*Article* **Detection Method for Tomato Leaf Mildew Based on Hyperspectral Fusion Terahertz Technology**

**Xiaodong Zhang 1, Yafei Wang 1, Zhankun Zhou 1, Yixue Zhang <sup>2</sup> and Xinzhong Wang 1,\***

<sup>1</sup> College of Agricultural Engineering, Jiangsu University, Zhenjiang 212013, China

<sup>2</sup> Basic Engineering Training Center, Jiangsu University, Zhenjiang 212013, China

**\*** Correspondence: xzwang@ujs.edu.cn; Tel.: +86-138-5298-9966

**Abstract:** Leaf mildew is a common disease of tomato leaves. Its detection is an important means to reduce yield loss from the disease and improve tomato quality. In this study, a new method was developed for the multi-source detection of tomato leaf mildew by THz hyperspectral imaging through combining internal and external leaf features. First, multi-source information obtained from tomato leaves of different disease grades was extracted by near-infrared hyperspectral imaging and THz time-domain spectroscopy, while the influence of low-frequency noise was removed by the Savitzky Golay (SG) smoothing algorithm. A genetic algorithm (GA) was used to optimize the selection of the characteristic near-infrared hyperspectral band. Principal component analysis (PCA) was employed to optimize the THz characteristic absorption spectra and power spectrum dimensions. Recognition models were developed for different grades of tomato leaf mildew infestation by incorporating near-infrared hyperspectral imaging, THz absorbance, and power spectra using the backpropagation neural network (BPNN), and the models had recognition rates of 95%, 96.67%, and 95%, respectively. Based on the near-infrared hyperspectral features, THz time-domain spectrum features, and classification model, the probability density of the posterior distribution of tomato leaf health parameter variables was recalculated by a Bayesian network model. Finally, a fusion diagnosis and health evaluation model of tomato leaf mildew with hyperspectral fusion THz was established, and the recognition rate of tomato leaf mildew samples reached 97.12%, which improved the recognition accuracy by 0.45% when compared with the single detection method, thereby achieving the accurate detection of facility diseases.

**Keywords:** tomato; leaf mildew; terahertz time-domain spectroscopy; near infrared hyperspectral technology; multi-source information fusion
