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© 2013 Plant Management Network.
Accepted for publication 3 June 2013. Published 14 August 2013.


Field and Growth Chamber Inoculations Demonstrate Persea indica as a Newly Recognized Host for the Laurel Wilt Pathogen, Raffaelea laurciola


Marc A. Hughes, Department of Plant Pathology, University of Florida, Gainesville, FL 32611; Gurpreet Brar, Department of Entomology and Nematology, University of Florida, Gainesville, FL 32611; Randy C. Ploetz, Department of Plant Pathology, Tropical Research and Education Center, University of Florida, Homestead, FL 33031; and Jason A. Smith, School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611


Corresponding author: Marc A. Hughes.  plantdochughes@gmail.com


Hughes, M. A., Brar, G., Ploetz, R. C., and Smith, J. A. 2013. Field and growth chamber inoculations demonstrate Persea indica as a newly recognized host for the laurel wilt pathogen, Raffaelea laurciola. Online. Plant Health Progress doi:10.1094/PHP-2013-0814-02-BR.


Laurel wilt, caused by the fungus Raffaelea lauricola, is a vascular disease of native and exotic trees in the Lauraceae in the USA (2). Recently, a study was conducted to investigate the attractiveness of Persea species to the laurel wilt vector, Xyleborus glabratus (redbay ambrosia beetle) (3). Hung bolts of the tree species P. indica were shown to be as or more attractive to X. glabratus than Persea borbonia (redbay), its primary host in the USA (3). In order to assess the susceptibility of P. indica to the laurel wilt fungus, field and growth chamber experiments were conducted. In May 2010, 12 trees of P. indica were planted at the University of Florida Plant Sciences Research and Education Unit in Citra, FL, and 10 were inoculated with 1.0 × 105 conidia of R. lauricola isolate RL4 (CBS 127349; GenBank accession no. HM446155) as described previously (4), with two trees serving as water-inoculated controls. After 6 weeks, all inoculated plants displayed symptoms of laurel wilt similar to those that develop on Persea americana (avocado), including wilt, defoliation, leaf necrosis, shriveling of small branches, and brown/black discoloration of sapwood. R. lauricola was consistently isolated from symptomatic sapwood (2,4).

In March 2012, six containerized seedlings of P. indica, two of Persea palustris (swamp bay), a preferred host of X. glabratus and R. lauricola, and a Quercus alba (white oak) non-host control were placed in a growth chamber (25°C, 12:12 diurnal light). On all plants, bark was scored with a 3-mm cork-borer, and to a single P. indica and P. palustris plant 3 empty gelatin capsules were attached to the lower stem (mock-inoculated controls). On the remaining P. indica plants and one each of P. palustris and the Q. alba, a single female of X. glabratus (reared from an infected redbay log) within an individual gelatin capsule with ventilation holes was affixed to the lower stem, with 3 capsules/beetles per plant. After 48 h, frass was visible and all beetles had successfully bored into the challenged plants of P. indica and P. palustris, whereas no boring activity or frass was visible on the Q. alba and mock-inoculated controls (Fig. 1). After 8 weeks, all X. glabratus-challenged plants of P. indica and P. palustris displayed laurel wilt symptoms, while the control plants remained healthy. Typical colonies of R. lauricola were isolated from four of the five plants of P. indica and the P. palustris. Total genomic DNA was extracted from a single spore isolate (PL1529) recovered from a symptomatic P. indica, and a partial ribosomal small sub-unit fragment was amplified and sequenced with primers NS1 and NS4 (5). A BLASTn search showed, complete homology with multiple R. lauricola GenBank accessions, including RL4 (100% similarity, and a total score of 1796). The sequence was deposited in GenBank under the accession KC333878.


 

Fig. 1. Growth chamber beetle challenge experiments: (A) X. glabratus entering the lower stem of P. indica; (B) P. palustris mock-inoculated control depicting placement of gelatin capsules onto host.


Persea indica is an ancient member of the Lauraceae, endemic to Macaronesia. This species is a dominant member of the laurel forests (laurisilva) in the Madeira and Canary Islands, and an ornamental in the Mediterranean climates of the USA and Spain (1). The knowledge that P. indica wood is attractive to the vector and the above experiments demonstrating pathogenicity indicates that monitoring efforts must consider this species as a potential host and reservoir for the pathogen and vector. The introduction of laurel wilt in areas where P. indica is native may prove destructive to the environmentally significant laurisilva forests of Macaronesia. In addition, P. indica trees in proximity to the P. americana (avocado) production areas of California and Spain may act as a host-bridge, bringing laurel wilt to these areas that have yet to be affected by the disease.


Literature Cited

1. Campos, P. S., and Pais, M. S. S. 1996. In vitro micropropagation of the Macaronesian evergreen tree Persea indica (L.) K. Spreng. In Vitro Cell. Dev. Biol.: Plant 32:184-189.

2. Harrington, T. C., Fraedrich, S. W., and Aghayeva, D. N. 2008. Raffaelea lauricola: A new ambrosia beetle symbiont and pathogen on the Lauraceae. Mycotaxon 104:399-404.

3. Pena, J. E., Carrillo, D., Duncan, R. E., Capinera, J. L., Brar, G., Mclean, S., Arpaia, M. L., Focht, E., Smith, J. A., Hughes, M., and Kendra, P. E. 2012. Susceptibility of Persea spp. and other Lauraceae to attack by redbay ambrosia beetle, Xyleborus glabratus (Coleoptera: Curculionidae: Scolytinae). Fla. Entomol. 95:783-787.

4. Ploetz, R. C., Perez-Martınez, J. M., Smith, J. A., Hughes, M., Dreaden, T. J., Inch, S. A., and Fu, Y. 2012. Responses of avocado to laurel wilt, caused by Raffaelea lauricola. Plant Pathol. 61:801-808.

5. White, T. J., Bruns, T., Lee, S., and Taylor, J. 1990. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. Pages 315-322 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis, D. H. Gelfand, J. J. Sninsky, and T. J. White, eds. Academic Press, San Diego, CA.