Davidson, J.

Garbelotto, M.

Hüberli, D.

Swain, S.

Van San, W.

English

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Susceptibility of Umbellularia californica to Phytophthora ramorumDaniel Hüberli1, Will Van Sant1, Steven Swain1, Jennifer Davidson2, & Matteo Garbelotto1Email: dhuberli@nature.berkeley.edu  Website: www.cnr.berkeley.edu/garbelotto/1Dept. of ESPM-ES, University of California, Berkeley, CA  94720, USA2Dept. of Plant Pathology, University of California, Davis, CA  95616, USAINTRODUCTIONPhytophthora ramorum is a newly described aerially disseminated phytopathogen that threatens the coastal oak forests in California (1). The coexistence of a number of different hosts in these forests allows the pathogen to become established. In particular, severe stem cankers on Quercus agrifolia (coast live oak) are strongly associated with heavily infested leaves of adjacent Umbellularia californica (bay laurel) (2). Often asymptomatic to low symptomatic bay trees are observed next to severely infested bays. We designed a bioassay to screen for variation in susceptibility to P. ramorum among populations of bay trees. By understanding the conditions most conducive for infection of bays, we can determine which oak woodlands have the highest potential risk of infection by the pathogen.Fig. 2. Leaf tip lesions that developed 14 days after inoculation with Phytophthora ramorum zoospores on representative leaves of Umbellularia californica collected from 4 locations in California.METHODSBioassay developmentBranches with 4 first expanded shade leaves were collected from various locations in California. Microcentrifuge tubes were attached to leaf tips (Fig. 1). To each tube, 300 µL of zoospores (2x104/mL) was added. Branches were incubated in moist biodomes for 2 weeks. Lesion area was measured.Two optimization studies were conducted:1. Exposure time: leaves were exposed to zoospores for 6, 12, 24, 36 and 48 h. Microtubes were carefully removed.2. Temperature: after inoculation, leaves were incubated at 12, 17, 27 and 29 °CInoculation studyBranches from 6 trees (3 reps/ tree) were collected from 4 P. ramorum infested locations in California. The locations were Big Sur (Monterey), China Camp State Park (Marin), Swanton Pacific Ranch (Santa Cruz), and Austin Creek State Recreational Area (Sonoma). Branches were incubated as above, but at optimized exposure time (12 h) and temperature (17-22 °C).DISCUSSIONThe bioassay is a reliable and practical screening system for assessing bay susceptibility. We found a range of susceptibility within and among infested locations in California. These and other locations will be assessed more intensively. It is hypothesised that a number of factors contribute to P. ramorumepidemics including host resistance, genetic structure of host species in the forests, pathogen variation, and environmental conditions. These issues are currently being assessed in bay as well as Q. agrifolia.RESULTSBioassay developmentExposure time: Leaves exposed to zoospores for 6 h had smaller lesions than those exposed for 12-48 h. Therefore, 12 h was used for the bioassay.Temperature: Largest lesions formed at 17-22 ºC. Towards the maximum and minimum temperature range, little or no differences wereevident in lesion size among leaves collected from different trees.Fig. 1. Method used to inoculate Umbellularia californica leaves on small branches with Phytophthora ramorum zoospores. Zoospores wereadded to microfuge tubes.Inoculation studyLesions within a location varied significantly among trees (Fig. 3). Variation between locations was also significant (Fig. 2, 3). Overall, leaves from Swanton Pacific had significantly smaller lesions, while those from Big Sur had significantly larger lesions. 020406080100120140Swanton Pacific Big Sur Austin Creek China CampLocationAverage lesion (mm²)Fig. 3. Average lesion area that developed 14 days after inoculationwith Phytophthora ramorum zoospores on leaves of Umbellularia californica collected from 4 locations in California; n = 3. REFERENCES1. Rizzo, D.M., Garbelotto, M., Davidson, J.M., Slaughter, G.W. & Koike, S.T. (2002) Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflorus in California. Plant Disease 86, 205-214.2. Swiecki, T. & Bernhardt, E. (2001) Evaluation of stem water potential and other tree and stand variables as risk factors for Phytophthora canker development in coast live oak and tanoak. USDA report - Forestry, March 28, 2001. Phytosphere Research, 1-31. 

Susceptibility of Umbellularia californica to Phytophthora ramorum

https://researchrepository.murdoch.edu.au/id/eprint/22031/1/Umbellularia_californica_and_Phytophthora_ramorum.pdf

Susceptibility of Umbellularia californica to Phytophthora ramorum

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