Fungicide Treatment of Avocado (Persea americana Mill.) Trees for Management of Laurel Wilt Disease

Laurel wilt is a lethal disease of Persea borbonia (redbay) and other tree species in the Lauraceae family. Laurel wilt has been found in the field on several other Lauraceae species including avocado (Persea americana), sassafras (Sassafras albidum), and other endangered species pondberry (Lindera melissifolia). The disease is caused by the fungus Raffaelea lauricola that is transferred into host trees by a non-native pest, the redbay ambrosia beetle (Xyleborus glabratus). The disease has spread to trees in forests, parks, orchards, and residential areas in the southeastern United States. Avocado, is the utmost important agricultural crop vulnerable to the disease. The disease threatens commercial avocado production in Florida, centered in Miami-Dade County ($54 million/year), as well as the National Germplasm Repository (NGR) for avocado in Miami (USDA–ARS). Elsewhere in the United States, major (California, $342 million in 2006) and minor commerce in the fruit (Hawaii and Puerto Rico) could be impacted if the disease continues to spread. Policies for reducing its impact in Persea species (especially P. americana) are been studied. The NGR in Miami, FL, is responsible for the conservation of the USDA–ARS avocado collection.  The collection contains over 300 accessions and includes all three races, as well as hybrids between them. Presently the collection is free from the ambrosia beetle and the fungus, it is believed to be only a matter of time before this disease reaches the collection. Macro infusion of fungicides such as Propiconazole® has shown to control the disease in oaks and other species.  Propiconazole was macro infused through the root system of the tree at the recommended doses. Approximately 270 mature avocado trees, one per accession, were selected for treatment based on tree health and infection status with Avocado Sunblotch Viroid (ASBVd). Plants were infused in February 2011 and again in June 2013. Following propiconazole application (at one day, and at 14 months, respectively), randomly selected branches above dbh of selected trees were collected. From each of the selected trees, segments were removed and placed in cold storage (40 °F) for further analysis for the presence of propiconazole. Then, the samples were sent to the University of Georgia for propiconazole analysis. The rate of application fluctuated significantly with the size of trees. Propiconazole was present in branches and fruits (below EPA standards) of trees injected 12–14 months after treatment. Results suggest that correctly applied fungicide may provide protection on avocado for periods beyond one year.