Segmentation and Detection of Cucumber Powdery Mildew Based on Visible Spectrum and Image Processing

Powdery mildew, as a kind of cucumber disease with high outbreak frequency, spreads very fast, often leads to yield reduction and can't achieve the expected economic benefits. Especially in serious years of disease outbreak, the reduction of cucumber in some areas was as high as 20%. This paper proposed a subinterval interval partial least squares regression (SI-PLSR) based on visible spectrum image for cucumber powdery mildew non-destructive detection. We used Canon EOS 800D and Ocean Optics USB2000+ optical fiber spectrometer to collect visible spectral images and reflectivity curves of 200 cucumber powdery mildew leaves. Firstly, we used wavelet transform and watershed algorithm to extract the target leaves from the real-time visible spectral images of cucumber powdery mildew leaves. Secondly, The Otsu algorithm optimized by Gauss fitting was used to segment the powdery mildew lesion. Thirdly, we established the PLSR in 350 similar to 1100 nm band and calculated the cross validation root-mean-square error (RMSECV). At the other hand, 350 similar to 1100 nm was divided into 20 sub-intervals, and established the PLSR independently. The sub-intervals of RMSECV smaller than the full band were selected to form the joint interval. Finally, the SI-PLSR model was established based on powdery mildew lesions images and joint interval. Results show that 188 target leaves were extracted from 200 susceptible leaves visible spectral images successfully of which 157 were more than 95% and 31 were between 90% and 95%. The success rate was 94.00%. The average misclassification rate of powdery mildew was 5.81%. The average false negative was 1.55% and the average false positive was 4.26%. PLSR was established for 20 sub-intervals, and the results showed that the RMSECV values of the 5, 6, 7, 11, 12, 13 and 19 sub-intervals were lower than those of the full-band modeling, indicating that the spectral information of these seven sub-intervals contributed greatly to the identification of powdery mildew, which was relative to the wavebands of 470 similar to 520, 530 similar to 580 and 700 similar to 780 rim showing peaks. Therefore, these 7 sub intervals should be selected to establish the joint interval. The principal component number of SI-PLSR model was 7. R-C, R-V and RMSEC, RMSEV were 0.9752, 0.9073 and 0.9195, 1.091. Compared with the full band PLSR model, the R-C and R-V of SI-PLSR was closer to 1, and the RMSEC and RMSEV were smaller. The above results showed that the SI-PLSR model proposed in this paper which effectively removed redundant information in visible spectral data and enhanced the stability of the model can be used to identify cucumber powdery mildew quickly and accurately, providing a method and reference for the diagnosis of cucumber diseases.