Spud Special: Rain sets back apid build-up

After spending  13 years involved in potato production in Scotland the only thing I have found predictable is you can almost guarantee whatever your problems are one season, then they will be completely opposite in the following seasons. It is, therefore, no surprise that aphids and virus, blight risk, powdery scab and potentially blackleg and slugs are back on the agenda, with a the consolation prize of good early establishment, good weed control, potentially less common scab and no need as yet to get the irrigation equipment out. Unfortunately all the signs are that the market will be the same or possibly even worse.

Heavy downpours last week has set back the aphid build up, but the majority of seed crops have had to commence spray programs at 50% emergence, to control the high levels of predominately willow-carrot aphid and leaf-curling plum aphid that are present in both suction traps and yellow water traps. Where varieties are highly susceptible to potato virus Y (PVY) and where there is known virus source in the seed or from neighbouring crops and volunteers then mineral oil @3% has been added to the pyrethroid in programs up to tuber initiation.

Myzus persicae are currently only being found in the odd water trap, so a neonicotinoid in combination with pyrethriod is being applied in second or third sprays, according to pressure. Even in ware crops there has been a build up of potato aphid in crops, with some in the South at two aphids per compound leaf last week and close to justifying an aphicide, but rain has now delayed this at least another week.

I expect we will see first blight reports for Scotland imminently, particularly as the blight pressure has come during canopy expansion where it is almost impossible to fully protect the plant even on a seven day program, which many growers tend to forget. Crops just emerging are getting  cymoxanil + mancozeb, with cymoxanil + mandipropamid the main choice for canopy expansion and dimethomorph + mancozeb for crops close to full canopy. If you can weather the storm now there will be opportunities to save money later in the program – let blight in and you’re in for an expensive year. For this season I am sticking to methiocarb for slugs and starting programs prior to canopy closure.

Seeing water in the drills is never a good sign for powdery scab or blackleg. The wet start has enabled inoculum to rapidly increase in the soil and unfortunately with no heat wave on the horizon, a potentially wet tuber initiation could mean wide spread infections. We are starting with very low risk from blackleg bacterial levels on seed, but the season is rapidly increasing this risk and it will be interesting to see what appears over the coming months.

I am leaving the challenges of Scotland to concentrate my attentions on the new challenges of processing potato agronomy and variety development for McCain. Hopefully there will be more requirement for suntan cream than water proofs for walking fields.


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