Department of Plant Pathology, University of Wisconsin, Madison, WI 608-262-1395, USA

Pathogens may act together to alter impact of disease on plants. The phenomenon of induced resistance provides many examples. In some cases, pathogens cause greater disease when plants are exposed to multiple infections than would result from the sum of the 'disease response' from each infection individually. In some cases, this might be thought of, hypothetically, as induced susceptibility. A problem with examining this phenomenon is defining and quantifying 'disease response'. Since disease is defined, in part, as departure from normal physiology, measuring the physiological response may help us understand the interaction caused by multiple infections - whether of a single pathogen or different pathogens. Basic whole-plant functions, such as photosynthesis, transpiration and/or respiration, are a starting point for quantifying disease on a physiological basis. Examples include the potato early dying syndrome caused by the interaction of Verticillium dahliae and Pratylenchus penetrans. The degree of synergism is measurable in terms of yield. This interaction is quantifiable by measuring gas exchange. The fungus alone reduces gas exchange by apparently reducing hydraulic conductivity. In the presence of the nematode it appears that additional physiological responses may occur.