Reevaluation of the Application Method and Efficacy of Propiconazole in Controlling Laurel Wilt in Avocado Orchards in South Florida

Propiconazole has fungistatic and fungicidal properties against Harringtonia lauricola, the causal agent of laurel wilt disease. Propiconazole injections are used by Florida, USA, farmers as a prophylactic method to manage the disease in avocado (Persea americana) trees, but its efficacy has remained questionable for more than a decade due to documented restricted mobility within the tree vascular system. This study was conducted to evaluate the absorption of propiconazole when using soil drenching as an alternative application method, assess the efficacy of propiconazole in controlling disease development when drenched or injected, and its synergistic effect on common cultural management practices used by the local farmers, such as branch removal and trunk cutting (“stumping”). To determine if propiconazole soil-drenching can provide better xylem coverage, potted and mature orchard trees were treated with different concentrations and artificially inoculated with the pathogen. Propiconazole translocation from the roots to above-the-ground tissue was confirmed in potted and orchard trees, but the concentrations in orchard trees were below the fungicidal threshold (1 ppm). Although none of the potted trees developed laurel wilt symptoms, all inoculated branches of the orchard trees did. Furthermore, noninoculated branches in more than 80% of the inoculated and propiconazole-treated orchard trees developed symptoms, even though the inoculated branch was cut at the early stages of disease development. To elucidate if propiconazole injections effectively control the disease, trees from a commercial orchard that were injected five times were challenged by artificial inoculation. Propiconazole concentration in trees was highly variable (ranging from < 0.01 to 294 ppm), but even in trees with a high concentration of propiconazole, inoculated and noninoculated branches developed symptoms. Even though drenched and injected trees were “stumped” soon after symptoms appeared in the noninoculated branches (4 to 5 months after inoculation), all of the stumps in the drenched plot and 80% of the injected trees, showed internal symptoms 5 and 4 months after the cut, respectively. Results demonstrate that the soil-drenching of propiconazole is an ineffective application method in orchard trees, and that the conventional injection does not prevent disease development after artificial inoculation. Moreover, because propiconazole does not prevent the movement of the pathogen to the trunk, the “stumping” of infected trees to reduce the disease in the orchard is an inadequate practice. This study highlights the critical need for other active ingredients with lower fungicidal thresholds, longer half-life, and higher xylem mobility.