New two-pronged potato weed routine works well – Farmers Guardian – Cam Murray, Andrew Goodinson

In the last of our potato agronomy series, Farmers Guardian asks Hutchinsons agronomists what we can learn from the challenging 2020 season ...

North: Cam Murray, East Lothian

Cropping profile: Salad potatoes, first and second earlies and maincrop

The exceptionally dry spring and unsettled summer presented some big challenges, but, at the time of writing (September 15), yields appear close to average with good quality.

Early pre-and post-emergence weed control was hampered by dry conditions in many areas, but a switch to a new routine proved successful, so a similar approach is likely next season.

The two-pronged strategy involves applying pendimethalin four to five days post-drilling to alleviate early pressure from polygonums and grasses, then metobromuron and aclonifen about a week before emergence.

Aclonifen’s persistence has been a real game changer for weed control and offers a good alternative to metribuzin where intolerant varieties are grown.

Blight pressure was exceptionally high through July and August, but the switch to Hutton Criteria reporting has helped keep incidence down.

The new warning systems more sensitive to conditions than Smith Periods, allowing growers to be more proactive in keeping blight out of crops with robust protectant fungicides.

There are also many good sources of blight reporting available, such as Bligtwatch and the Omnia disease forecasting option.

Cam Murray

This season has been a test bed for desiccation without diquat and learning how to best use the remaining alternatives, namely pyratlufen (as in Gozai) and carfentrazone (Spotlight).

For crops that had started senescing, a split dose 10 days apart worked well. However existing chemistry cannot take down large, green, indeterminate varieties (for example, Markies), so flailing followed by two treatments on remaining stems worked better.

Flailing is more intense and slower than spraying, but we have found desiccation takes about the same time as with diquat and, overall, fewer sprays are needed on indeterminate crops.

Uncertainty

Using high water rates and sunlight further helps maximise the efficacy of alternative chemistry.

There remains big uncertainty about long-term storage without chlorpropham especially as existing alternatives, namely ethylene and spearmint oil, will not suit everyone.

There is a solution in the form of DMN, which is already approved elsewhere in Europe, so it is frustrating UK growers still do not have this option.

West: Andrew Goodinson, Herefordshire

Cropping profile: Maincrop, 70 per cent processing, 30 per cent packing

After a difficult season, it is encouraging that yields are close to, or slightly better than, average, with good quality, however there has been some considerable yield variability.

Andrew Goodinson

The main cause, aside from aphid feeding, seems to be a legacy of soil structure damage caused by compaction on wet ground last autumn and winter. It again reinforces the importance of protecting soil health and many potato growers recognise the benefits cover crops can contribute to this.

Desiccation strategies without diquat have generally performed better than expected, helped by crops starting to senesce slightly earlier in hot, dry weather. Most have involved pyraflufen or carfentrazone to open the canopy ahead of flailing or a second spray seven days later.

Some crisping varieties treated with maleic hydrazide started senescing particularly early. And while this did mean slightly easier desiccation, the yield impact remains to be seen.

Effective

Using the correct water volume, reducing forward speed and setting up flails correctly have proven as important to effective desiccation as product choice.

Another issue highlighted this year is seed quality. There have been several reports of seed-borne diseases, such as silver scurf, rhizonctonia, or rots, reinforcing the need to pay close attention to quality when placing seed orders, not just price.

Consider visiting the seed supplier to ensure you are happy with the quality. Once seed is delivered next spring it is too late to do much about it if there is a problem.

Phosphate applications are something else to review, as many farmers in this area may be able reduce the amount applied and focus more on potassium and magnesium.

Soil phosphate indices are typically 3+ here, so there is plenty in the soil, providing crops establish quickly enough for roots to access it.

Although not widely used, placement fertiliser could help, especially on lighter soils with lower indices.

Finally with increasingly limited insecticide options in potatoes, accurate risk monitoring will be even more important next year, especially if there is a repeat of the aphid influx some areas witnessed this season.

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