Fourth consequence horizontal resistance lost

When breeding is conducted under circumstances in which the level of parasitism cannot be observed, the level of horizontal resistance tends to decline because of negative selection pressure. These circumstances include screening in the presence of unmatched vertical resistances, and screening under the protection of crop protection chemicals. This topic is discussed in detail below (see 6.6). The consequence of about a century of breeding in this way is that the levels of horizontal resistance in our crops are now generally low. This situation is possibly at its worst in potatoes (see 7.20.4), tomatoes, and cotton, but other crops spring to mind. The low level of horizontal resistance is revealed by attempts to cultivate these crops without any crop protection chemicals at all. (In this context, it should be mentioned that the relatively rare 'organic' farmer who does not spray his crops usually succeeds for the simple reason that the crops all round him are protected with crop protection chemicals. This means that the overall incidence of pests and diseases is low. Additionally, he may be using an heirloom cultivar which has high levels of horizontal resistance).

0 0

Post a comment