Assessing the stand size of bay trees (Persea spp.) after exposure to laurel wilt disease in a North Florida Preserve

Abstract

Although laurel wilt disease was fi rst reported in the United States in 2002 from redbay trees (Persea borbonia) around Savannah, Georgia it has rapidly spread throughout the southeastern coastal plain including Georgia and Florida. In the current study, transects were used to assess the spread and impact of the disease on two native bay trees redbay (P. borbonia) and swampbay (P. palustris) from north Florida in a semi-naturalized ecological preserve. Although tree size and mortality rates have been reported previously, this study provides the fi rst size-based static life tables for both species. While a signifi cantly higher percent (76%) of swampbay trees exhibited signs of laurel wilt disease compared to redbay trees (62%); redbay had more of its canopy damaged by the disease (41% vs. 32% for redbay vs. swampbay respectively); this resulted in a signifi cantly smaller stem diameter for P. borbonia compared to swampbay, both species are experiencing signifi cant declines due to the disease. Both species exhibited a Type III survivorship curve in which the vast majority of individuals were in the smallest size class (average stem diameter was only 2.5 and 3.6 cm for redbay and swampbay respectively). Although traditionally, population age (or size) structure that is heavily biased toward younger or smaller size classes suggests that the population is likely to expand in the future, for these bay trees high mortality rate due to beetle/fungal infestation of larger size classes is responsible for this trend; the smallest size classes are largely free from beetle infestation and laurel wilt disease because the stem diameter is likely insuffi cient to support beetle development. Results from this study suggest that swampbay is also highly susceptible to laurel wilt disease and its populations are likely to exhibit a similar (albeit slower) decline in Florida’s wetland and mesic ecosystems. Research Article Assessing the stand size of bay trees (Persea spp.) after exposure to laurel wilt disease in a North Florida Preserve Anthony M Rossi* and Christopher Bentzien Department of Biology, University of North Florida, Jacksonville, Florida, USA *Address for Correspondence: Anthony M Rossi, Department of Biology, University of North Florida, Jacksonville, Florida, 32224, USA, Tel: (904) 620-1934; (904)-620-2830; Email: arossi@unf.edu Submitted: 09 May 2019 Approved: 03 June 2019 Published: 04 June 2019 Copyright: © 2019 Rossi AM, et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited How to cite this article: Rossi AM, Bentzien C. Assessing the stand size of bay trees (Persea spp.) after exposure to laurel wilt disease in a North Florida Preserve. J Plant Sci Phytopathol. 2019; 3: 042-049. https://doi.org/10.29328/journal.jpsp.1001030 Introduction Laurel wilt disease is a generally fatal infection caused by a non-native fungus (Raffaelea lauricola) that is spread from tree to tree by the exotic redbay ambrosia beetle (RAB) (Xyleborus glabratus). Trees are inoculated with the fungus during gallery formation by adult beetles; females lay eggs in these galleries and the beetle larvae feed on the fungal hyphae eventually fungal growth plugs the water-conducting xylem vessels of the plant causing the leaves to brown and wilt; typically, the tree will eventually die from the infection [1,2]. Unlike leaf abscission in which the leaves develop a characteristic abscission zone and change color (as chlorophyll production ceases and secondary pigments such as carotenoids and anthocyanins become visible) and they are retained on the stem by the vascular tissue until wind or abrasion causes them to drop, laurel wilt kills the leaf by restricting its water supply causing the leaf to brown and die on the stem [3]. Infected trees respond to the fungus by producing tyloses and gums, which may reduce the spread of inections thoughout the plant, but they also inhibit the ability of the vessels to transport water [2]. For instance, tyloses are ballon-like swellings produced by parenchyma cells that project though pit cavities Assessing the stand size of bay trees (Persea spp.) after exposure to laurel wilt disease in a North Florida Preserve Published: June 04, 2019 043 in the vessel wall and partially or completely obstruct the vessel [3]. Progression of the disease is easily recognizable by the whole-sale browning of leaves that remain attached to the affected branches as well as the appearance of frass (referred to as sawdust tubes) created by beetle feeding which forms plugs or sticks visible on the external surface of the trees. Xyleborus glabratus transports the fungus in its mycangia, which are small specialized pouches that are associated with the mandibles [2,4]. Recent studies comparing fungi collected from the mycangia of RAB using molecular data and culturing medium that is selective to members of the Ophiostomatales, including R. lauricola, are consistent with populations from Southeast Asia [4,5]. Speci ically, fungal isolates from the mycangia of X. glabratus populations trapped in U.S. were similar to those from Taiwan and Japan; moreover, bacterial communities from X. glabratus mycangia collected in the U.S. were distinctly different from those of native species of xyleborine beetles, but similar to those from Asia [4,6]. These results are consistent with the hypothesis that within the U.S. both the beetle and its fungal symbionts were introduced into the U.S. from Asia; most likely via wooden pallets or packing materials that entered through Port Wentworth, which is part of the greater Savannah, Georgia metropolitan area, in the late 1990’s or early 2000’s [7-10]. Not surprisingly, introduced laurel species such as camphortree (Cinnamomum camphora), which are susceptible to laurel wilt, but are indigenous to southeast Asia and whose native distribution overlaps with that of RAB, have developed resistance to the disease [10,11]. For instance, camphor tree, which has become naturalized in the U.S., has shown signs of laurel wilt disease (e.g. browning of leaves and branch dieback), but affected trees typically recover from laurel wilt disease [10]. Laurel wilt has the potential to cause both ecological and economic damage because the laurel family of trees includes both non-commercial native trees such as redbay (Persea borbonia) and swampbay (P. palustris), but also economically important species such as avocado (P. americana). While loss of a commercial product such as avocado is relatively easy to estimate, ecological impacts of declining native bay trees is more problematic; redbay in particular is an important food plant for native animals such as the Palamedes swallowtail butter ly [5,12,13]. In the approximately two decades since its introduction to the U.S. laurel wilt disease has spread rapidly throughout the Southeastern U.S., especially along its primary east-coast interstate highway (I-95) which likely facilitates spread of the beetle and fungus via commercial deliveries along this busy traf ic corridor [14]. During 2005 RAB and the fungus were irst reported causing laurel wilt disease in redbay trees in north Florida from Timucuan Ecological and Historic Preserve, Duval County which includes the Jacksonville metropolitan region [5,8]. Fraedrich et al. [15], conducted a detailed study of redbay trees with a dbh > 2.5 cm within the preserve and found that percentage of cumulative mortality increased from approximately 10 to over 90% during a 16-month period and larger diameter trees died more quickly than those from smaller size class. Additionally, a 2008 study conducted in St. Johns County (just south of Duval County) of the state-listed endangered laurel tree, pondspice (Litsea aestivalis), reported 85% of the pondspice trees were already dead or dying from the disease [16]. Although southward spread of the beetle and the concomitant fungus was estimated at 20-30 miles (32-48 km) per year; the beetle was detected in central Florida coastal counties including Indian River and Brevard counties in 2005 and 2006 respectively and south Florida counties including Dade (which includes the Miami metropolitan area) in 2010 some ive years sooner than predicted [9,14]. Goals of this project were: 1) determine the extent of the disease affecting the two most common species of bay trees (i.e. P. borbonia and P. palustris) on the University of North Florida (UNF) campus (as part of the university’s land management strategy); 2) determine if the tree is infesting swampbay as well as redbay; 3) determine the effect of the disease on tree survival and size class. Assessing the stand size of bay trees (Persea spp.) after exposure to laurel wilt disease in a North Florida Preserve Published: June 04, 2019 044 Methods