Comparison of Proximal Remote Sensing Devices of Vegetable Crops to Determine the Role of Grafting in Plant Resistance to <i>Meloidogyne incognita</i>

Proximal remote sensing devices are novel tools that enable the study of plant health status through the measurement of specific characteristics, including the color or spectrum of light reflected or transmitted by the leaves or the canopy. The aim of this study is to compare the RGB and multispectral data collected during five years (2016–2020) of four fruiting vegetables (melon, tomato, eggplant, and peppers) with trial treatments of non-grafted and grafted onto resistant rootstocks cultivated in a <i>Meloidogyne incognita</i> (a root-knot nematode) infested soil in a greenhouse. The proximal remote sensing of plant health status data collected was divided into three levels. Firstly, leaf level pigments were measured using two different handheld sensors (SPAD and Dualex). Secondly, canopy vigor and biomass were assessed using vegetation indices derived from RGB images and the Normalized Difference Vegetation Index (NDVI) measured with a portable spectroradiometer (Greenseeker). Third, we assessed plant level water stress, as a consequence of the root damage by nematodes, using stomatal conductance measured with a porometer and indirectly using plant temperature with an infrared thermometer, and also the stable carbon isotope composition of leaf dry matter.. It was found that the interaction between treatments and crops (ANOVA) was statistically different for only four of seventeen parameters: flavonoid (<i>p</i> < 0.05), NBI (<i>p</i> < 0.05), NDVI (<i>p</i> < 0.05) and the RGB CSI (Crop Senescence Index) (<i>p</i> < 0.05). Concerning the effect of treatments across all crops, differences existed only in two parameters, which were flavonoid (<i>p</i> < 0.05) and CSI (<i>p</i> < 0.001). Grafted plants contained fewer flavonoids (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">x</mi><mo>¯</mo></mover></mrow></semantics></math></inline-formula> = 1.37) and showed lower CSI (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">x</mi><mo>¯</mo></mover></mrow></semantics></math></inline-formula> = 11.65) than non-grafted plants (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">x</mi><mo>¯</mo></mover></mrow></semantics></math></inline-formula> = 1.98 and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">x</mi><mo>¯</mo></mover></mrow></semantics></math></inline-formula> = 17.28, respectively, <i>p</i> < 0.05 and <i>p</i> < 0.05) when combining all five years and four crops. We conclude that the grafted plants were less stressed and more protected against nematode attack. Leaf flavonoids content and the CSI index were robust indicators of root-knot nematode impacts across multiple crop types.