Etiology and control of fusarial orchid diseases in Hawaii

Thesis (M.S.)--University of Hawaii at Manoa, 2007.Includes bibliographical references (leaves 98-110).ix, 110 leaves, bound ill. (some col.) 29 cmOverall, 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)

Improving Containerized Nursery Crop Sustainability: Effects of Conservation-driven Adaptations in Soilless Substrate and Water Use on Plant Growth and Soil-borne Disease Development

Containerized crop production faces increasing sustainability challenges with both soilless substrate and water use. To facilitate use of sustainable practices, we evaluated plant health impacts of two substrates, bark and wood fiber, which we contrasted with peat, a substrate that is slower to renew; this was overlaid with an analysis of the effects of water-saving-targeted irrigation reductions, compared with typical well-watered conditions. Health impacts were evaluated in two crops, considering both physiological and disease impacts for tomato with and without Phytophthora capsici, and chrysanthemum with and without Phytopythium helicoides. Substrate type was a strong determinant of plant health, wherein crops grown in a HydraFiber-peat mix (“fiber”) performed worse than those in bark and peat, with up to a 50% and 45% reduction in shoot biomass in tomato and chrysanthemum, respectively (P < 0.001). Tomato decline incidence from P. capsici was 3-6 times higher in fiber than other substrates, and fiber was the only substrate where the effect of P. capsici enhanced decline and rot development compared with noninoculated plants (P < 0.05). In bark, reduced irrigation consistently inhibited tomato and chrysanthemum growth and shoot water content (typically P < 0.001). In peat, whereas tomato growth was inhibited under reduced irrigation (P 5 0.012-0.013), chrysanthemum growth was often unaffected. Growth in fiber was uniformly poor regardless of irrigation regime for both crops, and an irrigation treatment effect was not typically apparent. Reduced irrigation enhanced pathogen effects in fiber and peat for tomato and fiber and bark for chrysanthemum (P < 0.05). This is perhaps the first study to evaluate HydraFiber interactions with disease and reduced irrigation and suggests that this product consistently incurs costs to crop productivity. However, the peat-replacing bark substrate has strong potential to optimize plant growth physiologically and via disease suppression and can be used under reduced irrigation without compromising economic productivity of the system

Monitoring for a new I3 resistance gene-breaking race of F. oxysporum f. sp. lycopersici (Fusarium wilt) in California processing tomatoes following recent widespread adoption of resistant (F3) cultivars: Challenges with race 3 and 4 differentiation methods

Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici (Fol), causes losses in tomato production worldwide, with major impacts on Californian tomato processing. Single-gene resistance is the primary management tool, but its efficacy has been compromised following the emergence of two successive resistance-breaking races, which, in California, emerged within 12 years of resistance deployment. Fol race 3-resistant (F3) processing tomato cultivars (containing the I3 resistance gene) were deployed in the state starting in approximately 2009. The emergence of a new resistance-breaking race (which would be called race 4) is imminent, and early detection will be critical to delay the spread while new resistance is sought. The detection of Fol race 4 is challenged by the lack of validated, rapid, and accurate diagnostic tools. In evaluating in planta phenotyping methods, this study found that rapid seedling phenotyping is not reliable and generates false positives for nonpathogens. Longer (10 weeks) mature plant assays are the most reliable, but may not be sufficiently timely. As an additional challenge, based on field and greenhouse studies, Fol race 3 can cause symptoms in resistant F3 cultivars at frequencies greater (30%) than expected for off-types (<2%). We developed a three-F3 cultivar in planta assay to overcome the challenges this posed to differentiating Fol race 3 and Fol race 4. Using the assay, we determined that all putative resistance-breaking cases were Fol race 3; Fol race 4 was not detected in these early survey efforts. These results highlight the need for developing rapid Fol race 4 detection tools and a better understanding of the factors underlying inconsistent I3 gene expression in Fol race 3

Phylogenomic analysis of a 55.1 kb 19-gene dataset resolves a monophyletic Fusarium that includes the Fusarium solani species complex

International audienceScientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. Previously (Geiser et al. 2013; Phytopathology 103:400-408. 2013), the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani Species Complex (FSSC). Subsequently, this concept was challenged by one research group (Lombard et al. 2015 Studies in Mycology 80: 189-245) who proposed dividing Fusarium into seven genera, including the FSSC as the genus Neocosmospora, with subsequent justification based on claims that the Geiser et al. (2013) concept of Fusarium is polyphyletic (Sandoval-Denis et al. 2018; Persoonia 41:109-129). Here we test this claim, and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species recently described as Neocosmospora were recombined in Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural and practical taxonomic option available