2.8.16
RELATIONSHIPS AMONG HEALTHY LEAF AREA, YIELD, REFLECTANCE, PHOTOSYNTHETIC EFFICIENCY AND SEVERITY OF ANGULAR LEAF SPOT OF BEAN

MG CANTERI1, M DALLA PRIA1, L AMORIM2 and A BERGAMIN FILHO2

1Universidade Estadual de Ponta Grossa, CEP 84010-330, Ponta Grossa, PR, Brazil; 2 Dep. de Fitopatologia, Escola Superior de Agricultura ‘Luiz de Queiroz’, Caixa Postal 09, 13418-900, Piracicaba, SP, Brazil

Background and objectives
The use of healthy leaf area in crop loss models give more consistent results than disease intensity [1]. Reflectance measurements can be used to estimate leaf area index [2]. The objective was to verify if reflectance readings were useful to estimate healthy leaf area and to study relationships among reflectance, leaf area variables, angular leaf spot severity and yield.

Materials and methods
Experiments were carried out with bean during two crop seasons (S1 and S2), using detached leaves and field plots. Leaf area, severity of angular leaf spot (Phaeoisariopsis griseola (Sacc.) Ferraris) and canopy reflectance were assessed weekly beginning 30 ;days after emergency (DAE). Reflectance readings were made with a hand-held multispectral radiometer at 8 wavelength, between 460 nm and 810 nm, at 50 nm intervals. The canopy reflectance at infrared band (IR), 810 nm and red band (R) 660 nm and 610 nm were used to calculate the vegetation index ND810.660 and ND810.610, where ND=(IR-R)/(R+R).

Results and conclusions
When disease severity was high, detached leaves showed high reflectance at 460, 510, 560, 660 and 710 ;nm and low reflectance at 760 and 810 ;nm. Any wavelength or vegetation index tested could not be used to estimate only disease severity at field. Yield presented linear relationship (P<0.01) when regressed against healthy leaf area duration (HAD) (R2=40.2% in S1 and 62.7% in S2), healthy leaf area absorption (HAA) (R2=44.8% and 73.5% in S2) and ND integrated over the growing season (AURND) (R2=50.1% in S1 and 75.4% in S2). The relationship between yield and area under disease progress curve (AUDPC) proved to be less consistent (R2=31,4% in S1 and 53.3% is S2). The ND810.660 was related to healthy leaf area index (HLAI) (R2=89.7% in S1 and 86.6% in S2) and to healthy leaf area absorption (HRI) (R2=87.7% in S1 and 77.0% in S2). Plots with the same HAD showed yield reduction for higher AUDPC values. This indicates a reduction in photosynthetic efficiency. The ND showed potential to be used to estimate HLAI. So, this relationship could be used, together with a control plot and disease severity assessment to angular leaf spot management programs.


References
1. Bergamin Filho A, Carneiro SMTPG, Godoy CV, Amorim L, Berger RD, Hau B, 1997. Phytopathology 87, 506-15.
2. Nutter Jr FW, Littrell RH, 1996. Crop Protection 15, 135-42.