Cocoa canker caused by Phytophthora palmivora

Canker disease of Theobroma cacao L. caused by Phytophthora palmivora (Butler) Butler gives rise to serious damage in Fiji and is common in the S.W. Pacific area. Fusarium decemcellulareBiick (imperfect stage of Calonectria rigidiuscula (Berk. & Br.) Sacc.) was constantly associated with P. palmivora in disorders of cocoa but was not found to be a primary pathogen. Most Amelonado cocoa trees were resistant to canker but there were differences in the reactions of other Forastero and of Trinitario types. The F₁ generation of a local Trinitario selection, itself resistant, showed segregation resulting in a proportion of very susceptible plants. Isolates of P. palmivora from black pod, canker and green twigs were equally pathogenic, both to pods and seedling stems. Sporangial dimensions, particularly the length/breadth ratio, differed from those reported elsewhere. It is uncertain whether the reason that cocoa canker is more serious in some countries than others is because of differences in the pathogenicity of P. palmivora strains to bark or the genetic differences of the cocoa grown.