Arboriculture & Urban Forestry 34(5): September 2008 Arboriculture & Urban Forestry 2008. 34(5):317–324. 317 Effect of Propiconazole on Laurel Wilt Disease Development in Redbay Trees and on the Pathogen In Vitro Albert E. Mayfield III, Edward L. Barnard, Jason A. Smith, Shawn C. Bernick, Jeffrey M. Eickwort, and Tyler J. Dreaden Abstract. Laurel wilt is a vascular disease of Lauraceous plants caused by a fungus (Raffaelea spp.) that is vectored by a recently introduced, nonnative ambrosia beetle (Xyleborus glabratus). The disease is devastating to redbay (Persea borbonia) trees in forests, parks, and residential landscapes in the southeastern United States, and management strategies for reducing its impact are needed. In this study, the systemic fungicide propiconazole completely inhibited mycelial growth of Raffaelea spp. in vitro at concentrations 0.1 parts per million (ppm) or greater and was fungitoxic at 1 ppm or greater, whereas the fungicide thiabendazole was less inhibitory. None of the ten mature redbay trees that received root-flare injections of propiconazole developed crown wilt symptoms for at least 30 weeks after being inoculated with Raffaelea spp., whereas nine of ten untreated control trees wilted in more than one-third of their crowns. Propiconazole was retained in the stem xylem for at least 7.5 months after injection but was more frequently detected in samples from trees injected 4.5 months earlier and was not well detected in small-diameter branches. Results suggest that propiconazole may be useful in preventing laurel wilt in redbay, but limitations and questions regarding duration of efficacy, rate of uptake, and efficacy under different levels of disease pressure remain. Key Words. Ambrosia beetle; fungicide injection; laurel wilt; Persea borbonia, propiconazole, Raffaelea; redbay; thiabenda- zole; vascular disease; Xyleborus glabratus. Laurel wilt is a vascular disease of redbay (Persea borbonia) and other plants in the family Lauraceae. It is caused by a fungus that is introduced into host plants by the redbay ambrosia beetle, Xyleborus glabratus (Fraedrich et al. 2008). The redbay ambro- sia beetle is native to Asia but was first detected in North America in a survey trap at Port Wentworth, Georgia, U.S., in 2002 (Rabaglia et al. 2006). Since then, both the beetle and its associated fungus have been consistently recovered from dead and diseased redbays in areas of extensive redbay mortality in South Carolina, Georgia, and Florida. The fungus is a previously undescribed species of Raffaelea, an asexual stage of fungi in the genus Ophiostoma. Female redbay ambrosia beetles carry spores of Raffaelea spp. in mandibular mycangia and transmit them to host trees by boring into the sapwood of stems and branches. As the pathogen moves systemically, affected trees exhibit rapid wilting of foliage and a black discoloration in the xylem. In Florida, laurel wilt has caused mortality rates of greater than 90% within 2 years among redbay stems greater than 2.5 cm (1 in) in diameter (Fraedrich et al. 2008). The laurel wilt pathogen has also been recovered from diseased plants of other Laura- ceous species in the southeastern United States, including sas- safras (Sassafras albidum), pondberry (Lindera melissifolia), pondspice (Litsea aestivalis), and avocado (Persea americana) (Fraedrich et al. 2008; Mayfield et al., in press). In addition to its potential ecologic impacts in natural forests and ecosystems, laurel wilt causes mortality of redbay shade trees in parks, residential neighborhoods, and other public land- scapes (Mayfield 2007). Management tools are desired by home- owners and park managers to help protect landscape redbays and other hosts from laurel wilt and to minimize the associated aes- thetic and economic impacts. Injection of systemic fungicides has been used to protect other tree species from similar vascular diseases. For example, injection of American elms (Ulmus americana) with the fungicides propiconazole and thiabendazole have been used to prevent mortality caused by Dutch elm disease (Haugen and Stennes 1999). Propiconazole injections have also been used to protect live oaks (Quercus virginiana) (Appel and Kurdyla 1992) and other species in the red and white oak groups (Osterbauer and French 1992; Ward et al. 2004; Eggers et al. 2005) from development of oak wilt. The objectives of this study were 1) to determine the mini- mum effective concentrations (MECs) of propiconazole and thiabendazole to inhibit the laurel wilt pathogen (Raffaelea spp.) in vitro; 2) to evaluate the effect of root-flare injections of propi- conazole on laurel wilt development in redbay trees; and 3) to evaluate the distribution and retention of propiconazole in stems and branches of injected redbays. METHODS Effect of Fungicides In Vitro The effect of propiconazole (Alamo; Syngenta Crop Protection, Inc., Greensboro, NC) and thiabendazole (Arbotect 20-S; Syn- genta Crop Protection, Inc.) on vegetative growth of the laurel wilt pathogen Raffaelea spp. was tested on fungicide-amended potato dextrose agar acidified with 50% lactic acid (APDA) in Petri plates (Difco Laboratories 1953). Fungicides were added to plates by mixing appropriate amounts of stock or diluted fungi- cide solution with the liquid APDA media before pouring plates. Treatments consisted of media amended with 50, 10, 1, 0.1, or 0.01 parts per million (ppm) of active ingredient for each fun- ©2008 International Society of Arboriculture
September 2008
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