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Research Project: Alternatives to Methyl Bromide: Mitigation of the Threat from Exotic Tropical and Subtropical Insect Pests

Location: Subtropical Horticulture Research

Title: Recovery plan for laurel wilt on redbay and other forest species caused by Raffaelea lauricola and disseminated by Xyleborus glabratus

Author
item Hughes, M - University Of Florida
item Smith, J - University Of Florida
item Ploetz, R - University Of Florida
item Kendra, Paul
item Mayfield, B - Forest Service (FS)
item Hanula, J - Forest Service (FS)
item Hulcr, J - University Of Florida
item Stelinski, L - University Of Florida
item Cameron, S - Forestry Commission
item Riggins, J - Mississippi State University
item Carrillo, D - University Of Florida
item Rabaglia, R - Forest Service (FS)
item Eickwort, J - Forest Service (FS)
item Pernas, T - National Park Service

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2015
Publication Date: 11/17/2015
Citation: Hughes, M., Smith, J., Ploetz, R., Kendra, P.E., Mayfield, B., Hanula, J., Hulcr, J., Stelinski, L., Cameron, S., Riggins, J., Carrillo, D., Rabaglia, R., Eickwort, J., Pernas, T. 2015. Recovery plan for laurel wilt on redbay and other forest species caused by Raffaelea lauricola and disseminated by Xyleborus glabratus. Plant Health Progress. 16(4):173-210.

Interpretive Summary: Laurel wilt is a highly destructive disease of members of the Lauraceae in the United States. The insect vector, the redbay ambrosia beetle (Xyleborus glabratus Eichhoff) was first captured in monitoring traps near Port Wentworth, GA in 2002 and first reported associated with mortality of redbay (Persea borbonia [L.] Spreng.) trees in 2003. Laurel wilt disease is initiated when X. glabratus introduces its fungal symbiont (Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva) into the sapwood of host trees. The fungus is carried within specialized pouches in the beetle’s mouthparts (mandibular mycangia), where it lives in a budding, yeast-like state. The fungal spores are introduced into the xylem as the beetle bores into the stem, leaving typical evidence of ambrosia beetle attack (small holes and boring dust). Host trees react to the fungal invasion with the production of gums and tyloses, which block water transport and cause crown wilt. Upon dissection of infected wood, xylem discoloration is readily evident. Laurel wilt has now been detected in seven southeastern states (AL, FL, GA, LA, MS, NC, SC), causing significant mortality to redbay populations. Redbay serves an important ecological role in forests, and the loss of this species has had significant effects on forest composition. Several other lauraceous hosts (sassafras, silk bay, swamp bay, pondspice [state endangered] and pondberry [federally endangered]) are susceptible to laurel wilt and have been affected by the disease to varying degrees. In addition, agricultural (avocado) and ornamental non-native members of the Lauraceae within the United States are susceptible, demonstrating that laurel wilt is more than a disease of native forests. Eradication of the vector and pathogen is improbable due to the ability of the vector to persist in small diameter stems and single females to establish new populations. Currently, management options within a natural forest setting are limited and spread of the disease into new areas (e.g. California, Mexico, and Central and South America) remains a threat. For these reasons it is essential to continue monitoring the spread of the disease, and continue to develop a better understanding of the biology of the beetle and pathogen as well as the epidemiology of the disease. In addition, further development of the following strategies may help to reduce the impact of laurel wilt in forests and urban settings, and limit the spread of the disease.

Technical Abstract: Genetic data can guide the management of plant conservation collections. Direct assay of an ex situ collection’s genetic diversity, measured against wild plant populations, offers insight for conservation efforts. Here we present a carefully chosen case study, Zamia lucayana, selected for its contrasts with a previous model for this type of assessment, Z. decumbens. These two species provide many comparisons, including: (1) longer vs. shorter generation times, (2) annual, abundant seed production vs. infrequent, irregular seed production; (3) one continuous population vs. several disjunct populations; and (4) ca. 1,000 extant plants vs. no more than 600. In line with these biological differences, structured ex situ collections of Zamia lucayana capture more genetic diversity (244 ex situ plants capture 94% of in situ alleles) than similar collections for Z. decumbens (205 plants capture 77% of in situ alleles). Systematic comparisons of the genetic data between these case studies provide basic recommendations. Foremost, careful consideration of the target species is essential when planning for capture of genetic diversity. Differences in species, accessions, populations, and timing all play a role in collection sampling strategy. For example, these data suggest that slow-maturing, infrequently coning species will require sampling in multiple years, whereas species with faster, frequent, and more abundant reproduction may not. Integrating this type of precise ex situ conservation assessment with in situ management, monitoring, and community outreach can "close the loop," ensuring these living treasures do not go extinct.