First Report of Phytophthora elongata causing Leaf Blight on Rhododendron in California

In February 2024, V-shape black necrotic leaf tips were detected on a Rhododendron x Cunningham White plant growing in San Rafael, CA, under Eucalyptus trees (37.9804603ºN, 122.5178767ºW). Symptomatic leaves tested positive using a genus-specific lateral flow assay (LFA) for Phytophthora (Agdia Inc, Elkhart, IN, USA). Tissue segments were surface sterilized using 70% ethanol, plated on selective PARPH media (Jeffers et al. 1986) and incubated at 20ºC. Following the plating of symptomatic leaf pieces, a single representative pure culture (NORS077) was isolated and retained as stock. Microscopic examination of the isolate grown on solid media revealed morphological structures diagnostic of Phytophthora elongata, specifically smooth-walled oogonia containing thick-walled oospores with paragynous antheridia (Fig. 1). The isolate was identified by sequencing the internal transcribed spacer (ITS) and elongation factor alpha regions using the primers ITS1/ITS4 (White et al. 1990; GenBank Accession No. PX640152.1) and ELONGF1/ELONGR1 (Kroon et al. 2004; GenBank Accession No. PX614640), respectively. A BLAST search revealed 99.88% identity with ex-type strain P. elongata CPHST BL 62 for both ITS (MG865485.1) and elongation factor alpha (MH358983.1). P. elongata (Peronosporaceae) belongs to clade 2e of the genus Phytophthora. It was first identified in Western Australia causing root and collar rot of Eucalyptus marginata (Rea et al. 2010). Pathogenicity tests were performed on whole plants (N = 10) of Rhododendron x Purple Passion using two methods: i) a zoospore solution (2.5 x 10 5 zoospores/ml) was sprayed on ten plants until runoff; ii) leaf tips of ten other plants were immersed in 7.5 ml of the zoospore solution in a 50 ml conical tube attached to the leaves during entire experiment. Ten leaves per plant were wounded for each assay. The inoculations were carried out in an open-field environment to simulate natural conditions. Plants were misted with water and kept individually in plastic bags for 72 hours to maintain humidity level. Control plants were treated with water only. Leaves inoculated with any of the four different treatments developed symptoms 10 dpi consisting of small black necrotic spots and lesions around the infected area (Fig. 2). Symptomatic leaves tested positive using the same Phytophthora LFA, and P. elongata was re-isolated from them. The identity of the isolates was confirmed morphologically and by sequencing the ITS region. No symptoms were observed on the control plants. While Phytophthora elongata was previously isolated from Rhododendron during a nursery survey in Maryland between 2010 and 2012, that study did not perform inoculation experiments to confirm disease infection (Bienapfl and Balci 2013). To our knowledge, this is the first report of P. elongata in the California and the first time Koch's postulates have been successfully fulfilled on Rhododendron. Although the disease symptoms observed on Rhododendron were limited and mild, this confirmation of P. elongata in the United States might be a significant threat to native ecosystems and the ornamental industry. Rhododendron could serve as a proven reservoirs for the pathogen which may facilitate its spread to more susceptible host plants. Given the destruction P. elongata has caused to Eucalyptus marginata forests in Western Australia, its potential infection into local environments still requires monitoring and management to prevent similar ecological damage.