First Report of Phytophthora cinnamomi Causing Root Rot of Southern Highbush Blueberry In California.

In 2015, several southern highbush blueberry plants (Vaccinium corymbosum L. cv. Ventura) growing in a production field in Oxnard, CA, were stunted, chlorotic, and had necrotic, blackened roots; two representative samples were collected and sent to the laboratory in Watsonville, CA, for isolation. Pieces of necrotic root tissue were disinfested in 10% bleach for 1 min, rinsed thoroughly, and were plated on corn meal agar with selective antibiotics (CMA-PARP). Multiple colonies were recovered and subcultured onto potato dextrose agar (PDA). After 5 days of incubation at room temperature (RT), all colonies appeared white and cottony and had a rosaceous growth pattern. One representative isolate was saved for characterization (isolate 14-009) and its microscopic characteristics consisted of coenocytic hyphae with clusters of grape-like chlamydospores, initially identified morphologically as Phytophthora cinnamomi Rands (Erwin and Ribeiro 1996). To confirm pathogenicity, ten 3-month-old, asymptomatic southern highbush blueberry plants (variety 92D9) were propagated in tissue culture and separated into control and test groups. Inoculum was prepared by growing isolate 14-009 in potato dextrose broth for 7 days on a laboratory shaker at room temperature, and mycelia was harvested and blended. Roots of the control and inoculated plants (five per group) were dipped for 45 min in water or suspended mycelium (0.02 g wet mycelium/ml) and planted in a 3:1 peat moss/perlite medium. Plants were placed in a growth chamber at 22°C with a 16/8-h light/dark cycle for 1 week. After 7 days, inoculated plants showed symptoms of yellowing, stunted growth and a 48% reduction in the number of leaves (mostly due to senescence), whereas control plants showed no symptoms. The roots of all plants were removed, weighed, and plated onto CMA-PARP. Cultures from both control and inoculated plants showed significant growth of Pythium spp.; however, isolates from three different inoculated plants also showed similar morphology to 14-009. All isolates were subcultured, and plant tissue samples were taken from each plant and then used for DNA extraction. DNA was extracted using a QIAamp DNA Mini Kit (Qiagen Inc. Valencia, CA), and amplified using the mitochondrial atp9-nad9 qPCR marker system developed by Bilodeau et al. (2014) to detect and quantify P. cinnamomi. PCR products were sequenced using the NAD9F and NAD9Rseq1 primers and a local BLAST was conducted using the program BioEdit v7.2.5. All atp9-nad9 sequences showed 99% identity to two isolates of P. cinnamomi from the UC Riverside’s World Oomycete Genetic Resource Collection (isolate Nos. P10203 and P10162 and GenBank Accession Nos. JF771728 and JF771727, respectively). P. cinnamomi is present in 24% of the commercial blueberry crops in Oregon (Bryla et al. 2008) and it is also an important pathogen on many crops (e.g., avocado) throughout California. It has been reported on California huckleberry (Vaccinium ovatum Pursh), a close relative to blueberry (French et al. 1989), but to the authors’ knowledge, P. cinnamomi has not been reported on blueberries in California. Many blueberry plantings are utilizing previous avocado fields or bordering existing ones in the Oxnard area. Due to its importance in other regions of the United States (Bryla et al. 2008), and other factors specific to California’s agricultural system, management of P. cinnamomi should be considered in agricultural production practices. © 2016 The American Phytopathological Society.