In order to facilitate accurate identification of Phytophthora species, we provide standards of procedures (SOPs) for isolation, maintenance, storage of isolates, methods for sporulation, and other tasks related to morphological identification. Each SOP is available as a downloadable PDF.
SOP-PID-06.01 Morphology: methods for isolation, maintenance, sporulation, and storage
SOP-PID-07.01 Morphology: quick reference guide for isolation and methods for production of asexual and sexual phases
Morphological identfication of Phytophthora species is based a number of characters of the asexual and sexual phases as well as colony morphology. To see how these stages fit in to the Phytophthora life cycle, please visit the What is Phytophthora? page.
Sporangia papillation and caducity are important characters for identification, as are the sporangiophore shape, and the presence/absence and shape of hyphal swellings and chlamydospores.
Sporangia papillation and caducity
There may be a thickening at the apex of the sporangium. In caducous species, the sprorangia break off readily. In noncaducous species, the sporangia are retained on the sporangiosphore at maturity. The length of the pedicel remaining on caducous sporangia may also be helpful for identification.
Sporangia may exhibit internal proliferation or external proliferation. External proliferation can be extended or nested, or both.
The sporangiophore can be unbranched (simple) or branched. Branched sporangiophores may be umbellate, simple sympodial, or compound sympodial. One species, Phytophthora litchii, has a specialized type of compound sympodial sporangiophore that appears erected, very similar to those of downy mildews.
Chlamydospores and hyphal swellings
Chlamydospores and hyphal swellings may be present or absent. When present, they can be intercalary (in the middle of a hypha) or terminal (at the end of a hypha), and can vary in shape and distribution.
Sporangia vary widely in shape, but this character is not very helpful for identification.
Aspects of the sexual phase that are helpful for identification include whether the species is homothallic or heterothallic, the shape of the gametangia (antheridia and oogonia), and the shape of the oospore.
The antheridium may be paragynous (next to the oogonial stalk) or amphigynous (around the oogonial stalk).
The oogonium may be smooth or ornamented and may have a tapered base.
Homothallism vs. heterothallism
Homothallic species are self-fertile and non-outcrossing, with both sexual mating types in a single culture. Heterothallic species have separate mating strains or types, so they are sterile without the interaction of different thalli of opposite mating types.
May be plerotic (no space between oospore wall and oogonium wall; the oospore fills the entire oogonium) or aplerotic (with space between the the oospore and oogonium walls), or slightly aplerotic.
The shape of the colonies produced on particular media may also be helpful for species identification.