GENETICS OF HORIZONTAL RESISTANCE TO DISEASES OF CROPS

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Abstract
Summary: (1) Horizontal resistance (HR) to diseases of crops has the following leading features: it is polygenically controlled and must be studied by biometrical‐genetic methods; it is pathotype‐non‐specific and essentially ‘durable’; it has several components (e.g. infection rate, latency period, sporulation potential) which tend to be correlated; its use is relevant to the control of all classes of pathogen (fungi, bacteria, viruses, insects, nematodes); though often unrecognized, it is very common and generally the means by which ‘minor’ diseases remain minor. HR may be contrasted with ‘vertical resistance’ (VR) which is procured by major genes and is pathotype‐specific. VR has uses in particular circumstances but has often failed against airborne epidemic pathogens such as many rusts; hence the importance of understanding HR.(2) In this review the genetic evidence of HR is reviewed and examples are summarized in appendices. Very diverse crops, places and pathogens are represented. So far as may be judged, HR is indeed universal, and found or constructable wherever sought. The most important genetic evidence is based on generation mean and variance analysis, general and specific combining ability, offspring‐parent regression and response to selection. Useful supplementary evidence comes from historical observations, continuity and repeatability of resistance and yield‐related effects.(3) The main conclusions are that HR is universally available, usually highly heritable and responsive to selection, already keeps a host of crop diseases down to acceptably low levels and has socio‐economically attractive features that are likely to increase its use in the future. In particular, it offers long‐term stability of performance that must be valuable to small farmers in the Third World and is environmentally attractive because successful breeding programmes minimize the need for environmentally damaging chemical protectants. ‘Green’ pressures are likely to favour HR and some relevance to public policy is thereby implied.
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