The work described in this thesis involves the development of assays for the detection of two crop pathogens, potato virus Y (PVY) and Rhynchosporium commune (R. commune). Infection of crops by these pathogens results in significant economic losses and reduced yields. Therefore, efficient control of their spread is paramount. It was proposed in this work to develop immunoassays and molecular techniques for PVY and R. commune to provide novel additional detection and control capabilities for these important pathogens. An antibody library was generated in order to isolate PVY-specific scFv. The library was screened using a combination of ELISA, phage display, and biopanning approaches. From this screening, scFv were isolated and their PVY-binding abilities characterised in ELISA. Anti-PVY mAbs were structurally engineered into recombinant scAb and scFv fragments. The performance characteristics of the parental antibodies and recombinant derivatives were determined through a combination of immunoblotting, ELISA and SPR. Subsequently, dot blots and SPR-based immunoassays were developed for the enhanced detection of PVY. An isothermal nucleic acid amplification-based immunoassay for the detection of PVYrelated nucleic acids was also generated. This method combines recombinase polymerase amplification with lateral flow immunoassay technology. The developed assay could detect PVY nucleic acids at high sensitivity from both DNA and RNA templates. A recombinant scFv library was generated for the isolation of antibodies with R. commune specificity. Additionally, anti-R. commune avian pAbs were generated and characterised. This work led to the development of paper-based immunoassays for the detection of R. commune-associated antigens. Both the immunoassay and molecular diagnostic approaches offer a means of early detection of the target pathogens such that their burden in the field can be ascertained and tailored treatment can be implemented at early stages of disease outbreaks.