*3.2. Characteristics of Reflectance Spectra*

The spectral characteristics of plants are affected by their internal tissue structure, biochemical composition, and morphological features, which together constitute the biophysical and biochemical responses of plants to light. Figure 4a shows the spectra of the red and healthy leaves and SI of red leaves. Evidently, the variation in the spectral reflectance of healthy leaves is smaller than that of red leaves. In the 580–670 nm range, a new reflection peak appeared in the red leaf spectra, which was rather different from that in the healthy leaf spectra. The large SI values were mainly distributed in the 595–764 nm regions, and SI gradually increased with increases in Anth, reaching the maximum value exceeding three. At Anth values of 0.28, 0.58, and 0.68, SI showed obvious bimodal characteristics, and when the Anth value reached 0.75, the bimodal characteristic disappeared. The band correspondingtothemaximumSIvalueappearednear700nm.

**Figure 4.** (**a**) Spectra of healthy and red leaves and SI of red leaves; (**b**) spectra and red edge of red leaves with different Anth.

Selected spectral characteristics of infected leaves with different degrees of reddening (Anth = 0.29, 0.58, 0.68, and 0.75 <sup>μ</sup>g·cm<sup>−</sup>2) compared with the spectral characteristics of healthy leaves (Anth = 0.06 <sup>μ</sup>g·cm<sup>−</sup>2) are presented in Figure 4b. The differences were mainly concentrated in the visible and near-infrared ranges (400–750 nm). The spectral curve of healthy leaves (Anth = 0.06 <sup>μ</sup>g·cm<sup>−</sup>2) showed an obvious absorption valley in the blue band at 450 nm and in the red band at 680 nm, as well as a strong reflection peak in the green band at 550 nm, which is consistent with the typical reflectance spectral characteristics of green plants. However, the spectral curve of red leaves exhibited obvious bimodal characteristics in the visible range (380–760 nm), with the left peak near 550 nm and the right peak near 640 nm. At Anth values of 0.29 and 0.58 <sup>μ</sup>g·cm<sup>−</sup>2, the reflectance of the left peak was higher than that of the right peak. However, at higher Anth values of 0.68 and 0.75 <sup>μ</sup>g·cm<sup>−</sup>2, the left peak disappeared, and the right peak increased sharply, forming a characteristic that is obviously different from the spectrum of healthy leaves. With the increase in Anth, the reflectance of the right peak continued to increase, whereas the absorption valley at 680 nm weakened until disappearing. The first-order differential of the original spectrum in the 640–760 nm range was calculated in Figure 4b. With an increase in Anth, the position of the red edge (wavelength corresponding to the maximum value of the first-order differential spectrum) moved in the short-wave direction; this trend is similar to that observed in cotton with different degrees of aphid damage [40]. As such, the higher the Anth is, the more severe the MDMV infection is, resulting in poor photosynthesis and low consumption of long-wave photons.

### *3.3. Correlation between Anth and Spectral Reflectance*

The correlation between spectral reflectance and Anth of the two types of leaves was examined, as shown in Figure 5. The spectral reflectance of red leaves was negatively correlated with Anth in the 520–563 nm region but positively correlated with Anth in the 400–519 and 564–1000 nm regions. In the 400–449 and 587–753 nm regions, there was a strong correlation (P< 0.01), with the maximum correlation coefficient of 0.76 at 695 nm. The spectral reflectance of healthy leaves was positively correlated with Anth in the 400–1000 nm, and all correlations were strong (P< 0.01), with the maximum correlation coefficient of 0.68 at 554 nm.

**Figure 5.** Correlation between Anth and spectral reflectance of healthy and red leaves.

In the 444–620 nm region, the correlation between the reflectance and Anth was stronger in healthy leaves than in red leaves, due to the strong reflection of Chl in this region, while anthocyanins had little effect on the reflectance of the spectrum in this region. The maximum difference in the correlation coefficient was recorded near 550 nm, coinciding with the position of the reflection peak of the healthy leaf spectrum shown in Figure 4b. In the 400–443, 621–748 and 776–1000 nm regions, the correlation between reflectance and Anth was stronger in red leaves than in healthy leaves due to the strong reflection of anthocyanins in this region. The maximum difference in the correlation coefficient was recorded near 680 nm, coinciding with the position of the right reflection peak of the red leaf spectrum shown in Figure 4b.
