The 'vertifolia effect' is a loss of horizontal resistance that occurs when the plant breeding is conducted either in the presence of a functioning vertical resistance, or under the protection of a fungicide or insecticide. It happens because the level of horizontal resistance can be assessed only by the level of parasitism, and this is possible only if the parasitism is present and active. Individual plants with a relatively high level of horizontal resistance are in a minority. If the level of resistance is invisible because there is no parasitism at all, susceptible individuals, which are in the majority, stand a better chance of being selected on the basis of their other attributes.
This loss of horizontal resistance to potato blight began soon after the discovery of the first fungicide, called Bordeaux mixture, in 1882. This fungicide made the breeding much easier because so many seedlings were no longer being killed by blight. The gradual loss of resistance was not recognised at the time and, even if it had been, no one would have cared, because it was believed that resistance was not necessary, as the crops could now be sprayed with Bordeaux mixture. All potato cultivars bred after about 1885 were increasingly susceptible. At about this time, in North America, farmers began to spray against Colorado beetle. The only insecticides they had were very nasty compounds of lead, arsenic, cyanide, and mercury. Later, with the use of vertical resistance, the vertifolia effect continued. And then DDT was discovered, as well as many other synthetic insecticides and fungicides. Because of the vertifolia effect, which has now been operating for a century or more, modern cultivars of potato have less horizontal resistance than the cultivars of 1880. Our task is to ensure that they have much more horizontal resistance than those old varieties.
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