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Disease Detection and Losses

Infrared Thermometry for Determination of Root Rot Severity in Beans. J. C. Tu, Research plant pathologist, Agriculture Canada Research Station, Harrow, Ontario N0R 1G0; C. S. Tan, research soil scientist, Agriculture Canada Research Station, Harrow, Ontario N0R 1G0. Phytopathology 75:840-844. Accepted for publication 1 March 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-840.

Bean plants were grown in soil infested with root rotting fungi (Fusarium, Pythium, and Rhizoctonia) and in pasteurized soil. Differences between leaf and air temperatures taken with an infrared thermometer between 1400 and 1500 hours on sunny days showed that green leaves of diseased beans were warmer than those of healthy ones even if the soil moisture was maintained at field capacity. Depending on the severity of root rot, leaves of diseased plants appeared to be either normal or slightly less turgid. Increases in root rot severity correlated closely with increases in leaf temperature (P ≤0.01). This was most apparent at early stages of plant growth (one or two trifoliate leaves). Diseased plants grown under water stress had much higher leaf-air temperature differentials than healthy ones. If plants were stressed almost to the wilting point and water was added to return the soil to field capacity, decreases in leaf temperatures of healthy plants were larger than those of diseased plants. In diseased plants, decreases in leaf temperatures subsequent to watering were inversely related to the severity of root rot. Thus, leaf temperatures could be used not only to detect the presence of root rots but also to monitor disease severity in individual plants in soil without visually examining the roots. This technique can be used to screen segregating progeny from crosses of resistant and susceptible cultivars.