The most destructive disease of potato is blight. The potato famine in Ireland in 1845 and 1846, when blight ruined the crops and thereby caused the deaths of hundreds of thousands from famine, and led to the emigration of many more, illustrates how terrible the consequences of the disease can be if uncontrolled.
Blight is caused by a fungus (Phytophthora infestans) which indicates its presence on the potato foliage and stems by the appearance of brown spots, with the development, on the underside of the leaf, of whitish mould round the edges of the spots, easily seen if the leaves are examined during humid weather. (the way I recognise blight is to look for freckles, then a crinkled brown margin to the leaves - this occurs at an early stage; later on the entire leaf goes crackly and brown and rolls up). Spores break away from the infected parts and are spread into the air by wind and washed onto the ground by rain. Of those which drift in the air, some settle on other potato leaves, and when the weather is suitable germinate and produce more spores in a very short space of time. The haulm of the whole field of potatoes, in favourable conditions, can be entirely killed in a week or two, so rapid is the spread of the disease. The tubers depend for their growth on healthy foliage, and anything which affects it has a direct effect on the yield. Tubers harvested may also be infected with the disease and this affects their keeping qualities.
The figures quoted overleaf for a crop of Majestic potatoes giving a total yield of about 10 tonnes to the acre in Yorkshire (1941), will serve to indicate how the weight of tubers increases during the growing period:
Date, followed by total weight of tubers per acre in tons
In this case the potatoes evidently grew with little or no check by blight. But if the haulm had been killed by blight on 19 August there would have been no increase in crop after that date, and the loss due to blight would have been 4 tons to the acre. If haulm destruction had occurred later the loss would have been less; if earlier, greater still. Once the haulm is dead there can be no further increase of crop.
The progress of potato development varies with the date of planting, variety, seasonal conditions and cultivations, while the date at which blight occurs varies according to the district and the weather.
The only way to protect the leaves from the disease is to coat to them with a very thin covering of copper, which kills the spores before they are able to attack the leaf. The compound may be applied in the form of a spray or as a dust, and so long as an adequate amount of copper remains on the foliage, the disease cannot establish itself. With Bordeaux mixture copper deposits will remain on the foliage for several weeks, despite a good deal of rain.
In areas where blight can be expected every year it is a mistake to delayed applying spray until the disease is established. If spraying is to be fully effective the first application should be made shortly before the disease usually makes its appearance, or at any rate as soon as the first few spots are seen in the district. In other words the spray is a protective spray. This should be followed up after three or four weeks by a second application to protect the new foliage which has developed since the earlier spray, and also to reinforce the copper left from before. As a general rule, potatoes are ready for the first spray when the foliage is just meeting between the rows. In normal seasons two sprayings are usually adequate though in abnormal years more may be needed.
Where blight on the haulm is not prevented by spraying, the infection of the tubers can cause extensive loss in some years. Tuber infection arises from the blight spores which fall to the ground or are washed from blighted haulms into the ground by rain. The spores to a large extent are filtered from the rainwater by the soil, but some will be washed through crevices on to the tubers and cause infection. To minimise this risk, good earthing up is advantageous, though the best precaution is to keep the haulm free from blight in the first place.
The above information was summarised from a much longer article in the book " Copper compounds in agriculture" published by the copper development Association in 1948.
Nigel Deacon, Diversity website
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