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Storage Bulletin - November 2014

Publication Date: 
20 November 2014

As ambient temperatures start to fall, especially at night, the risk of structural condensation within stores increases. There is continual moisture release into the store environment by evaporation through the skin and from respiration by the crop. This means the storage environment typically equilibrates at humidities in excess of 90%.
Depending on the level of insulation in a store, it usually only needs a relatively small change in ambient temperature to result in structural condensation. Typically, within the normal temperature range used in potato stores, a 1.5°C change in temperature will result in condensation. The cold surfaces of, for example, metal Z-purlins are often the first to show beads of moisture.
In chipping/processing stores, where the threat is perhaps highest, the use of roof space heating will alleviate the risk as adding electrical heat lowers the humidity of the store air. This should be thermostatically controlled to instigate heating if the air temperature in the roof space drops more than about ¾-1°C below the crop. Using perforated polythene ducting with a roof space heater is a good way of distributing the warm air evenly around the store. If heating is not available use local, recirculative ventilation to minimise the formation of condensation.
Reports of skin set problems have been widespread this autumn and are a likely source of disease ingress, which has resulted in a significant and increasing number of cases of breakdown in storage since harvest (see our October bulletin).
This problem is not a new one; it is a known factor which is exaggerated in some seasons, especially where there is a need to re-handle indeterminate or late-maturing varieties.
Work was carried out back in 2001-2005 by Milne et al., as part of a project funded by the former British AHDB Potatoes, which broadly concluded:
  • If a variety is known to have a tendency for poor skin set or unsetting, then handle as gently and as little as possible. 
  • Ideally, handle it within two hours of harvesting as skin set can deteriorate progressively from lifting.
  • After harvest, try to avoid handling the crop until the skins have become fully set. This may take weeks in some cases to be achieved.
  • Temperature has an effect on skin set but it is not the only factor contributing to poor skin set in the field or unsetting of skins.
  • The environment around the tuber has a part to play in achieving skin set, although it is not known exactly how or why and it is therefore difficult to manipulate it with confidence. However, dry soils produced a better skin set than wetter ones and it may be reasonable to assume that crops from wetter soils are more prone to skins unsetting.
This will be our last storage bulletin before the Christmas break and it is therefore timely to remind users of CIPC that stewardship best practice requires judicious use of the chemical at all times. Please don’t make further applications to processing crops unless you can demonstrate a clear need for re treatment. The limit for 2014/15 is 58 grams CIPC/tonne and there is a 14 day withholding period after final treatment before sale. Data from work by Sutton Bridge CSR has shown that, in many cases where second doses are applied, they are in fact made too early or are even unnecessary. If an initial dose has been correctly and evenly applied, CIPC will be uniformly available and will offer control at levels as low as 2 ppm. As ever, consult your BASIS advisor before making any CIPC application. 
Remember, as was the case last year, second applications can no longer be made to fresh crops being held in cold stores at 5°C or below. 
Further guidance is available from the Potato Industry CIPC Stewardship website at 
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