Integrative Analyses of Host-Pathogen Relations

The invasion of host tissues by pathogens initiates a complex and variable developmental pattern between host and parasite which continues throughout the course of their coexistence. Differences in the physiological responses and morphological structures of various host genotypes affect their susceptibility or resistance to invasion and its consequences; similar variations in pathogens influence their growth rates and virulence. Other sources of variation include the physical environment and the timing and level of infection relative to the current physiological and morphological states of the host organism. Host plants that are <:;rowded for space, deprived of nutrients, or in mature phenological states may respond almost as different species compared to uncrowded, well-nourished, or young plants. The developmental patterns are determined to a large degree by measurable attributes of the community, but it is difficult to view these attributes as deterministic. We are hindered by lack of knowledge, by the complexities of the system, and by the fact that a considerable part of the variation is stochastic, depending, for example, upon spore dispersal, infection success, and plant-to­ plant variation among hosts. Studies of pathogenicity within such systems are needed for several pur­ poses. We need to understand more clearly the mechanisms of disease develop­ ment and the way and degree to which host plant performance is altered. In managed forests, grasslands, and crops, it is also important to have some means of assessing losses of economic production. Crop loss assessment is emerging as a specialized field of technology ( l0, 41, 84) for insurance adjustments, marketing predictions, and crop protection actions.