The fourth disadvantage to breeding for vertical resistance has already been mentioned. It is insidious, and largely unappreciated, but dangerous for this very reason. This is the decline in the level of horizontal resistance that slowly but inexorably occurs during breeding for vertical resistance. Vanderplank (1963) first recognised this phenomenon, and he called it the 'vertifolia effect' after a potato variety of this name. It was only after its vertical resistance had broken down that it was discovered that the Vertifolia potato was quite unusually susceptible to blight, because it had a remarkably low level of horizontal resistance.
The vertifolia effect can also occur when the screening population is protected with crop protection chemicals. This is because horizontal resistance can only be observed and measured in terms of the level of parasitism. If there is no parasitism during the breeding process, because of a functioning vertical resistance, or because the screening population is protected with crop protection chemicals, the level of parasitism, and the level of horizontal resistance, cannot be observed.
Individuals with high levels of horizontal resistance are relatively rare in a breeder's genetically mixed, screening population. They represent an extreme of a normal distribution. When the horizontal resistance cannot be observed, this means that individuals with only low or moderate levels of horizontal resistance are more likely to be selected, because of their other attributes. In the course of many breeding generations, the average level of horizontal resistance in the breeding population decreases until it reaches dangerously low levels. This explains why the breakdown of vertical resistance is so very damaging in most modern cultivars. The second line of defence, the horizontal resistance, is largely lacking.
This cryptic loss of horizontal resistance also explains why many modern cultivars need such large quantities of chemical pesticides if they are to be cultivated at all. Not a few breeders, who abandoned resistance breeding years ago, have been protecting their screening populations with crop protection chemicals. This makes the breeding work incomparably easier, but it also leads to this unappreciated decline in the level of horizontal resistance. It leads to a progression of cultivars that are increasingly susceptible to a widening range of parasites, and that require an escalating need for pesticide protection. We have been losing horizontal resistance to crop parasites for most of the twentieth century, and most modern cultivars have considerably less resistance than the cultivars of 1900.
It is in this connection that the recognition of genetic flexibility and inflexibility (see 1.15) is important. Being genetically inflexible, our crops cannot gain horizontal resistance during the cultivation process. Changes in the level of horizontal resistance can occur only with the genetic flexibility of the breeding process. If there is a vertifolia effect, all control over the level horizontal resistance is lost.
From these arguments, it is clear that screening for horizontal resistance can occur only after vertical resistance, if it occurs, has been matched, and only in the absence of all crop protection chemicals.
The vertifolia effect has had an unexpected consequence in the seed trade. Traditionally, farmers kept some of their own crop for seed, and they bought new seed only if they wanted a new cultivar. However, with a gradually increasing susceptibility to both pests and diseases, this practice of using your own seed became risky. Sustained advertising by seed producers and seed merchants persuaded commercial farmers of the need to buy new seed for every crop. Indeed, in some countries, they are required by law to do so. But, given good levels of horizontal resistance, this wasteful practice would be unnecessary.
It need hardly be added that much of the hostility to horizontal resistance comes from the chemical corporations and the seed trade, both of which positively require a degree of susceptibility. These commercial organisations also provide much research funding, and they have considerable control over what kind of research is being conducted.
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