β Scribed by H. J. Toxopeus
No coin nor oath required. For personal study only.
At the start of a breeding programme mostly a cross is made between the resistant plant and a plant belonging to a commercial variety of the crop in question . If both belong to the same species the exchange of genes can be easily brought about . If, however, the resistance is present only in other more distantly related species, difficulties mostly arise .
In consequence of successful breeding work races of the parasite able to break down the resistance can come to the fore . It is of the utmost importance to combine the genes for resistance from all available sources so as to produce barriers that cannot be broken down or at least only with great difficulty . Therefore not only the easy line of intraspecific crossing should be followed but also the much 'more difficult one of interspecific and even of intergeneric crossing .
As in most cases resistance is dominant, the method of repeated backcrossing can be used with great profit . In each step of the backcross programme large numbers of seedlings should be raised, selecting very carefully the parent material that will be used for the next step, even if this selection takes several years . As a consequence the number of subsequent backcrosses can be restricted and the danger of loss of valuable genes for resistance and for other characters is very much reduced .
If resistance is recessive the backcross method cannot be applied to its full extent . The Fl is susceptible and by inbreeding more or less resistant seedlings can be obtained. If the resistant source is a wild species this program of outbreeding followed by inbreeding has to be repeated and progress will be very slow .
A point of major importance is the availability of quick methods for mass-testing . These methods need not be more reliable than is demanded by the kind of material that has to be tested .
The discovery of resistance, with a few exceptions, is the beginning of a breeding programme . The objective of such a programme is to combine resistance with a complex of characters that make a plant suitable for use as a crop .
The damage caused by a disease is seldom so severe that any new variety is accepta-Lecture read at the A-course "Resistance in agriculture", 12-14 Jan . 1959, organised by the Royal Society of Agricultural Science and the Netherlands Society of Graduates in Agriculture at Wageningen .
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