Sources of Error

A number of common errors can occur in the understanding of horizontal resistance, and they must be clearly recognised by the amateur breeder, because they can be dangerously misleading. The first concerns the belief, held by many organic farmers, that organic farming produces plants that have an improved resistance to many parasites. However, good nutrition affects the physiology of the plant, but it does not affect the genetics of the plant. And resistance to parasites is controlled...

Recurrent Mass Selection

The main feature of recurrent mass selection is that it deals with quantitative variables, such as horizontal resistances. And it raises the level of many desired variables simultaneously, including all the horizontal resistances to all the locally important parasites. The method of recurrent mass selection is to grow a large screening population of seedlings which all genetically different. The best 10-20 individuals in that population are selected, and they become the parents of the next...

Grafting

Recurrent mass selection requires very large numbers of seedlings, almost without limit. Grafting potato scions on to tomato rootstocks is a technique for producing very large numbers of true seeds of potato. Selected potato scions can be grafted on to tomato rootstocks to induce flowering. Being unable to form tubers, the potato scion grows upwards as a vine, with a bunch of flowers every few inches. These flowers can be used for controlled cross-pollination. One good graft can produce up to...

The Vertifolia Effect

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...