Spud Special: Slow emergence raises rhizoctonia risk


Continued cool and often windy weather has made the timing of herbicides a real challenge, even for those using the split approach of applying residuals early, with a follow up with diquat just prior to emergence. Usually once the plant emerges and gets into rapid growth it will out compete and smother any remaining weeds. Not the case this year, with black bindweed and cleavers likely to cause problems.

Cleavers are relatively easy to control with rimsulfuron (Titus), but this is not the case with black bindweed unless post-emergence metribuzin can be added and that is only possible with a few varieties. Interestingly, should some weeds escape control, remember that two or three weekly low-dose (eg 0.1 kg/ha) metribuzin applications starting soon after emergence have proved very effective in trials carried out by SRUC over the last few years, in holding back weeds whilst the crop grows away.

Slow emergence also brings about other issues, particularly that of rhizoctonia and I am finding it fairly easily this year, typical symptoms being coiled sprouts and burnt off shoot tips/stolons (see below). In my experience once the crop emerges it will tend to grow away from the problem, but damage to roots and stolons may already be significant, so additional help with foliar nutrition may be beneficial. This would also be the case in fields where there are issues with high potato cyst nematode (PCN) counts.


Another point that I will be raising with clients is if the entire nitrogen requirement hasn’t already been applied, should we be reducing top dressing amounts? Particularly on those varieties that are late maturing and where achieving skin set can be an issue.

Recent rainfall seems to have encouraged slugs to the surface, certainly backed up by the numbers I am seeing as I walk round grass margins and through cereal crops to get to potato fields. Often out of sight, out of mind, but should never be ignored, therefore where slugs are likely to be a problem I am recommending applications of either ferric phosphate (eg Sluxx) or methiocarb (eg Karan) as soon as possible, with another application just prior to canopy closure. Remember all methiocarb products must be applied before 19September 2015. It is also worthy of note that ferric phosphate has no buffer zone restrictions and can be applied to headlands to act as a barrier to slugs coming from field margins and neighbouring crops.

So far weather conditions have not been conducive to blight, even the aggressive blue 13 and pink 6 strains. No doubt once the weather warms up and irrigation starts, which will provide leaf wetness, blight will start to appear, particularly in light of the amount of potatoes dumped or in stock feed piles, which will provide inoculum.

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