Case Study: Gardner’s grid system and plant selection efficiency in cotton (Verhalen et al., 1975)
5.8 Discussion
including Myzus persicae, a major virus vector (Russell, 1978). Hairy cultivars of cotton are more susceptible to some Lepidoptera than others, e.g. glabrous cot- ton cultivars resistant to Helicoverpa spp.
are susceptible to Spodoptera littoralis (Norris and Kogan, 1980). Glandless cot- tons are more susceptible to Helicoverpa spp. and blister beetles Epicauta spp.
but are resistant to the boll weevil Anthonomus grandis. These resistant/sus- ceptible relations are a result of an imbal- ance in the plant pathosystem because of an emphasis on one particular pest. One problem is simply being replaced by another: an undesirable situation but one exacerbated by the lack of a holistic approach to breeding resistant plants.
Durable resistance to insects is possible, provided breeders select for horizontal rather than vertical resistance and prefer- ably for horizontal resistance that is not selected for, purely on the basis of a single mechanism of resistance (durable major gene, horizontal resistance). Horizontal resistance is particularly applicable to insects because they have a high dissemi- nation efficiency and tactical mobility, which suggests the general lack of a verti- cal subsystem. This makes breeding for horizontal resistance relatively simple but it still requires a major shift in approach by many breeders. The problem lies in the reluctance of breeders to consider a new line of reasoning and a consequent change in breeding methodology. With the advent of genetic manipulation, which represents an even more extreme single gene based approach, it is unlikely in the near future that horizontal resistance breeding will gain much attention.
In the same way that genetic manipula- tion is not a panacea for insect pest control, there are of course situations in which breeding for horizontal host plant resis- tance would be totally inappropriate. For instance, crops with a high commercial value that have special qualities, e.g. wine grapes, date palm and pineapple, the essential characteristics of which could be lost during the breeding process. Also in
highly commercial crops the costs of con- trolling the pest insect may be small rela- tive to the crop value, so there is little incentive to use horizontal resistance, even if it were possible. Robinson (1987) argues that the agricultural value of horizontal resistance tends to be inversely propor- tional to the commercial value of the crop (Fig. 5.21). Therefore breeding for horizon- tal resistance will be most appropriate to subsistence farming and pasture crops where profit margins will not permit expenditure on insecticides. The need for developing higher yielding, high quality food crop cultivars resistant to major pests for the benefit of farmers in developing countries has long been recognized. The use of resistant cultivars provides the most appropriate means of control for subsis- tence farmers and is one form of new tech- nology that does not require the farmers to make fundamental changes in their way of life or farming methods.
shown in developing insect resistant culti- vars. Resistance to insects is often only par- tial (de Ponti, 1983) and since this can produce problems for the breeder, the development of resistance to insects has taken second place to that of resistance to pathogens where large discrete differences have been found. Secondly, the progress in plant breeding technology and procedures has mainly centred around the temperate zones where most of the major crop prob- lems are associated with pathogens. Also, for the years in which plant breeding gained in importance, insect pests were being controlled effectively and cheaply by insecticides, so that insect resistant crop plants were considered unnecessary, or at least of only secondary importance. The role of resistance to either pathogens or insect pests though, has always been sec- ondary to that of improving crop yield and quality, and it is this obsession with maximizing yield that has fashioned the approach of breeders to that currently followed today.
Where resistance has been incorporated into crops either by vertical resistance from conventional breeding or genetic manipula-
tion, then it will always be susceptible to resistance breakdown. This could involve farmers in the ‘boom and bust cycle’ evi- denced within pathogen resistance – it can be very costly! The costs to the farmer and to the region or state can be enormous if a resistant cultivar breaks down, especially if the crop is a staple food. An epidemic can result in widespread damage and some- times total crop loss, so that individual farmers can lose their income, source of sta- ple food and seed for the following year’s crop. Governments may find that they have to spend more money on insecticides, on importing alternative foodstuffs or request- ing aid from other countries. As far as the farmer who utilized the resistant crop culti- var is concerned, the costs of a resistance breakdown can vary from a mild nuisance to economic and social disaster. The real- ization of the potential for the latter places a grave responsibility on the breeders and gene manipulators of resistant cultivars.
The adoption of horizontal resistance as a long term durable method of control is unlikely until the evidence for its value is overwhelming. However, in the present cli- mate of enthusiasm of genetically manipu- Fig. 5.21. The value of horizontal resistance to different types of agriculture (after Robinson, 1987).
lated crops, horizontal resistance research will not receive the funding it requires to prove its case. The consequence will
inevitably be a benefit to the commercial seed producers but not to sustainable resis- tance in crop plants and farmers.