Influence of the yellow flower colour in remote sensing of Brassica spp.

This report concerns spectral reflectance of two Brassica crops at intensive yellow flowering and when all flowers had fallen off. The first crop was oil seed rape (Brassica napus L. var. oleifera Metzger) with various infection levels of club root disease (Plamodiophora brassicae Wor.) in a field experiment in 1995. The other crop was oil seed turnip rape (Brassica rapa L. var. oleifera Metzger) in an ordinary field in 1996 with uneven growth but visually healthy. Rape seed had been cultivated in the experimental field on one, two, four and eight occasions, respectively, during the last eight years. In 1995 there was a substantial difference in the frequency of club root infection between the entries. The crop was studied using a hand-held Cropscan radiometer with eight spectral bands (460, 507, 559, 613, 706, 761 and 813 nm; band width 30 nm). The plots were first measured at full flowering stage (26 July). The yellow color substantially influenced the spectral signatures. There were good correlations between club root frequency and spectral reflectance at 460, 761 and 813 nm as well as for the ratio of near infrared (IR) and blue. Radiometry two weeks later demonstrated good correlations also at red spectral bands (R) and conventional ratios of IR and R. Supplementary spectral reflectance measurements made in 1996 of flowering oil seed turnip rape (Brassica rapa L. var. oleifera Metzger) of various growth density confirmed the above findings. The experiment demonstrated the advantage of using the blue as well as IR reflectance of a Brassica canopy in an intensive yellow flowering growth stage. It may extend the time period for remote sensing of these important crops by about two to four weeks. The experimental errors (LSD) were considerable smaller for remote sensing than for conventional visual disease examination of 200 randomly selected plants per plot.