Taxonomic classification
Phytophthora ramorum belongs to kingdom Chromista and is classified as an oomycetes (water mould).
Description, Natural Habitats, and Pathogenicity
Phytophthora ramorum is a plant pathogen that has been identified as the causative agent of sudden oak death. The disease has been observed in central coastal California and southwestern Oregon. Phytophthora ramorum now appears to be eradicated in Oregon and Canada. In addition to causing high oak mortality, Phytophthora ramorum may also infect some ornamental plants. It has been shown to cause disease on nursery crops in Europe. By now, 23 species in 12 plant families are known to be naturally infected by Phytophthora ramorum.
Similar to other water moulds, Phytophthora ramorum requires moist environment for active growth and reproduction. The spores of the fungus are found on surfaces of the susceptible leaves. Windblown rain and contaminated soil serve in transmission of the fungus from one plant to other. The laurel tree, Umbellularia californica is the major reservoir of the fungus in California forests. During infection, irregular, necrotic leaf lesions are observed instead of regular distinct leaf spots. The stem may also initially or eventually be involved and become necrotic.
Microscopic Features
The major microscopic structures that constitute the morphology of Phytophthora ramorum are septate hyphae, semi-papillate sporangia, zoospores, and chlamydospores. Zoospores are bi-flagellate spores that are found inside the sporangia and have the ability to swim in water. Chlamydospores are resistant to dryness, extreme temperatures, and other unfavorable environmental conditions and thus help the fungus to survive under these conditions. These chlamydospores are typically large (22-72 µm) and terminal. They are initially hyaline and may get cinnamon brown in color by maturation. Phytophthora ramorum is an heterothallic fungus and produces the sexual spores, oospores by mating of two different types, A1 and A2.
Diagnosis and Management
The plant parts (leaves and stems) that appear symptomatic should be sampled and placed in a plastic bag. The sample is kept cool and away from direct sunlight and transferred to the laboratory. Cultivation and examination of morphological features, ELISA, and PCR methods are used for diagnosis. Pimaricin-ampicillin-rifampicin-PCNB (PARP) selective medium and carrot piece agar may be used for cultivation and isolation. Available ELISA kits detect various Phytophthora species and are not specific for Phytophthora ramorum.
Once an infection has been established, the fungicides are not beneficial. However, they may help in prevention of the disease. Fungicides may be applied to susceptible host crops under seasonal conditions that predispose to the development of infection, such as rainy winter and spring weather. Prevention of introduction of the pathogen to the nursery via an infected plant material is crucial in avoidance of spread of the disease. The new incoming host propagative material should be carefully examined by agricultural inspectors. The infected hosts and the infected leaves that drop should be removed and the nearby native hosts should be periodically inspected for appearance of any symptom related to the disease.