Sudden Oak Death and Phytophthora ramorum

Fungi can infect humans, animals, and plants. Those which are plant pathogens may cause severe damage and kill many trees or crops. Oaks are among the susceptible hosts and may get infected by various fungi. Among these are Ceratocystis fagacearum which has been found to infect oaks and cause "oak wilt" in 21 states in the US and cause considerable damage particularly in the Midwest. It has also been reported in Texas which is outside its main range.

Phytophthora ramorum, on the other hand, has been shown to cause sudden oak death in central California and has recently re-drawn attention. Ceratocystis fagacearum and Phytophthora ramorum have not been related to any human disease so far and remain as phytopathogens.

Sudden Oak death was first reported in 1995 in central coastal California when tens of thousands of tanoak, coast live oak, California black oak, Shreve oak, and madrone were affected. In 2000, the cause of the oak death was found to be a fungus, a new Phytophthora species, Phytophthora ramorum.

By 2001, the disease was observed in eight counties at the central California coast from Monterey County to Mendocino County. By 2004, it was detected in 12 counties in California and in Curry Co., southwestern Oregon. The leaves of the infected trees turned from green to pale yellow or brown within a few weeks and the trees gradually lost their leaves and faded out.

The causative agent of the disease, Phytophthora ramorum is classified in kingdom Chromista and is a water mould. It is one of the over 60 species of genus Phytophthora.

Hyphae, sporangia containing bi-flagellated zoospores, and chlamydospores constitute its microscopic morphology. These structures may be observed on infected leaf disks. Windblown rain and contaminated soil carry the fungus from one plant to other and give rise to the spread of the infection.

The fungus sporulates on California bay laurel trees which serve as the reservoir. Cultivation of the organism on selective culture media, such as PARP (pimaricin-ampicillin-rifampicin-PCNB) medium, ELISA, and PCR are the laboratory methods that may be used for diagnosis.

Since the fungicidal agents are not beneficial once the infection has developed, prevention of the introduction of the pathogen into the forest or the nursery via an infected plant material is the crucial strategy in disease management. Despite being ineffective when used for treatment, the fungicides may help in prevention of the infection.

Fungicidal agents may be applied to highly susceptible crops when environmental factors such as rainy winter and spring that predispose to development of the disease exist. The newly coming host propagative material should be carefully inspected to prevent the introduction of infected plant material into the nursery and the infected hosts should be removed.

Related reading

www.suddenoakdeath.org

www.na.fs.fed.us/spfo/pubs/fidls/oakwilt/oakwilt.htm