First Report of Camphor Tree (Cinnamomum camphora) as a Host of Phytophthora ramorum

Cinnamomum camphora (Lauraceae) is an evergreen shade tree grown in many parts of the United States, including California. From 2007 to 2011, an arborist working in a residential neighborhood in Mill Valley (Marin Co.) noticed several camphor trees with branch dieback and decline. Affected trees had patchy, irregular cankers on the branches and shoot blight. Cankers were black and most had horizontal fissures. Cankers were most abundant in the inside and lower portions of the canopies. In 2011, samples sent to Bartlett Tree Laboratory tested positive for Phytophthora sp. using the Agdia ELISA Phytophthora kit (Agdia, Elkhart, IN). In February 2009 and April 2011, camphor leaf samples were collected by Sacramento Co. inspectors during an annual nursery inspection for Phytophthora ramorum and submitted to CDFA. The normally bright green leaves were reddish with small necrotic spots surrounded by green halos. Camphor samples from Marin Co. were also collected and sent to CDFA in September 2011. An organism with coralloid coenocytic hyphae, chlamydospores, and ellipsoidal semi-papillate sporangia grew on CMA-PARP (4) from both Marin and Sacramento Co. samples. Morphologically, it matched the description of P. ramorum (3). rDNA sequences of the internal transcribed spacer (ITS) region of the Marin (GenBank KC473521) and Sacramento (KC473522) isolates, amplified using primers ITS1 and ITS4 (4), were 100% identical to P. ramorum by a BLAST query (AY038058). Microsatellite loci placed the Marin isolate in the NA1 clonal lineage, while the Sacramento isolate belonged to the NA2 lineage (2). Pathogenicity of both isolates was tested on 5 trees grown in 18.93-liter pots. Three leaves on each tree were inoculated with 6-mm agar plugs taken from the margin of 7-day-old cultures grown on V8 juice agar (V8). Leaves were wounded with a sterile pushpin and two colonized plugs of each isolate were covered with a freezer tube cap filled with sterile dH 2 O and attached to the leaves with a pin-curl clip (4). Three branches of the same plants were wounded and inoculated with a 3-mm colonized agar plug for each isolate and secured with Parafilm. An equal number of leaves and stems were treated with uncolonized V8 plugs as controls. Plants were sprayed with dH 2 O, covered in large plastic bags, and placed in a growth chamber at 18°C. After 4 days, the bags, caps, and plugs were removed from the leaves. Black lesions were seen 7 days after inoculation on most leaves and 10 to 14 days on inoculated branches. After 32 days, P. ramorum was isolated from leaf lesions and canker margins onto CMA-PARP. No Phytophthora spp. grew from the controls. The experiment was repeated once with similar results. Overall, leaf and stem lesions were larger with the NA2 lineage isolate than the NA1 lineage isolate, which is consistent with previous research (1). Leaf abscission was seen in 30% of the leaves inoculated with the NA2 lineage isolate but none of the NA1 or control leaves. To our knowledge, this is the first report of P. ramorum on camphor in nursery and landscape settings. Mill Valley is known for its mild temperatures and abundant summer fog. Optimal weather conditions likely led to the spread of P. ramorum from infected neighboring forest hosts to camphor in Mill Valley, rather than from an introduction of infected nursery plants. References: (1) E. Elliott et al. For. Pathol. 41:7, 2011. (2) E. M. Goss et al. Phytopathology 101:166, 2011. (3) S. Werres et al. Mycol. Res. 105:1155, 2001. (4) L. E. Yakabe et al. Plant Dis. 93:883, 2009.