agronomy Article Pre-Symptomatic Disease Detection in the Vine, Chrysanthemum, and Rose Leaves with a Low-Cost Infrared Sensor Ioannis Vagelas , Athanasios Papadimos and Christos Lykas *   Citation: Vagelas, I.; Papadimos, A.; Lykas, C. Pre-Symptomatic Disease Detection in the Vine, Chrysanthemum, and Rose Leaves with a Low-Cost Infrared Sensor. Agronomy 2021, 11, 1682. https://doi.org/10.3390/ agronomy11091682 Academic Editor: Leire Molinero-Ruiz Received: 29 June 2021 Accepted: 20 August 2021 Published: 24 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Agriculture Crop Production and Rural Environment, University of Thessaly, Fytokou St., N. Ionia, GR-38446 Magnesia, Greece; vagelas@uth.gr (I.V.); apapadimos@uth.gr (A.P.) * Correspondence: chlikas@uth.gr Abstract: Thermography is a technique based on infrared imaging, which is used nowadays to detect plants under stress caused by biotic and abiotic factors. In many cases, temperature changes have already been correlated with pathogen attacks. In this sense, thermography offers the ability of early disease detection in plant pathology. In this work, a low-cost AMG8833 Grind-Eye infrared camera combined with a 1080P RGB web camera was used to develop an integrated infrared and RGB imaging system, to record temperature changes on vine, chrysanthemum, and rose plant leaf surfaces. Vine and chrysanthemum leaves were infected with Phomopsis viticola and Septoria ssp. respectively, respectively, whereas rose plants leaves were infected with Colletotrichum spp. as well as with Podosphaera pannosa. Measurements were performed using the integrated imaging system on infected and uninfected leaves, as well as on PDA plates with active and non-active mycelium. According to the results, vine leaf tissue infected with P. viticola and rose plants leaf tissue infected with P. pannosa had a pre-symptomatic (four days after infection) decrease in temperature up to 1.6 and 1.1 ◦ C, respectively, compared with uninfected tissue. In contrast chrysanthemum leaf tissue infected with Septoria ssp. and rose plant leaf tissue infected with Colletotrichum spp. had a pre- symptomatic (four days after infection) increased temperature up to 1.1 ◦ C and 1.0 ◦ C, respectively, compared with uninfected tissue. In vitro measurements showed that the active fungi mycelium had approximately 1.1 to 2.1 ◦ C lower temperature than the non-active mycelium. The results above show that the integrated infrared and RGB imaging system developed in this work can be used to detect early disease infection before visible symptoms appeared, facilitating the decision-making process. Keywords: thermal camera; thermography; biotic stress; early detection; ornamentals; decision making process 1. Introduction Early detection of plant disease infection plays a crucial role in their management and prevention strategies [1–3]. Most plant diseases are caused by fungal organisms and express characteristic visible disease symptoms on plants (e.g., lesions, blight, cankers, galls, wilts, rots, and damping-off), within a period after their infection. These symptoms may include changes in color and/or function of the plant as it interacts with the pathogen, or visible signs of a pathogen, e.g., conidia of Erysiphales [4]. For example, when rose plants are infected with powdery mildew, neither the disease symptoms (e.g., the red blister-like areas on the upper leaf surface) nor the pathogen are visible early on. The symptoms of the infection appear much later as white powdery, when the mycelium of Podosphaera pannosa, which is the causative agent of this disease, is developed on the aerial parts of the plant [5,6]. Consequently, experts, consultants and farmers must have extremely good observation skills for early disease diagnosis, which is not always possible. In addition, the time since host infection to disease symptoms expression (incubation period), is not fixed and is strongly dependent on the pathogen, the host species, as well as the climate parameters (air temperature and relative humidity; [7–10]). The characteristic disease symptoms are vital Agronomy 2021, 11, 1682. https://doi.org/10.3390/agronomy11091682 https://www.mdpi.com/journal/agronomy