrients such as boron or manganese depending on soil type and crop need.


Looking at adult flea beetle and at this time of year the effect of flea beetle larvae, ADAS has conducted research showing that when either the cotyledons, leaf 1 or leaf 2 are severely damaged, the final green leaf area achieved by the crop was either no different or better than crops which were not damaged, showing the compensatory effects of the oilseed rape crop. Conversely, this does not take into account the problems which may be experienced further down the line from larval damage.


Where oilseed rape has had high flea beetle pressure and has now made it through to this stage, some of these crops are now showing signs of high flea beetle larvae in the plant petioles. Research has shown that when more than five larvae per plant can be found, yield benefits of about 0.34 tonnes/hectare can be achieved by including an insecticide in autumn or spring. Taking the plant and dissecting is the best way to determine this figure.


The five larvae per plant figure is based on old studies, new varieties could well withstand more larvae pressure. When the larvae are in the plant petioles, insecticides won’t directly kill the pest, the larvae do, however, burrow in and out of the petiole and can therefore pick up the chemical residues on the plant surface and may be controlled to some degree.

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