Preface One of the main characteristics of life is the phenomenon of self-perpetuation, ensured by three major control activities, steady-state control, reproduction, and adaptation. When the steady-state control becomes inoperative, or operates inadequately for any reason, the organism concerned suffers disease. According to Paul Weisz, therefore, a disease may be defined as "...any structural or functional breakdown of steady-state controls, that is a temporary unsteady state". Then, also a plant disease may be looked upon as the result of a disturbance of the regulation mechanism of the plant, caused by any "stress"-factor, be it biotic or abiotic. It is, essentially, from this point of view, that interest in the study of plant diseases is gradually shifting towards an attempt to understand the physiological and biochemical backgrounds of the disease process. Without hesitation, the assumption may be made that in the future a further shift in emphasis will be noticed from a more descriptive type of research to an approach aimed at the understanding of basic metabolic processes of host and parasite. This idea is reflected in the subjects chosen as topics of the symposium. Increasing knowledge on metabolic changes as a result of infection of plants, and on the causes of natural and induced resistance will lead to a better insight into host-parasite relationships, which might provide a more rational base for controlling plant diseases and for obtaining disease resistant varieties. In the Proceedings of this symposium the papers have been arranged in the same order as they have been read during the symposium. The Organizing Committee deeply regrets that two of the speakers, Dr Farkas (Budapest, Hungary) and Professor Shaw (Saskatoon, Canada), at short notice, were unable to attend the symposium. After replacement of their papers* by two other ones, the programme contained the following contributions.

September 12 (Chairmen: Professor A. J. P. Oort, Wageningen, The Netherlands; Professor R. K. S. Wood, London, United Kingdom) A. J. P. Oort: Welcome and introduction (not published) H. F. Linskens: Host-pathogen interaction as a special case of interrelations between organisms R. Heitefuss: The significance of changes in nucleic acid metabolism for the relations between host and obligate parasites G. Wolf: On the incorporation of ~2p into the various nucleic acid fractions of rustinfected primary leaves of wheat A. Fuchs: Metabolism of aromatic compounds in healthy and rust-infected primary leaves of wheat (not published here; see Can. J. Bot. 45 (1967):863-889 and 21372153) R. C. Staples: Protein synthesis by uredospores of the bean rust fungus * G. L. Farkas: Problems of regulationof enzymelevels in diseasedtissues, withparticular regard to peroxidasesand ribonucleases. M. Shaw: Cellbiologicalaspects of host-parasiterelations. VI

J. Raa: Polyphenols and natural resistance of apple leaves against Venturia inaequalis A. Matta and I. A. Gentile: The relation between polyphenoloxidase activity and ability to produce indoleacetic acid in Fusarium-infected tomato plants A. Ballio, A. Carilli, V. di Vittorio, and A. Graniti: Purification of a toxin from culture filtrates of Gloeosporium olivarum Alm.

September 13 (Chairmen: Dr R. Heitefuss, Gb'ttingen, Germany; Professor W. H. Fuchs, G6ttingen, Germany) H. Veldstra: On the biochemistry of interference with normal plant development, with special reference to growth regulators D. F. Bateman: The enzymatic maceration of plant tissue M. Dean and R. K. S. Wood: Maceration by Erwinia aroideae A. Mahadevan and M. Koti Reddy: The effect of phenolic compounds on growth, polygalacturonase production and activity of Fusarium oxysporum f. vasinfectum F. Grossmann: Studies on the therapeutic effects of pectolytic enzyme inhibitors B. Osterroht: The effect of rufianic acid on tomato wilt in relation to calcium supply O. M. van Andel: Shifts in disease resistance induced by growth regulators A. Kaars Sijpesteijn and H. D. Sisler: Studies on the mode of action of phenylthiourea, a chemotherapeutant for cucumber scab J. Dekker: The development of resistance in Cladosporium cucumerinum against 6-azauracil, a chemotherapeutant of cucumber scab, and its relation to biosynthesis of RNA-precursors

September 15 (Chairman: Dr D. F. Bateman, Ithaca, N.Y., U.S.A.) B. J. Deverall, I. M. Smith, and S. Makris: Disease resistance in Vicia faba and Phaseolus vulgaris A. K. Sinha and R. K. S. Wood: Resistance to vascular wilt parasites B. H. H. Bergman and J. C. M. Beijersbergen: A fungitoxic substance extracted from tulips and its possible role as a protectant against disease L. A. Hadwiger: Changes in plant metabolism associated with phytoalexin production W. L. Klarman: The importance of phytoalexin in determining resistance of soybeans to three isolates of Phytophthora At the end of the symposium, one afternoon was devoted to a panel discussion, chaired by Professor G. J. M. van der Kerk (Utrecht, The Netherlands); members of the panel were: Professor F. Grossmann (Giessen, Germany), Professor H. F. Linskens (Nijmegen, The Netherlands), Dr R. C. Staples (Yonkers, N.Y., U.S.A.), Professor H. Veldstra (Leiden, The Netherlands), and Professor R. K. S. Wood (London, United Kingdom). A condensed report of the discussions has been added at the end of these Proceedings. The Organizing Committee:

A. J. P. Oort, chairman A. Fuchs, secretary J. Drijver, treasurer (LA.C.)

O. M. van Andel J. Dekker A. Kaars Sijpesteijn VII