Susceptibility to Laurel Wilt and Disease Incidence in Two Rare Plant Species, Pondberry and Pondspice

Fraedrich, S. W., Harrington, T. C., Bates, C. A., Johnson, J., Reid, L. S., Best, G. S., Leininger, T. D., and Hawkins, T. S. 2011. Susceptibility to laurel wilt and disease incidence in two rare plant species, pondberry and pondspice. Plant Dis. 95:1056-1062. Laurel wilt, caused by Raffaelea lauricola, has been responsible for extensive losses of redbay (Persea borbonia) in South Carolina and Georgia since 2003. Symptoms of the disease have been noted in other species of the Lauraceae such as the federally endangered pondberry (Lindera melissifolia) and the threatened pondspice (Litsea aestivalis). Pondberry and pondspice seedlings were inoculated with R. lauricola from redbay, and both species proved highly susceptible to laurel wilt. Field assessments found substantial mortality of pondberry and pondspice, but in many cases the losses were not attributable to laurel wilt. R. lauricola was isolated from only 4 of 29 symptomatic pondberry plants at one site, but the fungus was not recovered from three plants at another site. R. lauricola was isolated from one of two symptomatic pondspice plants at one site, and from five of 11 plants at another site, but not from any plant at a third site. Insect bore holes, similar to those produced by Xyleborus glabratus (the vector of laurel wilt), were found in some pondberry and pondspice stems, but adults were not found. Damage caused by Xylosandrus compactus was found in pondberry stems, but this ambrosia beetle does not appear to be a vector of R. lauricola. Xyleborinus saxeseni adults were found in a dying pondspice with laurel wilt, and R. lauricola was recovered from two of three adults. Isolates of R. lauricola from pondberry, pondspice, and X. saxeseni had rDNA sequences that were identical to previously characterized isolates, and inoculation tests confirmed that they were pathogenic to redbay. Because pondberry and pondspice tend to be shrubby plants with small stem diameters, these species may not be frequently attacked by X. glabratus unless in close proximity to larger diameter redbay. Widespread mortality of redbay (Persea borbonia (L.) Spreng.) has been observed in the coastal plains of South Carolina and Georgia since 2003 (3). The mortality is due to a disease known as laurel wilt caused by Raffaelea lauricola T.C. Harrin., Fraedrich & Aghayeva, a fungal symbiont that serves as a food source for the redbay ambrosia beetle, Xyleborus glabratus Eichhoff (Coleoptera: Curculionidae: Scolytinae) (3,9,10). The beetle is native to Asia (e.g., India, Japan, and Taiwan), where it is often associated with plant species in the Lauraceae (e.g., Cinnamomum osmophloem Kanehira, Lindera latifolia Hk.f., and Litsea elongata (Nees) Hk.f.) (11,31), and the beetle brought the laurel wilt pathogen with it from Asia (11). The redbay ambrosia beetle was first detected in the vicinity of Savannah, GA in 2002, and mortality of redbay in nearby Hilton Head, SC was noted soon thereafter (3). The pathogen causes a vascular wilt disease similar to the wilt caused by the related Dutch elm disease fungi (Ophiostoma ulmi and O. novoulmi), which also have vectors in the subfamily Scolytinae (3). Wilted redbay trees of suitable diameter sustain large populations of X. glabratus in outbreak areas (3,5). Female X. glabratus carry R. lauricola and other fungal symbionts in specialized mandibular sacs called mycangia, in which the symbionts grow in a yeast phase (8,9). The beetle may introduce the pathogen into healthy trees during what are believed to be aborted attacks, often on branches too small for development of beetle brood (3). Since the initial discovery of laurel wilt on redbay (3), the disease was diagnosed on other plant species in the Lauraceae occurring in coastal plain areas of the Southeast. Most notably, sassafras (Sassafras albidum (Nuttall) Nees) and avocado (Persea americana Mill.) were reported as hosts in areas with diseased redbay, and both species proved susceptible to R. lauricola in inoculation studies (3,16). The disease has been a serious concern for the conservation of two other indigenous members of the Lauraceae, pondberry (Lindera melissifolia (Walter) Blume) and pondspice (Litsea aestivalis (L.) Fern.), both of which are rare because of habitat loss. Pondberry is a dioecious, deciduous, rhizomatous, clonal shrub that grows up to 2 m tall and occurs at the margins of ponds, swamps, and sinks (1,22). Pondberry is listed as a federally endangered species and is extremely rare in the Southeast. In the 1990s, only five populations of pondberry were identified in South Carolina (25), and four populations were identified in Georgia (20). Pondspice is a dioecious, deciduous, multibranched shrub that grows 0.5 to 3 m tall, and this species also grows at the margins of ponds and swamps (18,22). This species is known to occur in only 13 Georgia counties (20) and five counties in South Carolina (22). Pondspice is listed as a candidate for the federal endangered species list and is considered threatened in Georgia and South Carolina. R. lauricola has been isolated from pondberry and pondspice plants with wilt-like symptoms (3,24), but there is no information on the susceptibility of these species, and the potential impact of the disease on these species is not known. The objectives of this Corresponding author: Stephen Fraedrich, E-mail: [email protected] Accepted for publication 12 April 2011. doi:10.1094 / PDIS-11-10-0841 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2011.