Many ware growers have been frustrated in Scotland during the early part of April, watching progress in East Anglia and unable to turn a wheel themselves. Other than the odd field that went in through March prior to change in the weather, very little planting has taken place till until last weekend. This means that one of three key planting weeks for main crop has been lost and we need a good run from now for the majority of the area to be planted in the optimum window.
Conditions should be fine for a mid-April start to seed crop planting and there is no reason to delay this season. Potentially, both blackleg risk, and also virus risk due to an early predicted aphid flight, may well increase rapidly during the season. Earlier planted crops are more likely to be desiccated prior to potential pectobacterium contamination from rainfall and have more mature plant resistance to assist insecticide programs for virus control.
Early planting applies equally to pre-basic seed crops, which have traditionally been left till last, but once soil temperatures have reached 8C and conditions are good don’t delay. The more information we can get on aphid flights the better this season and growers need to get their yellow water traps out as soon as they have planted so the risk can be fully quantified.
Once the seed is in the ground emergence should be rapid in present conditions, with the real prospect of seeing April planted crops emerge within three weeks, compared to last season’s six weeks or more. So thoughts turn to herbicides.
One interesting debate is over timing of the residual herbicide and even the possibility of splitting the contact and residual. While splitting has its merits for efficacy in practice, particularly with the heavy reliance on contract spraying in Scotland, it’s usually difficult enough getting a herbicide on once. With current weather patterns, the prospect of long periods without rain prior to emergence seems to be more frequent. Therefore, once sprout development has come through the lag phase (>2 inches after from tuber) and emergence should be within 10 days, there is an advantage to applying the herbicide in the optimum conditions with moist surface to the ridge to get maximum effect from the residual. If conditions are dry, then the contact will have to do 80% of the job, so leave application as late as possible to maximise kill.
In terms of products, metribuzin at varying rates depending on weed spectrum and variety from 0.5-0.8kg/ha is still the main stay, with the addition of linuron and diquat in most situations where cleavers and annual meadow grass are not issues. Annual meadow grass was a more wide spread problem last season, but is mostly confined to being a serious issue north of Aberdeen after burn off. Flufenacet + metribuzin at 1.75-2.0l/ha is the product that can provide long enough persistence to prevent this, but metribuzin needs topping up to control difficult broad leaf weeds. Regardless of what residual you apply, if the annual meadow grass has emerged then 3.0-4.0l/ha of diquat is required to control it.