Department of Biological Sciences, IENS, Lancaster University, LA1 4YQ, UK

Background and objectives
Every student of botany learns by painful experience that clear conclusions drawn from laboratory experiments frequently become confused, and sometimes unsupportable, when the work is taken into the field. The contrast between laboratory and field is particularly acute for plant pathology because in any study there are at least two organisms, each with its own set of environmental limits and optima. The symposium focuses on the impact of infection, through plant growth, on both the yield of crops and the reproductive success of wild plants. It shows that the environment affects resistance to infection, and thus the occurrence and amount of infection, through indirect effects on plants that are additional to any direct effects on the pathogen.

The programme highlights major components of the abiotic environment, temperature, light, water and nutrients. However, the effects of these are integrated in planta through effects on photosynthesis and, thus, the growth of biomass. There is uncertainty about the extent to which pathogens have effects beyond the areas they colonize. There remains much to be learned about the effects of pathogens on the partitioning of biomass to those organs that facilitate survival during periods environmental stress (e.g. roots or seeds) and the possibility that effects arising during seasonal stress may differ from those occurring during cumulative or episodic stress.

In their potential interactions with plant pathogens, few components of the biotic environment have received much attention. An exception is the considerable interest of many researchers in the possibility of protecting crops against disease by pre-infecting them with avirulent or nonpathogenic types, but few have investigated the effects of multiple infections involving virulent pathogens. There is a small but growing body of evidence that endophytic or mycorrhizal infections can interfere with pathogenic infections.

Results and conclusions
Our own work shows how herbivory of Rumex spp. by Gastrophysa beetles can induce resistance to both biotrophic and necrotrophic pathogens [1]. Pathologists have not yet addressed the fundamental question: How do defence mechanisms of plants in the field respond to the frequent and various challenges from the biotic environment?

Studies of the effects of ultraviolet B radiation on foliar diseases, such as Septoria on wheat, demonstrate that our ability to predict the impacts of climate change are all too often limited by our understanding of the effects of the current ranges of environmental variables on disease [2].

1. Hatcher PE, Ayres PG, 1997. In: Gange AC, Brown VK, eds. Multitrophic Interactions in Terrestrial Systems. Oxford: Blackwell Scientific, pp. 133-149.
2. Rasanayagam MS, Paul ND, Royle DJ, Ayres PG, 1995. Mycological Research 99, 1371-1377.