Potato blight - continued
Infection of potato as with blight can occur during lifting, and this can be avoided. In spite of spraying with copper compounds to control blight, there is always likely to be some blighted foliage at the end of the growing season. As spores cannot exist for long in the absence of moisture, lifting should be done in dry weather. A more effective precaution, which should always be taken in addition, is not to lift the crop until the haulm has been dead for about a fortnight. If for any reason it is desired to lift the crop early the haulm should be destroyed by sulphuric acid, or tar oil preparation, copper sulphate dust, copper sulphate and salt, copper chloride, or other agent which kills the spores at the same time as the foliage, or else the haulm should be cut down and removed immediately.
After removing or killing the haulm, the potatoes should not be lifted for about a fortnight, by which time any blight spores remaining on the soil will be dead. In this way the tubers will be kept free from disease, and the loss from blight in storage avoided.
Blighted tubers not only do not keep, but often by rotting in the clamps they cause a rise of temperature which will bring about other rots in sound tubers stored with them. Sometimes entire clamps collapse from this cause and all too often a considerable portion of the crop has to be thrown out, frequently so badly affected that it is useless even for feeding to pigs. It was estimated early in 1944 that 20 per cent of the 1943 crop in this country was ruined in storage, which represents well over a million tons of potatoes. And this does not include the potential loss of hundreds of thousands of tonnes due to the premature stopping of growth through a blight on the haulm. In Jersey the chief loss from blight occurs in the tubers after lifting and this sometimes amounts to half the crop.
During the years 1940 - 1944 a comprehensive series of field trials was carried out in Devon and Cornwall by E.C.Large. The results of 103 trials showed an average gain in yield of 2.75 tonnes per acre after two sprayings.
Blight is found all over the world were the potato is grown and similar measures we have to be taken to control it as in this country. Many places have much more humid warm weather than we experience here, and to obtain a reasonable crop, spraying several times in the season is necessary. For example, in Ireland, where the weather is nearly always favourable to blight, and where the potato crop is of immense importance, spraying with Bordeaux or Burgundy mixture is routine practice.
The Ministry of Agriculture recommend (1947) that in South and south-west England and in Wales spraying the haulm with copper fungicides should be regarded as a routine summer operation, and that spraying or dusting should also be carried out in South Lincolnshire and other coastal districts where blight occurs every year. In the northern and drier Eastern areas blight may not occur till so late in the season and that yield is scarcely affected, and in such places it will be enough to take preventive action if the weather conditions demand. There are however certain varieties of potato which are very susceptible to blight, among which are King Edward, Up-to-Date, and British Queen, and these should be sprayed wherever they are grown.
The above information was taken from a much longer article from the book "copper compounds in agriculture" published by the copper development Association in 1948.
Nigel Deacon, Diversity website
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