Talking Roots with Darryl Shailes: Testing times for the potato crop

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Well – it has happened. The blight epidemic for 2016 is well under way, with plenty of red spots on the Fight Against Blight maps. There are infections reported from East and West, North and South and everywhere in between.


I suspect the actual number of blight infections will be higher than is being reported, so there will be plenty of inoculum for the rest of the season to keep the risk very high if the weather stays conducive to blight.


The first Smith Period Warning for my postcode came through on May 5, with another on June 1 and another on June 13, so not too many considering the amount of blight now being reported in crops. However, as we now know the modern blight strains – Blue 13, Pink 6 and the rest – do not respect the defined infection criterium that is a classic Smith Period.


It appears a trend is now developing where potato blight epidemics seem to happen earlier in the season than was the case prior to 2007, the one we always seem to refer to when talking about blight.


The 2007 epidemic which caught the industry by surprise. Blight spiralled out of control rapidly, several crops were lost and the rapid defoliation leading to reduced photosynthetic capacity meant reduced yields in many fields simply due to blight, which is in itself quite unusual. Strange as it may seem, there wasn’t a lot of tuber blight in stores that year, maybe due to the huge amount of blight sprays which were applied, or that the blight infection was so early most infected tubers had rotted away by the time harvest came.


We have come a little way since 2007 – not with many new blockbuster products – but how we are prepared to reduce the interval between sprays much more readily. At that time, 10-day intervals were more the norm and seven days more the exception. Now most of the industry accepts the only way to stay in control in such a high risk period as we’ve just experienced is with application intervals down to four-five days with robust products being preferred.


Of course, in a season like this where there has been between 150mm and 200mm of rain across most of the country in June, even the best-laid plans can be destroyed. Whatever equipment you have and blight sprays available to you, if it is always raining or you cannot travel as the fields are under water, then keeping the crop protected becomes a serious problem.


This excessively wet weather has not only led to difficult spraying conditions, but several other things which may now affect canopy longevity, and hence yield.


Has the rain washed away much nitrogen? Work done at Brooms Barn on sugar beet suggests the amount of rain over a short period of time can move available nitrogen down the soil profile by 1.5 metres. With more and more growers placing nitrogen and the jury out on whether nitrogen should be split or not, will it all be accessed? Many growers have spun more on and some applied foliar urea, but will it be effective in maintaining canopies and yields?


There are also reports of canopies showing P and K deficiency symptoms, again most likely due to waterlogging and growth cracks being reported. Also we know internal rust spot can become a problem where the normal water relations are adversely affected due to waterlogged soils and high humidity.


So all in all not an easy time for the potato crop, let us hope for some sunshine and dry weather to get things back on an even keel.


Arable Farming
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