We used Raman spectroscopy to characterize the effects of chitin treatment and fungal inoculations on Arabidopsis thaliana and Brassica vegetables. Chitin, a recognized fungal pathogen-associated molecular pattern (PAMP) elicited positive Elicitor Response Index (ERI) in wild-type Arabidopsis in a dose-dependent manner. Mutant plants lacking chitin receptors (cerk1 and lyk4/5) displayed minimal ERI whereas fls22 mutant deficient in the bacterial-specific flg22 receptor was hyper-responsive. These results confirm critical role of chitin receptors in the activation of downstream pathways and highlighting distinct responses in two separate pattern-triggered immunity (PTI) systems. Inoculations of Colletotrichum higginsianum and Alternaria brassicicola induced significant changes in Infection Response Index (IRI) values with the former giving positive IRI at 12 -24 hours post-inoculation whereas the latter exhibited a transient negative IRI before transitioning to positive values. Notably, shifts in Raman spectra could predict fungal infection prior to the appearance of visible symptoms, establishing Raman shifts as a potential early diagnostic marker. Comparative analyses of infected Brassica vegetables indicated varied sensitivity to fungal pathogens and revealed a correlation between symptom severity and IRI values. Furthermore, randomized controlled trials validated the reliability of Raman technology for early, pre-symptomatic detection of fungal infections, achieving an accuracy rate of 76.2% in Arabidopsis and 72.5% in Pak-Choy (Brassica rapa chinensis). Principal component analysis can also distinguish Raman spectral features associated with fungal and bacterial infections, emphasizing their unique profiles and reinforcing the utility of Raman spectroscopy as a tool for early detection of pathogen-related plant stress. Our work supports the application of non-invasive diagnostic techniques in agricultural practices enabling timely intervention against crop diseases.
