Temperature index is a proportional representation of the latent period. Temperatures at or above the optimum temperature for the disease cycle will yield 1 and the lower the temperature the infinitely longer the latent period.
temperature_index(Tm, opt_Tm = 21)numeric, proportion representing the speed of the latent period in relation to the temperature optimum
numeric, temperature for any given hour
numeric, the lowest temperature optimum at which all temperatures above will have the same (fastest) latent period.
For equations and original documentation, see Wolf and Verreet (2005) and Wolf et al. (2001)
Wolf, P. F. J., and J. A. Verreet. “Factors Affecting the Onset of Cercospora Leaf Spot Epidemics in Sugar Beet and Establishment of Disease-Monitoring Thresholds.” Phytopathology® 95, no. 3 (March 2005): 269–74. https://doi.org/10.1094/PHYTO-95-0269.
Wolf, P. F. J., F.-J. Weis, and J.-A. Verreet. “Threshold Values as Indicators of Fungicide Treatments for the Control of Leaf Blotching Caused by Cercospora Beticola (Sacc.) in Sugar Beets.” Journal of Plant Diseases and Protection 108, no. 3 (2001): 244–57.