In Vitro Efficacy of Various Fungicides Against Fungi Associated with Leaf, Stem, and Soilborne Diseases of Proteaceae in Hawai‘i, U.S.A.

Species in the Proteaceae have been important ornamental plants cultivated for both cut flowers and foliage in Hawaiian agriculture since the 1960s. Symptoms ranging from spots on leaves and flowers to branch lesions and cankers, as well as dieback, have been reported on various Proteaceae in Hawai‘i and attributed to various fungi. Nurseries can experience significant economic losses if symptoms are not controlled. Few studies are available regarding the effectiveness of chemical treatments to reduce disease incidence and severity. The efficacy of six ornamental fungicides from various Fungicide Resistance Action Committee (FRAC) groups were evaluated in vitro for their ability to inhibit the mycelial growth of nine fungal isolates of six known or suspected protea pathogen genera ( Diaporthe, Alternaria, Neofusicoccum, Neopestalotiopsis, Phomopsis, and Calonectria). Isolates were recovered from symptomatic stems and leaves, as well as soil surrounding diseased Proteaceae grown at the Maui Agricultural Research Center (MARC), Kula, HI, and from 10 commercial farms on Maui. All fungicides evaluated in the study inhibited the in vitro growth of some, if not all, of the nine isolates to some capacity. Tebuconazole was the most effective fungicide, reducing mycelial growth of all isolates by ≥85% or more compared with the nonamended control plates. Rotation of these fungicides could potentially be used for disease management against a broad range of pathogens infecting Proteaceae in Hawai‘i.