AppleScab degradation model

A second approach to simulate the degradation of the fungicide application over time is derived by the AppleScab model (Arneson et al., 1978). This approach simulates the actual dose of the fungicide at day d after the application (Dd, unitless, 0-1)  as:

where k (unitless) is the constant rate of fungicide degradation and d0 is the day of fungicide application. Values of k can be estimated from data on chemical residues and were estimated for a number of fungicides (Baude et al., 1973). The impact of precipitation on fungicide degradation is then estimated according to the Equation below:

Where Tenf is the tenacity factor of the fungicide, and it is used to reproduce the variable sensitivity of the chemicals to wash-off by precipitation. The model considers an increasing difficulty in removing the fungicide residues from the plant surface as long as the degradation proceeds:

Finally the model computes the updated value of Dd+1 by combining the previous equations as

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