Characterization and management of different Fusarium species associated with orchids cultivated in Hawaiʻi

Abstract

Commercially grown orchids are among the most valuable ornamental crops produced in the United States. Hawaii is one of the three largest orchids-growing states in the nation, producing the greatest variety of tropical species and unique intergeneric hybrids. A decrease in orchid production has been observed in Hawaii in association with spot and blights of shoots and flowers, as well as root rot that result in rapid death of juveniles and slow decline of mature plants. Such decline is now commonly observed in nearly all nurseries across the Oahu and the Island of Hawaii. Fungal pathogens are speculated to be one of the important sources of diseases in orchids in Hawaii. In the present study 16 fungal genera were isolated from 60 plant samples collected from Oahu and the Island of Hawaii. Fusarium was the most prevalent genus, with 78% of the isolates and frequently associated with several orchid cultivars, including over 9 genera and hybrids. Eleven Fusarium species, namely F. proliferatum, F. oxysporum, F. solani, F. subglutinans, F. poae, F. begoniae, F. bulbicola, F. anthophilum, F. denticulatum, F. circinatum and F. semitectum were found. Fusarium oxysporum was identified and was the most commonly isolated species from all the samples followed by F. proliferatum and F. solani. All Fusarium species were tested for pathogenicity on four orchid genera. Among the 11 Fusarium species, 6 were found pathogenic on 4 orchid genera. Three species, F. proliferatum, F. oxysporum and F. solani, showed moderate to high virulence on Dendrobium, Cymbidium and Miltonia. Fusarium circinatum and F. poae were low to moderate virulent on Dendrobium and Cymbidium whereas F. begoniae caused very low virulence on Dendrobium and Miltonia. Cattleya was susceptible to F. oxysporum only. Pathogenicity assays of Fusarium isolates on different cultivars of orchids resulted in identification of pathogenic Fusarium species was followed by testing efficacy of three fungicides, Pyraclostrobin, a.i. at 25%, Azoxystrobin a.i. at 50% and Triticonazole, a.i. at 20% on mycelial inhibition on four most prevalent Fusarium species namely-F. proliferatum, F. oxysporum, F. solani and F. subglutinans. Triticonazole was the most effective fungicide in the in vitro tests and was further evaluated for its efficacy in on Dendrobium plants in vivo against the most aggressive species of Fusarium, F. proliferatum under greenhouse conditions. This dissertation provides a thorough study to identify pathogenic Fusarium species on orchids and provides information regarding pathogenicity on orchid genera other than Dendrobium, and viable chemical methods to manage Fusarium species on orchids in Hawaii.