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Appendix A

This figure shows a graph that is known as the "bell-shaped" or Gaussian curve. It represents a "normal" distribution of a quantitatively variable character, which might concern height in men, or the level of resistance in plants. This variable is represented on the horizontal axis of the graph, and it ranges from the minimum to the maximum. The percentage of the population, which possesses a particular value of that variable, is represented on the vertical axis of the graph, with the highest percentage at the top. The dotted line is the mode. In a symmetrical curve, the mode is also the mean, or the average, and it is exactly halfway between the minimum and the maximum.

Appendix B

Each of these two diagrams represents a potato clone and its reactions to eight races of the potato blight fungus Phytophthora infestans. The top diagram represents a clone with the resistance gene R1 and it is susceptible to any blight race that possesses the matching gene for parasitic ability. It is also resistant to any race that lacks this gene. The bottom diagram represents a clone with the resistance gene R2 and it is susceptible to any blight race that possesses the matching gene. It is also resistant to any race that lacks this gene. Differences in this kind of resistance are parallel to the vertical axis of the diagram and it is accordingly called vertical resistance. Vertical resistance is thus qualitative resistance in its inheritance and its effects. It is due to a gene-for-gene relationship. (After Vanderplank, 1963).

This diagram represents three potato clones which have no vertical resistance genes at all. Each clone is accordingly susceptible to every one of the blight races. However, clone C is more susceptible or, conversely, less resistant, than Clone B, and Clone B is more susceptible than Clone A. Differences of this kind of resistance are parallel to the horizontal axis of the diagram and it is accordingly called horizontal resistance. Horizontal resistance is not due to a gene-for-gene relationship, and it is normally inherited polygenically. It is consequently quantitative in both its inheritance and its effects. (After Vanderplank, 1963).

Appendix C

This diagram is called Pascal's triangle and it represents the probability of "either-or" events, such as heads or tails in the tossing of a coin, and boy or girl in one-child births. The left-hand column shows the number of events, and is represented by n. For example, if a coin is tossed twice, n =2 and there is one chance of two heads, one chance of two tails, and two chances of one head and one tail (i.e., head and tail, tail and head). These probabilities are called binomial coefficients and the triangle is easily constructed because each coefficient is the sum of the two immediately above it, to its right and left. For any number of events, the largest binomial coefficient occurs with n/2 events. For example, in a family of eight children, the total possible combination of four boys and four girls is 70. With a gene-for-gene relationship that has twelve pairs of genes, the maximum number of locks and keys occurs when each lock and key has n/2 (i.e. 12/2 = 6) genes. From the triangle, it will be seen that there are 924 possible 6-gene locks and keys.

Appendix D: CABI

The following institutes belong to Commonwealth Agricultural Bureaux International (CABI), and they offer crop parasite identification services, in addition to publishing abstracting journals, books, monographs, etc. Breeding clubs may care to write to them, enquiring about the services offered, and their charges.

International Mycological Institute
Bakeham Lane, Egham
Surrey, TW20 9TY
England
Tel: 0784-470111    Fax: 0784-470909

For a fee, this institute will provide an authoritative identification of a fungal or bacterial plant pathogen. It also publishes an abstracting journal (Review of Plant Pathology) and various books and monographs.

International Institute of Entomology
56, Queen's Gate,
London, SW7 5JR
Tel: (071) 584-0067    Fax: (071) 581-1676

For a fee, this institute will provide an authoritative identification of an insect specimen. It also publishes an abstracting journal, and various books and monographs.

International Institute of Parasitology,
395A, Hatfield Road,
St Albans, Herts., AL4 0XU
England
Tel: 833151    Fax: 868721

For a fee, this institute will provide an authoritative identification of a nematode specimen. It also publishes an abstracting journal, and various books and monographs.

Bibliography

Beek, M.A. (1988); Selection Procedures for Durable Resistance in Wheat, Agricultural University Wageningen Papers 88-2, 114pp.

Large, E.C. (1940); The Advance of the Fungi. Jonathan Cape, London, 488pp.

Pimental, D., & Lehman, H. (Eds), (1993); The Pesticide Question. Chapman & Hall, New York & London, 441pp.

Robinson, Raoul A. (1987); Host Management in Crop Pathosystems. MacMillan Publishing Company, New York, (now marketed by McGraw-Hill), 263 pp. Spanish Ed., translated by Roberto Garcia Espinosa (1989); Manejo del Hospedante en Patosistemas Agrocolas, Colegio de Postgraduados, Montecillos, Edo. de Mexico, 281pp.

Simmonds, N.W., (1979); Principles of Crop Improvement. Longman, London & New York, 408pp.

Simmonds N.W. (1991); Genetics of Horizontal Resistance to Diseases of Crops. Biol. Rev. 66, 189-241

Van der Plank, J.E., (1963); Plant Diseases; Epidemics and Control. Academic Press, New York & London, 349pp.

Van der Plank, J.E., (1968); Disease Resistance in Plants. Academic Press, New York & London, 206pp.

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