..in the early 1900s, farmers had a theory that degeneration in their potatoes was associated with the longevity of the potato, constantly propogated by tubers and denied the rejuvenation of sexual reproduction. They were on the right track - degeneration was not caused by old age, however, but was a concomitant of it, when the potatoes were grown in districts infested by greenflies. The farmers knew that, except in one or two favoured localities, the yield fell off when they saved seed-potatoes from their own crops, for planting year after year.
In England the farmers took to sending to Scotland for their seed, and thus the Scottish seed-potato trade had developed, a long time before the word 'virus' came to have any specialised meaning. Was there any real reason why Scottish seed-potatoes should do any better than those grown elsewhere? The farmers thought so, and backed their conviction with cash.
After 1920, entomologists began studying the habits of aphids infesting potato crops. They found that the Peach Aphis, Myzus persicae, was the one chiefly responsible for spreading the Leaf Roll disease. The number of peach trees in England, however, was insufficient to account for the prevalence of Peach Aphis on the potatoes, and one exceedingly important discovery was made by Maldwyn Davies of Bangor during the winter of 1931. He found the greenfly, in its wingless stage, in great abundance on savoys at a provincial market. He then went out with his specimen bottle into the market - gardens, and at thirty-six places visited in North Wales he found savoys harbouring the aphids over the winter. The insects found shelter in their wrinkled leaves. Like other parasites, they had more than one host plant - they also hibernated on brussels sprouts and cabbages, which were also green over the winter, and they moved to the potatoes from May onwards.Thus the aphids which spread virus disease among the potatoes were most prevalent near to market gardens.
Then there was the question of the dispersal of the aphids by wind. It might appear at first sight that a high wind would blow them all over the country. But here another piece of Maldwyn Davies's work was very instructive. He collected some of the live aphids and attempted to blow them off glass surfaces in the laboratory with a hair-dryer. He found that the insects were very reluctant to fly when the wind velocity exceeded three to four miles an hour, and that when it reached six to seven miles an hour they just hung on for dear life. Their natural provisions for holding fast in a wind which might smash them to pieces could be seen through the glass using a microscope.
Through this and other experiments it was established that the spread of aphids was favoured only by very gentle movements of air, and that where the windspeed often exceeded five to six mph very few aphids were found on the potato crops. High humidity also inhibited the insects' flight, and they did not like the cold. They would not multiply below about 55 degrees Farenheit.
Many puzzling phenomena were now explained. Sundry diseases of the potato were spread by aphids; insects relatively harmless in themselves, but pestilent because they sucked the juices of diseased potato plants and then flew off to feed on the healthy, which they injected with the viruses they had picked up.
In sheltered, warm localities and near to towns and and market gardens, as over much of the south of England, the potatoes were greatly infested by the aphids and they degenerated rapidly. The yield of tubers fell off when the locally-grown seed of one year was used for planting in the next. In parts of Scotland, where seed- potato crops were grown away from other cultivations, especially in bare, exposed or windy places neat the sea, the aphids found conditions unfavourable, and healthy potato crops could be grown year after year from local seed without showing appreciable degeneration. By taking out the few affected plants found in the fields here and there the seed stocks could be kept fairly free from virus disease. The high repute of Scotch seed-potatoes was nothing to do with any special skills of Scottish seedsmen; it was largely due to Scotland being too cold and windswept to be any sort of paradise for greenflies.
Paraphrased from "The Advance of the Fungi" by E.C.Large, 1940.
Nigel Deacon, Diversity website
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