Detection and differentiation of nitrogen-deficiency, powdery mildew and leaf rust at wheat leaf and canopy level by laser-induced chlorophyll fluorescence

The objective of the present study was to test whether the detection of chlorophyll fluorescence in high frequency multipoint scanning mode may enable identification of nitrogen(N)-deficiency and pathogen infections such as leaf rust (Puccinia recondita) and powdery mildew (Blumeria graminis) in winter wheat. Fluorescence readings at 690 nm (F690) and 730 nm (F730) were taken in the light under constant environmental conditions at leaf and canopy level. In order to identify the sources of heterogeneity of fluorescence signals at canopy level, fluorescence recordings from upper and lower leaf sides were additionally compared. F690/F730 was tendentiously lower on the lower leaf sides in all treatments. Throughout the experiment, N-deficient wheat plants displayed lower chlorophyll content and increased F690/F730 ratio. Pathogen infected plants showed a significantly enhanced fluorescence ratio, associated with chlorophyll degradation in the infected areas, only after appearance of visual symptoms. The results of cross-validation analysis indicated that with chlorophyll fluorescence measurements, samples with pathogen infections may be misrecognised as N deficiency and vice versa. Classification rates of N deficiency and leaf rust could be improved when heterogeneity (standard deviation) of F690 and F730 was considered in addition to the mean as a parameter for discrimination.