*3.1. Time-Series Variation of Spectral Reflectance*

Figure 3 shows the spectral-reflectance changes of healthy and diseased leaves on different days after infection at the jointing stage. The average DS of the wheat leaves increased gradually over time. Compared with healthy leaves, the spectral reflectance of diseased leaves was higher in the VIS region but lower in the NIR region, which is consistent with previous studies [17]. Moreover, the spectral signature in the NIR region differed more obviously between normal and diseased leaves at late infection stage.

**Figure 3.** Reflectance changes of healthy (black line) and diseased (red line) wheat leaves after infection at the jointing stage. Note: X axis: Wavelength (nm); Y axis: reflectance; "12d (25%)" represents the 12th day after infection with an average DS of 25%.

### *3.2. Selection of the Sensitive Features*

In this study, we used a new feature-selection algorithm SPA for selection of sensitive wavelengths for wheat PM, and then we calculated 8 texture features based on the selected sensitive wavelengths for developing the NDTIs. Meanwhile, sensitive VIs were selected and analyzed to compare with the performance of NDTIs. The results of feature selection are as the below.
