Toward Fungicidal Management of Laurel Wilt of Avocado

Laurel wilt threatens commercial and residential production of avocado (Persea americana) in Florida. Laurel wilt on redbay (P. borbonia) was controlled previously with macroinfusions (injections) of Alamo, an injectable formulation of propiconazole. To determine whether Alamo macroinfusion would be cost effective in commercial avocado production, economic analyses were conducted for various macroinfusion scenarios and a standardized production situation in southern Florida. Under prevailing conditions, macroinfusion was not cost effective. In the interest of identifying alternative means to manage the disease, other fungicides and application measures were evaluated. In all, 20 fungicides in 15 chemical groups and 10 fungicide groups were examined in vitro. In vitro inhibition of the radial growth of the pathogen Raffaelea lauricola was determined on fungicide-amended malt extract agar; demethylation inhibitors (DMIs; fenarimol, myclobutanil, propiconazole, prothioconazole, triadimenol, triadimefon, and triticonazole), quinone outside inhibitors (azoxystrobin, pyraclostrobin, and fluoxastrobin), and a quinone inside inhibitor (fluazinam) had the greatest impact on radial growth (the concentration at which growth was reduced by 50% was ≥0.1 μg ml^-1). In greenhouse studies, the most inhibitory products in vitro, plus thiabendazole and two products that were not tested in vitro, flutriafol and a potassium salts mixture of phosphorus acid, were tested for disease suppression on artificially inoculated, potted 'Simmonds,' a susceptible avocado cultivar. In general, soil drench applications of the above DMIs and thiabendazole but not azoxystrobin, pyraclostrobin, fluazinam, or the phosphorus acid salt provided significant control of disease (P < 0.05). Topical branch or trunk applications of propiconazole, and triadimenol in 2% Pentrabark, a bark-penetrating surfactant, were also effective at lower rates than were used in drench applications. Comparable levels of disease suppression were achieved when propiconazole was applied at 11% of the rates that were used in soil drenches. Although topical fungicide applications in bark-penetrating surfactants would be a less expensive practice than macroinfusion, moving sufficient concentrations of propiconazole or other fungicides into host xylem will be difficult in trees that are larger than the potted plants that were tested in these trials. Ongoing work examines means by which this goal might be met on fruit-bearing trees in the field.