Relationships between measures of reflectance and health leaf area, soybean rust severity and soybean yield

The aim of this research was to verify if some measures of reflectance could detect variations in the healthy leaf area which could be used as parameter to estimate damages in the production due to soybean rust, and to determine the influence of the disease on photosinthetic efficiency of plants under field conditions. The experiment was lead in the experimental area of Embrapa, in Londrina, PR. The experimental design was randomly blocks, with eight treatments (different fungicide application times) with four replications. The fungicide used was azoxistrobin + ciproconazole (60+24 g i.a./ha) + mineral oil (0.5%). The reflectance readings were made at eight wave lengths, between 460 and 8 10 run, with intervals of 50 tun. It was estimated the Normalized Difference Vegetation Index (NDVI), where NDVI=(810-660)/ (810+660). The yield presented linear increments (P < 0.01) with Healthy Leaf Area Duration (HAD) (r(2)=37.7%), with Healthy Area Absorption (HAA) (r(2)=21.6%), with Area Under Disease Progress Curve (AUDPQ (r(2)=25.4%) and with Integral of Normalized Difference (AUR(NDVI)) (r(2) =54.8%). Significant difference in Photosynthetic Efficiency (RUE) (P < 0.05) was observed between the control plot and the treatment with three fungicide applications (first at R,), which was the only treatment that differed significantly (P < 0.05) in production from untreated control. The sensitivity of the measures NDVI and RUE presented a potential to detect variations among the treatments. However additional studies are still needed to verify and eliminate some possible interferences, such as the calibration of the radiometer and other parameters before it can be used for taking of decisions.