In November 2006, trees of Italian alder (Alnus cordata) were observed declining in association with bleeding trunk cankers in a commercial landscape in Foster City, CA. A species of Phytophthora was isolated on PARP selective medium from the leading edge of the cankers. The Phytophthora species was homothallic with primarily paragynous antheridia and had oospores that were mostly globose and aplerotic. Sporangia were produced from mycelia on plugs of carrot piece agar in soil extraction solution and were semi-papillate and ovoid to ellipsoid in shape. The intergenic transcribed spacer region of rDNA from an alder isolate matched with 100% identity to isolates in GenBank of Phytophthora siskiyouensis, a recently described species associated with tanoak and found in the soil and waterways of coastal Oregon. Pathogenicity was tested on young alder trees growing in pots. Pathogenicity was confirmed on Italian alder trees and potential pathogenicity was demonstrated on red and white alder trees. Accepted for publication 11 March 2009. Published 13 April 2009.

The potential aerial spread of Phytophthora ramorum, causal agent of sudden oak death and Ramorum blight, from infected plants in a quarantine research nursery at the National Ornamentals Research Site at Dominican University of California (NORS-DUC) to the environment was monitored weekly for five years (2011 to 2016) using a sentinel system. Phytophthora ramorum was never detected on any of the sentinel plants (Rhododendron, Viburnum, and Loropetalum spp), indicating very limited aerial spread under suboptimal meteorological and environmental conditions. An infection experiment with host plants placed in the immediate vicinity of symptomatic plants proved the potential for short-distance (1 to 2 m) aerial transmission of P. ramorum. Other Phytophthora spp. causing symptoms similar to P. ramorum were detected during the rainy season (January to May) on the sentinel plants, among them potentially two novel species. These data reveal how sentinel monitoring at NORS-DUC allows for seasonal assessments of disease incidence and provide longitudinal data to assess the threat of P. ramorum movement in nurseries.

Golden or giant chinquapin, Chrysolepis chrysophylla (Fagaceae), is a slow growing, evergreen shrub or tree, native to the west coast of the US. In April 2015, several declining chinquapin trees were identified on Bolinas Ridge near Mt. Tamalpais in Marin Co. CA, a Phytophthora ramorum infested region. Branch dieback was observed but no bole or branch cankers were observed. Discolored xylem tissue (5-mm2) was cultured on corn meal agar selective medium, CMA-PARP. An organism morphologically resembling P. ramorum was isolated from one water sprout piece. The ITS sequences obtained from pure cultures, symptomatic branches, and water sprouts were identical, and one was deposited into GenBank. The ITS sequence and cox2 sequence obtained from a culture were a 100% match to the ex-type of P. ramorum. Koch’s postulates were tested on 3 small plants (0.5m tall, 3.8-liter pots). Plants were inoculated with 6-mm plugs taken from the margin of a 7-day-old P. ramorum colony, growing on V8 juice agar. Plugs were placed on wounded stems, approximately 2 cm above the soil line, and wrapped in Parafilm. An equal number of plants were treated with uncolonized V8 agar plugs as controls. The potting mix of 2 additional plants of the same size was infested with 200 ml of a zoospore suspension (105 spores ml-1) which was drenched around the base of the stem. Each base was scored with a razor blade before applying the suspension. Two plants were scored and drenched with sterile water as controls. After 7 days, plug-inoculated plants began to wilt and leaves began to turn grayish green. After 15 days, plants began to collapse and isolations were made from stem cankers and roots onto CMA-PARP. After 12 days, leaves of plants that were soil-drenched with zoospores were a dull green color and after 38 days, dry and brittle. Lesions were seen on the roots and stems when isolations were made at 35 days. P. ramorum was consistently isolated from stems of wound-inoculated plants and from roots and stems of soil-inoculated plants. No P. ramorum was isolated, nor symptoms were observed on any control plants. P. ramorum causes sudden oak death on many members of the Fagaceae as well as ramorum blight on more than 130 hosts (Cobb et al., 2020). To our knowledge, this is the first official detection of P. ramorum causing disease on C. chrysophylla in wildlands. Unlike other Fagaceae hosts of P. ramorum, external bole cankers were not seen on infected chinquapin trees. Branch isolations onto PARP media was difficult and detection by PCR from symptomatic vascular tissue was more efficacious. Losses of chinquapin in US forests due to P. ramorum would reduce forest diversity and could cause the loss of habitat for many animal species.

Reliable data on the transmission of airborne plant pathogens are crucial for the development of epidemiological models and implementation of management strategies. The short-distance vertical transmission of the forest pathogen Phytophthora ramorum from a symptomatic California bay laurel (Umbellularia californica) to healthy containerized rhododendrons (Rhododendron caucasicum × R. ponticum var. album) was monitored for five winters (2016/17 to 2020/21) in a field experiment in Northern California. Transmission events were observed during four winters at a frequency of 1 to 17 per season, but not during the extremely dry winter of 2020/21, and were positively correlated to total rainfall rates. The first leaf symptoms were detected around mid-December and reached the highest numbers in January of most years. Only limited symptom development was observed in the spring, with the last detections in May. The exposure time (the time between the first rainfall after placing a bait plant under the bay laurel and development of symptoms) varied between 3 and over 150 days, with an average between 14 and 21 days. P. ramorum was detected from water samples collected from the canopy of the symptomatic California bay laurel. No horizontal pathogen spread was detected from symptomatic to healthy rhododendrons placed at a distance of 2 to 6 m.