Soybean rust (Asian rust) is a disease that occurs in soy cultures, negatively affecting pod formation and final grain weight and reducing value and product quality. Early identification of fungus in the plants prevents severe farming losses and spread to neighboring cultures. In this paper, a fast response sensor was developed based on surface plasmon resonance to detect Asian rust in soybean leaf extract at early stages of the disease. The antibody anti-Phakopsora pachyrhizi (pathogen) was covalently immobilized on a gold substrate via a self-assembled monolayer (SAM) of thiols using cysteamine-coupling chemistry. This immunosensor presented a linear response range for the antigen from 3.5 to 28.0 microg mL(-1) (r(2)=0.996). The effects of the antibody amount and the surface blocking to minimize non-specific adsorption on immunosensor response were evaluated. These studies provide new perspectives on using SPR technology for the development of a highly sensitive sensor for agricultural applications.