First Report of Dieback Caused by Phytophthora ramorum on Golden Chinquapin, Chrysolepis chrysophylla, in California

Abstract

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.