The U.S. Department of Agriculture (USDA) estimated that 185 million geraniums were produced in 2000 with a total wholesale value of $250 million (USDA, 2003). Geraniums are susceptible to pathogens [e.g., water molds (Pythium and Phytophthora) (Daughtrey, 2003a; Moorman, 1998)] that can cause major disease problems. Many species of Pythium cause damping-off of germinating seeds, infect young roots, cause root rot in older plants, and, by proliferating into the stem base and growing upward, cause black leg symptoms on geraniums (Daughtrey, 2003b). Aboveground symptoms of Pythium infection include yellowing and browning of lower leaves, wilting, and plant death (Daughtrey, 2003b). However, mild root rot, causing only plant stunting, might go undetected unless compared directly with healthy, uninoculated plants (Hausbeck et al., 1989). Growers may attribute poor growth in asymptomatic plants to inadequate nutrition or insufficient watering, often resulting in overfertilization and overwatering (Jones, 2001). Wet soil conditions can favor the production, survival, and dissemination of zoospores and sporangia.
Root rot can reduce efficient water uptake by roots compromising the evaporative cooling ability of the plant (Mengistu et al., 1987). Therefore, leaf temperature is a potentially sensitive indicator of plant moisture stress.
Using a simplified energy balance model (Monteith, 1977), a sensitivity analysis of leaf temperature and transpiration can be performed. Assuming some typical values, a geranium leaf that is 4 inches wide might absorb 1000 W·m−2 of radiation if the plant is experiencing ≈80% full sun [(Lambers et al., 1998) both long and shortwave radiation]. Assuming realistic values for other environmental parameters of 25 °C temperature, 50% relative humidity, and a leaf temperature 3 °C above air temperature (Jifon and Syvertsen, 2003), the plant loses ≈100 W·m−2 through transpiration. If transpiration decreases by only 5% and all other environmental conditions remain the same, leaf temperature would rise by nearly 0.1 °C. Further decreases in transpiration result in a linear increase in leaf temperature of ≈0.02 °C per percent decrease in transpiration.
The objective of this study was to determine if changes in geranium leaf temperature, measured by infrared (IR) transducers aimed at the plant canopy or individual leaves, correlate with root infection by pathogenic water molds. If successful, leaf temperature could be used to nondestructively indicate infection of greenhouse-grown plants, perhaps before visible symptom development.
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