Technical Abstract: Diagnosis of incipient disease based on visual symptoms of geraniums (Pelargonium × hortorum L. H. Bailey) exposed to water mold pathogens is often difficult, especially when the plants are maintained under optimum growing conditions. Such plants tend to be asymptomatic until late in the infection cycle when control methods are less effective and aesthetic value is diminished. To circumvent such a problem and to be able to predict the susceptibility of the plants to infection, we used infrared transducers to measure temperature in addition to other parameters in leaves of geranium plants exposed to a number of soil pathogens that are commonly associated with greenhouse production. Differences in leaf temperature among treatments were noticeable by week 2 and they were the greatest in week 7. However, visual disease symptoms were detected 3 weeks after inoculation. Results of this study suggest that leaf temperature measurements are a versatile, non-destructive way of rapidly examining whether plants are under pathogen stress.