Institute of Grassland and Environmental Research, Aberystwyth, SY23 3EB, Wales, UK

Background and objectives
Stem nematode (Ditylenchus dipsaci) damages white clover, (Trifolium repens) reducing its persistence and yield in perennial temperate grassland [1]. Effects are especially severe when infestations reduce the number of growing points surviving the winter. This handicaps the clover during regrowth in spring, making it more vulnerable to the effects of competition from the companion grasses. Our objectives were to describe the biology of the interaction and to model its dynamics, especially in relation to temperature, to assess factors which might be manipulated to enhance clover survival and to develop a predictive simulation model of nematode/plant relationships.

Materials and methods
We used susceptible white clover genotypes in all experiments, with stem nematode inocula maintained on tissue-cultured plants [2}. Experiments characterized infestation and nematode multiplication at a range of temperatures to establish basal temperatures and rates of population increase. Host interactions were investigated in pot experiments in controlled daylength and temperature and in simulated sward experiments where temperatures were continuously recorded. A mechanistic, thermal-time, energy budget simulation model was developed.

Results and conclusions
Rates of development of the nematode in white clover were directly proportional to temperature, with a basal temperature Tb=3C, and a thermal constant for development of gravid females from newly laid eggs of S=270 accumulated day degrees above Tb. Maximal rates of egg production were 0.8 and 3.1 eggs per female per day at 10 and 20C, respectively. Culturing a nematode population for 90 generations at 7.5C above the mean annual temperature of its original location did not affect the temperature response characteristics of the population. Nematodes spread within host tissues at about 0.2 mm of stolon/day. This rate was temperature dependent but was independent of the numbers of nematodes within the infested locus. Plants developed local symptoms (cellular hypertrophy and tissue swelling) in response to single nematodes, but stolon morphology was affected only when more than 100 nematodes were present within close proximity to an active meristem. Rates of nematode population growth were controlled to sustainable levels by pre-adult juveniles entering a diapause-like state and, at the same time, rates of egg-laying declined. Based on these experiments, the model realistically predicted temporal change in nematode population size in infested plants and its predictions were validated by the field experiments. From these studies and simulations, we conclude that nematode activity in the field in western Europe is limited by low temperature, whereas clover growth is limited by low light intensity. Consequently, warmer winters will lead to greater damage to infested plants. Two aspects of the nematode-plant interaction which are significant for future control measures are (1) the host-dependent mechanisms that limit rates of increase in the size of the infestation locus, and (2) the mechanisms which induce diapause and reduced egg laying. The simulation model has the potential to be generalized to describe other relationships between migratory pathogens and plants.

1. Cook R, Evans DR, Williams TA, Mizen KA, 1992. Annals of Applied Biology 120, 83-94.
2. Griffith GS, Cook R, Mizen KA, 1997. Journal of Nematology 29, 356-69.