The motive of the author to embark upon the research reported here was the need to develop new concepts to approach the problem of crop losses, and eventually the problem of their prevention by means of disease control and plant breeding. The introduction of ecophysiology as an approach, explained above, was the first result. An ecophysiological treatment of processes like growth and development of plants in health and disease necessitates adjustments of current thoughts on experimental techniques and on organization in research (Van der Wal and Cowan, 1974).
In order to measure plant or aegricorpus responses concurrently with environmental factors in climate chamber and field experiments during periods Of months, a great variety of instruments had to be bought, modified, or devised. The climate chamber had to be adjusted to allow for high light intensities with a view to growing wheat under conditions approximating those of early summer in the Netherlands; the yield of the spring wheat 'Kolibri' was c. 0.5 kg.m -2
, which is nearly equivalent to the average field production of the country. Regulations for the prevention and control of contamination by aphids and mildew without any use of chemicals were issued after detailed experimentation (Van der Wal, unpubl.). Several instruments have been developed, usually with the help of others; this has led to two publications (Schurer and Van der Wal, 1972; Tegelaar and Van der Wal, 1974). These efforts resulted in the experimental designs described in the appendix. Field experiments, conducted in the same period as the climate chamber experiments reported here, will be published later. It is felt that the techniques to record growth of pathogens are still inadequate.
The results reported in the appendix show that the 'state' of the plant at the time of infection is a major determinant of the plant's future behaviour, which plant breeders and pathologists can express in terms of resistance and tolerance. The conceptual framework presented above may contribute to a future revision of crop husbandry and crop protection practices.