Responses of spectral features to powdery mildew at single and multiple stages based on the independent t-test.

*indicates difference is significant at p-value = 0.05; ** indicates difference is significant at p-value = 0.01; *** indicates difference is significant at p-value = 0.001. “S” is short form for “stage”. The multi-stage form of spectral features were calculated using a normalization process, e.g., S2S1 represents (SpectralFeatureS2-SpectralFeatureS1)/(SpectralFeatureS2+SpectralFeatureS1).