Infrared Imaging to Assess the Wild Rocket (Diplotaxis tenuifolia) Response to Fusarium Wilt in the Early Stages of Infection

Wild rocket is a baby-leaf vegetable crop devoted to the high-convenience food chain for the preparation of ready-to-eat salads. Healthiness and ecological management of crops are the requirements increasingly demanded by the market. On the other hand, very intensive cultivations under greenhouse are prone to fungal soil-borne pathogen attacks, such as Fusarium oxysporum f. sp. raphani, the causal agent of wilting. Control strategies currently include chemical and nonchemical means, but the implementation of digital systems able to support detection of outbreaks and promote optimization of interventions may concur to increase efficacy of management strategies. Thermography, based on the recording and analysis of thermal energy emitted by plant canopy in the infrared spectral range, is a well-known imaging technique able to monitor plants at both proximal and remote scales providing information on incipient plant stresses. In this work, wild rocket plants subjected to artificial infection with pathogen conidia performed by a dipping or flooding method were monitored during the pathogenesis by both passive and active thermographic approaches. Canopy temperature changes were compared with symptom severity indices measured by visual inspection on both inoculated and control plants. As key result of this study, depending on the inoculum concentration, passive thermography allowed classifying infected plants 30–48 h post inoculation (hpi) by dipping, and in coherence with the disease symptoms detection by visual inspection. Similar results were found in the case of the flooding inoculation, where infected plants were detected by thermography at 48 h.p.i. Active thermography revealed a decrease in leaf heat capacity attributable to the fungal infection and the subsequent tissue colonization over time. These findings constitute a solid base of knowledge about the thermal imaging applied to wild rocket affected by Fusarium wilting, and they can contribute to the development of new remote sensing systems for the detection of primary outbreaks.