Table 2.

Hyperspectral vegetation indices (dimensionless) used in this study for wheat STB disease detection.

Index	Description	Application	Formula*	References
SRRed−edge	Simple ratio at red edge	Chlorophyll	R750/R710	Zarco-Tejada et al., 2001
CIRed−edge	Chlorophyll index at red edge	Chlorophyll	(R750 – R700)/R700	Gitelson et al., 2003
NDVI	Normalized difference vegetation index	Green biomass, LAI	(R800 – R680)/(R800 + R680)	Sims and Gamon, 2002
ND705	Normalized difference at 705 nm	Chlorophyll	(R750 – R705)/(R750 + R705)	Sims and Gamon, 2002
mSR705	Modified simple ratio at 705 nm	Chlorophyll	(R750 – R445)/(R705 – R445)	Sims and Gamon, 2002
mND705	Modified ND705	Chlorophyll	(R750 – R705)/(R750 + R705 – 2*R445)	Sims and Gamon, 2002
SIPI	Structure insensitive pigment index	Pigment ratio between carotenoid and chlorophyll a	(R800 – R445)/(R800 – R680)	Peñuelas et al., 1995b
PRI	Photochemical reflectance index	Physiology, photosynthesis	(R531 – R570)/(R531 + R570)	Gamon et al., 1997
WI	Water index	Water content	R900/R970	Peñuelas and Inoue, 1999
NDWI	Normalized difference water index	Water content	(R857 – R1241)/(R857 + R1241)	Gao, 1996
PSRI	Plant senescence reflectance index	Senescence	(R678 – R500)/R750	Merzlyak et al., 1999
RRDIRed−edge	Ratio of reflectance difference index at red edge	Chlorophyll	(R745 – R740)/(R740 – R700)	Yu et al., 2015
NPQI	Normalized phaeophytinization index	Disease, chlorophyll degradation	(R415 – R435)/(R415 + R435)	Peñuelas et al., 1995a
CAI	Chlorophyll absorption integral	Disease, chlorophyll	${\int }_{600}^{740}R(EQ)$	Oppelt, 2002; Laudien et al., 2003
HI	Healthy index	Disease	(R534 – R698)/(R534 + R698) – $\frac{1}{2}$*R704	Mahlein et al., 2013
DSI	Leaf rust disease severity index	Disease	6.9*(R605 – R455) – 1.2	Ashourloo et al., 2014a

^*Ri denotes the reflectance at wavelength i nm; EQ = Ri/Ei, where Ei denotes the envelope (straight line connecting 600 and 740 nm) reflectance at wavelength i nm.