| Summary: | The first section of this thesis records the testing and standardising of techniques. Extensive experiments demonstrated that the Seinhorst mistifier technique could be used quantitatively. An in vitro bioassay technique, using onion seedlings, was developed for studying the behavour of Ditylenchus dipsaci and for use as a post treatment assay of its "infective potential". An in vivo bioassay technique was used for determining the stem eelworm population of soil. The seasonal change in the "infective potential" of eelworm populations was due to a combination of a physiological change in the eelworms and a change in the proportion of fourth stage larvae to adults. The eelworms infected the narcissus generally during the cool, wet months (November to early March) and did so via the epidermis of the young foliage. Host reaction to the presence of the eelworms was most severe when the eelworms infected the active meristem, and the build up of the eelworm population in the plant increased rapidly at a point which was probably associated with the physio- logical condition of the host plant. This build up of eelworms in the foliage and bulb reached its maximum at the height of the narcissus growing season. The migration of eelworms into the soil occurred throughout the year and mostly via the base of the bulb; the rate of migration being directly related to the eel- worm density in the host. Controlled laboratory experiments showed an interaction between the effects of humidity and temperature on eelworm movement through sand. The random movement of eelworms through the soil was partially modified by the local attractive forces of host plants. The spread of eelworm infection, which depended on soil conditions and the population density in the host reservoir(s), was relatively slow in the absence of external agencies. Eelworm wool was an additional aid to dispersal but its viability and infective potential were affected by temperature and humidity.
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