The first time I became aware of plant viruses was in 2002. I had planted some fresh seed potatoes from a registered supplier the year before and the yield was excellent. The climate was just right - lots of light, plenty of rain at the right time, and a long growing season.
The next year I used some of my own seed, of the same varieties, and everything was fine until mid July. At that point I noticed some of the leaves going crinkly, and a lot of the new shoots drying up - an indication that the plants had stopped growing. Some of the leaves went mottled. The yield, when the tubers were taken out three weeks later, was down by over 75%, and the tubers were poor quality. For example, Golden Wonder tasted more like poorly-stored Picasso; the superb frying quality had gone, and the texture was spongy rather than crisp when cut with a knife. I consulted someone more knowledgeable than me, and was told "virus damage".
What follows is taken from a book from sixty years ago, when knowledge of viruses was just beginning...it starts by describing an experiment involving the Leaf Roll virus, widespread over England.
Healthy potato plants, inoculated with the virus of Leaf Roll by aphids which had fed on diseased plants, became affected in all of their parts except their true seeds. By planting infected tubers the disease was continued. The plants were stunted, their leaves rolled up into trumpet shapes, and the next tuber crop was small. All the potato plants in cultivation in the 1930s were susceptible in some degree. The only way to stop the Leaf Roll decimating the crop was to use seed from the aphis-free reservations every year or, at the most, every two years.
There were other viruses too - causing degeneration and dropping of yield: mosaic viruses, including Crinkle, then there was Curly-Dwarf, Paracrinkle, Leaf-Drop Streak. In Kenneth Smith's book of 1937, eighteen viral diseases of the potato were described. They were caused by a collection of viruses acting separately or in combinations of two or more at a time. In the early thirties only four viruses other than Leaf Roll had been distinguished: A, F, X and Y. Each of these existed in a number of strains, differing in their virulence to different potato varieties.
X viruses were not known to be carried by insects.They sometimes mottled the leaves or caused a simple Mosaic. They were often carried passively, causing no marked symptoms of disease. Every King Edward potato since the first seedling was raised contained some of virus X. All American varieties were said to contain it, too, and most suffered no harm. But X was lethal to Epicure and Arran Crest.
The Y-viruses, on their own, caused vein-mosaic or vein-banding.
X plus Y together caused Leaf-Drop Streak, probably the most serious virus disease after Leaf Roll.
X plus A caused Crinkle. And so it went on ....
Many of the varieties which farmers had chosen in the past owed part of their practical superiority to the fact they were good carriers of some of the viruses, but were unaffected by them. Then it was found that if a potato was inoculated with a certain mild strain of X, inoculation ten days later with a virulent strain of the same virus caused no ill effect. Preventive inoculation of susceptible potatoes might be a possibility.
There was a hope that some day a means might be found of inoculating against virus Y, which was the most deadly of the known viruses. But the one simple and practical measure for all the potato virus diseases was to plant clean seed. There was a need for the Ministry of Agriculture to over-ride commercial opposition and to issue Health Certificates for seed potatoes.
It took an expert to understand the significance of the certificates actually issued during the thirties, which were more concerned with trueness to name of variety (or locality of origin) than with freedom from disease.
Paraphrased from The Advance of the Fungi, by E.C. Large, 1940
Nigel Deacon, Diversity website
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