SPAD and hyperspectral images for sensing chlorosis affection in peach trees

The peach tree is one of the fruit species with the highest production and area cultivated in Spain. For peach cultivation, the range of available rootstock is high, since several species, or even hybrids among them, can be used as rootstocks. Peach varieties are markedly influenced by the rootstock. In fact, many works show this influence, both in productive and physiological aspects. The nutritional diagnosis of the peach tree, usually based on the mineral analysis of leaves, is often late for the application of corrections before harvesting. On the other hand, the leaf diagnosis of ferric chlorosis is imprecise in many cases, due to the narrow range of variation in leaf iron concentrations. SPAD measurements are commonly used for chlorophyll content sensing in leaves, showing relation with chlorosis development. However, the SPAD is measured locally by transmission. Hyperspectral images could be able to sense the evolution and the distribution of ferric chlorosis on the leaf surface, to clearly study the development and the variability of such affection. Thirty-two trees, belonging to seven different rootstocks, and three leaves per tree were considered for hyperspectral measurements. SPAD measurements were acquired from their leaves and flowers. Mineral composition (Al, B, Ca, Cu among others) was also determined. The SPAD appears correlated with several minerals in leaf: Ca, S, Fe, Cu and Sr, restricted to Ca and Sr when comparing the SPAD with the mineral composition in flower. There appears to be a very large dispersion among SPAD values for the same rootstock which points the existence of isolated individuals with leave chlorosis. According to the visual identification of experts SPAD>=32 was set to sound trees, SPAD<24 was labelled as trees severely affected by chlorosis, and intermediate chlorosis level in-between. Comparing the average spectra of sound leaves with affected ones, three main features were observed; a) a general increase of reflectance from 500 to 600 nm, with a peak around 550, b) a red-edge displacement to the left, and c) an overall weak decrease of reflectance at NIR range probably due to the increase of the chlorosis affection. Several authors notices that red-edge moves towards left with the decreasing of chlorophyll content. Besides, a global increase in the visible range can also be caused by a decrease in chlorophyll due to nutritional stress.