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Etiology and control of fusarial orchid diseases in Hawaii

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Title: Etiology and control of fusarial orchid diseases in Hawaii
Authors: Swett, Cassandra L.
Keywords: Orchids -- Diseases and pests -- Control -- Hawaii
Fusarium diseases of plants -- Control -- Hawaii
Issue Date: 2007
Abstract: Overall, control efforts are focused on breaking the disease cycle. The disease cycle is initiated when spores disperse from a contaminated source to a healthy host. For Fusarium species, two types of asexual spores, macroconidia and microconidia, are often the primary means of dispersal, although sexual spores (ascospores) can also playa role (21). In nurseries, ascospore dispersal is by wind (45), and conidia can spread by both wind and water splash, primarily from overhead watering (21, 45). Vectors, such as insects and tools which contact infected material, also contribute to spread (2). Once in contact with the host, the spore germinates, penetrates the tissue, and enters the host. The pathogen infects the tissue, feeding on cellular material, growing, and producing spores that are then dispersed, perpetuating the disease cycle. If a host is absent, thick walled survival spores, called chlamydospores, can develop from cells in the hyphae and/or the macroconidia (87). Chlamydospores can allow for survival in plant parts (such as root matter) or in the soil for many years (87). Spread of Fusarium in greenhouse or field grown orchids can be reduced by various cultural practices, including removal of infected material (2, 28, 34), regulation of watering and wind (when possible) (21,45,56), and management of vectors (96), among other sanitary practices. Fusarium infection and reproduction can also be prevented through the use of resistant cultivars (2, 77). Fusarium germination growth and infection can also be inhibited by anti-fungal compounds, including chemical fungicides and biological controls (2). Some of the most successful practices in controlling fusarial diseases on nursery crops have involved the use of anti-fungal compounds, and the various types are reviewed below, together with examples of efficacy in controlling Fusarium in nursery systems. Chemical Fungicides: Chemical fungicides are toxic compounds which inhibit fungal germination, infection, growth, and reproduction. Compounds are either broad spectrum, with efficacy against many fungi, or are specific to certain fungi. Most fungicides are preventative, protecting the surface tissues on which they are applied, and inhibiting fungal germination (2). However, some fungicides are systemic, and enter the plants vascular system, reducing fungal growth and reproduction within the host, and curing very early stages of infection. Most systemic fungicides function either by releasing antifungal toxins into host's vascular system, or by inducing a systemic resistance response in the plant. Many of these fungicides are "traditional" compounds, which persist in the environment, and have higher mammalian toxicity than newer compounds. As an alternative to traditional fungicides, there are several compounds available, characterized as reduced risk, which have low mammalian toxicity and degrade quickly in the environment. Although many of these reduced risk compounds can be effective, they are often expensive and necessitate frequent applications. There are many effective fungicides currently available for Fusarium disease control in nursery crops. Effective traditional fungicides include captan and carbendaxim, found to inhibit gladiolus corm rot caused by F. oxysporum (76), carboxim (Vivetax), which was inhibited F. oxysporum wilt of gladiolus, (63), and thiram, a successful control of cucumber root and stem rot caused by F. oxysporum (77). Strobilurins are compounds that inhibit fungal mitochondrial respiration, and have been effective in Fusarium control. The strobilurin Kreoxim-methyl (Sovarn/BAS 490), a traditional compound, was found to control F. oxysporum on carnation and cyclamen (76). Azoxystrobin, developed from a compound originally isolated from mushrooms, is a broad spectrum reduced risk compound marketed as either Heritage (for ornamentals) or Quadris (for vegetables). Heritage is registered for control of Fusarium on more than 100 ornamental crops, including chrysanthemums (Chrysathemum spp.), carnation (Dianthus caryophyllus), geranium (Pelargonium spp.), and rose (Rosa spp.) (40, 76). This compound is reported to have equal or greater efficacy than traditional chemical compounds, such as benomyl, in Fusarium disease control on several ornamentals (76). Biological Control: In many cases, fungicides are not the preferred means for control of Fusarium, as efficacy is often poor (22, 61, 62, 72). One alternative to chemical fungicides is the use of biological controls (2, 22, 77). Biocontrols reduce disease levels by inhibition of the pathogen germination, growth, and reproduction. Inhibition can be either direct, through toxin production or nutrient competition, or indirect, by activation of a systemic resistance response in the host (2). Fungal, bacterial, and actinomycete biocontrols have all been able to effectively control Fusarium diseases in nursery systems. On cucumbers, root and stem rot caused by F. oxysporum was reduced by the fungi Gliocladium catenulatum (Prestop WP or Prestop Mix), Trichoderma harzianum and T. virens (SoilGard), the bacterium Pseudomonas chlororaphis strain 63-28, and the actinomycete Streptomyces griseoviridis (Mycostop) (77). Trichoderma virens was also effective in reducing root rot levels caused by F. oxysporum on gladiolus (63). On Cymbidium orchids, a weakly virulent Fusarium species, HPF-l, was able to induce a systemic resistance response in the host that suppressed leaf spot caused by F. proliferatum and F. subglutinans, and both bulb and root rot caused by F. oxysporum (45). The actinomycete Streptomyces kasugaensis, reduced Fusarium wilt of Cymbidium caused by F. oxysporum, by production of an macrolide toxin (47, 49). Integrated Control: The most effective control of fusarial diseases has frequently been achieved through the integration of several different control methods (13, 18, 22, 72, 73, 77). Effective control of F. oxysporum on greenhouse cucumbers was achieved through the integrated use of greenhouse composts as pathogen-suppressive potting media, and the application of both the chemical fungicide thiram, and the bacterial biocontrol, Pseudomonas chlororaphis strain 63-28 (77). On gladiolus, the application of carboxim together with Trichoderma virens, reduced disease levels more than either treatment alone (63). On carnation, cyclamen, and chrysanthemum, the most effective management of Fusarium wilt was achieved through the use of resistant varieties, steam sterilization of planting material, clean plant propagules, and fungicides, particularly benzimidazoles (28, 34). For effective control of Fusarium diseases on orchids, many different control methods may therefore need to be employed together. Overview of Objectives: The first objective of this research was to provide a comprehensive review of the of Fusarium diseases on orchids, the taxonomic considerations in determining the etiology of Fusarium orchid diseases, and possible measures available for control of Fusarium pathogens on orchids (Chapter 1). The second objective was to conduct a statewide survey to determine the fungi associated with new orchid diseases in Hawaii, their frequency and distribution, and their associations with symptoms and hosts (Chapter 2). The third objective was to determine the etiology of Fusarial diseases, by conducting pathogenicity tests on Dendrobium orchids, in fulfillment of Koch's postulates (Chapter 3). The fourth objective was to survey the incidence of Fusarial diseases in the field, which are caused by the established pathogens (Chapter 3). The fifth objective was to characterize the strain populations of the most common Fusarium orchid pathogen (Chapter 3). The sixth and final objective was to determine effective control measures for the Fusarium pathogens found, examining both biological controls and chemical fungicides (Chapter 4).
Description: Thesis (M.S.)--University of Hawaii at Manoa, 2007.
Includes bibliographical references (leaves 98-110).
ix, 110 leaves, bound ill. (some col.) 29 cm
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections:M.S. - Tropical Plant Pathology

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