Ability of the Redbay Ambrosia Beetle (Coleoptera: Curculionidae: Scolytinae) to Bore into Young Avocado (Lauraceae) Plants and Transmit the Laurel Wilt Pathogen (Raffaelea sp)

Abstract

Laurel wilt is a vascular disease of plants in the Laurel family (Lauraceae) that has caused ex tensive mortality of redbay (Persea borbonia (L.) Spreng) trees in South Carolina, Georgia, and Florida. The disease is caused by a previously un described species of Raffaelea, a fungal symbiont of the non-native redbay ambrosia beetle, Xyle borus glabratus Eichhoff (Fraedrich et al. 2008). Xyleborus glabratus was initially detected in the US. near Savannah, GA in 2002 (Rabaglia et al. 2006). The beetle carries spores of the laurel wilt fungus in mandibular mycangia and inoculates the xylem of host trees by boring into the wood. In redbay, the fungus moves rapidly through the xy lem, plugging the flow of water and causing trees to die in a matter of weeks or a few months. Af fected trees are characterized by a dark discolor ation in the outer sapwood (Fraedrich et al. 2008). Laurel wilt continues to devastate populations of redbay in the southeastern coastal plain and the distribution of the disease has rapidly expanded (Johnson et al. 2008). In the summer and fall of 2006, 16 small avo cado (Persea americana var. americana Mill.) trees were planted on Ft. George Island, FL, a site with a high incidence of laurel wilt. By 12 Feb 2007, X glabratus and its associated Raffaelea sp. were obtained from two dying trees of the West In dian cultivar Donnie (A.E.M., J.E.P. & J.H.C., un published data). In September 2007, a large avo cado tree in a homeowner's yard in Jacksonville, FL also was discovered to be diseased with laurel wilt (Mayfield et al. 2008). In growth chamber ex periments, some avocado plants wilted after arti ficial inoculation with the laurel wilt pathogen (Fraedrich et al. 2008). Furthermore, avocado wood has been demonstrated to be attractive toX glabratus in field trapping trials (Hanula et al. in press). Concern exists that laurel wilt and X. gla bratus could negatively impact the commercial avocado industry in south Florida and beyond. There are 3 distinct races of avocado, West In dian (WI) (Persea americana var. americana Mill), Guatemalan (G) (P. americana var. guatemalensis Williams) and Mexican (M) [P. americana var drymifloia (Schlect. and Cham.)]. In Florida, West Indian and West Indian-Guatemalan hybrids are commercially grown; in California, mostly Guate malan-Mexican hybrids are grown. Two no-choice experiments were conducted to determine (1) whether Xyleborus glabratus would bore into healthy young avocado plants of various genetic backgrounds, and (2) whether these bee tles could transmit the laurel wilt pathogen (Raf faelea sp.) into those plants. Stem sections (ap prox. 30 cm long and 10 cm in diameter) from wilted redbay trees infested with X. glabratus were collected from Jennings State Forest, Clay County, FL in early May 2007 and placed in rear ing containers at the Florida DACS Division of Plant Industry in Gainesville, FL. Emerging adult females were collected, sealed in Petri plates and transferred to a quarantine green house at the University of Florida, Indian River Research & Education Center in Fort Pierce, FL. Experimental plants were obtained in 3.8-L pots from a commercial nursery near Homestead, FL and ranged from 1.2-4.4 cm in basal diameter and 43.0-94.0 cm in height. Experiment 1 included 4 plants from each of 4 avocado cultivars represent ing different genetic backgrounds: 'Hass' (G-M hybrid), 'Simmonds' (WI), 'Monroe' (G-WI hybrid), and Winter Mexican' (M). These particular culti vars were of interest because 'Hass' is a widely grown commercial cultivar in California, 'Sim monds' and 'Monroe' are popular Florida culti vars, and 'Winter Mexican' is a pure Mexican race cultivar. Within each cultivar, 2 plants were ran