S.A. Tjosvold1, D.L. Chambers2, J.M. Davidson3, and D.M. Rizzo3
Phytophthora ramorum, the causal agent of the disease commonly known as Sudden Oak Death is a prevalent pathogen in California with its effects evident in 12 counties and found on 14 different oak, tanoak and non-oak hosts. In Santa Cruz County, several streams run through affected woodland. It is important to know if and when P. ramorum is found in these streams. Potentially the pathogen could be dispersed from affected drainages downstream for long distances. In some cases, stream water is used for irrigation by some nurseries and landowners situated along these streams. Water contaminated with inoculum might be inadvertently dispersed to nursery hosts or other landscape hosts. In addition, if P. ramorum could be reliably detected from streams draining through diseased woodland, this detection technique might be used as a novel technique to identify and delineate potentially affected woodland areas that are not easily accessible.
Sampling of stream water for the presence of Phytophthora ramorum inoculum occurred approximately weekly, from April 2001 to April 2002, in 7 rivers and creeks in Santa Cruz County. All streams were perennial except one that dried in late spring. All northern streams drained through woodland known to contain Sudden Oak Death. The two most southern streams drained through woodland that contained hosts of Sudden Oak Death but the pathogen has not been confirmed in those regions. The samples were taken downstream from the suspected diseased watersheds. Approximately 8 liters of water were sampled at each site. In the laboratory, the water samples were baited with pears, inspected for characteristic P. ramorum lesions, and pear tissue from suspicious lesions were transferred to PARP selective media for pathogen identification.
P. ramorum was detected in all streams except the two most southern streams, Corralitos Creek and Aptos Creek. The pathogen was found almost entirely during the period following the beginning of winter rainfall and through early spring (January 2002 through March 2002). There was no P. ramorum detected in streams during the dry months except on one occasion; P. ramorum was detected in September 2001 in the water collected from the San Lorenzo River following a short rain in the river's drainage.
