BIOLOGY, EPIDEMIOLOGY, AND INTEGRATED MANAGEMENT OF DISEASES CAUSED BY FUSARIUM IN POTTED ORNAMENTALS

R. J. McGovern, W. H. Elmer, D. M. Geiser, and B. K. Harbaugh

INDUSTRY NEEDS ADDRESSED AND OBJECTIVES

Fusarium species cause common, persistent, and damaging diseases in most major potted and field-grown ornamentals. Severe outbreaks of Fusarium crown and stem rot, Fusarium wilt, and Fusarium root and crown rot caused, respectively, by F. avenaceum, F. oxysporum, and F. solani have caused extensive losses in lisianthus in the US. A decline in caladium production over the past decade that has led to a shortage of tubers for use as potted plants is strongly linked to a high incidence (>90%) of F. solani and ossibly F. oxysporum. Fusarium wilt of cyclamen (F. oxysporum) is highly destructive and economically limiting to the production of quality cyclamens, and its incidence has steadily increased in production facilities across the US. Root rot and vascular wilt caused by F. oxysporum also represents a continuing threat for the production of chrysanthemum. Despite their importance, only sporadic research on diseases caused by Fusarium has recently occurred.

Objective:

This multi-year, interdisciplinary project will generate useful information on the biology, epidemiology, genetics, detection, and integrated management of diseases of potted ornamentals caused by Fusarium. Model pathosystems to be examined include caladium (F. solani and F. oxysporum), chrysanthemum (F. oxysporum), cyclamen (F. oxysporum), and lisianthus (F. avenaceum, F. solani, and F. oxysporum).

PROGRESS REPORT FOR FY 2000-2001

1. Biology and Epidemiology of Fusarium

   Knowledge of pathogen survival, spread (including insects and other vectors), and the conditions that favor infection is the starting point for effective disease management. We have gained such useful information on the following diseases:

   a.) Tuber Rot (F. solani, F. oxysporum) in Caladium. Because infection of commercial caladium tubers by Fusarium spp. is widespread, we obtained tissue-cultured plantlets to establish a source of pathogen-free tubers for research. Fusarium solani was initially identified as the causal agent of tuber rot in caladium (Knauss.1975. Plant Dis. Reptr. 59:975-979). However, of 118 Fusarium isolates obtained from rotted tubers, 53 are F. oxysporum and the remainder F. solani. Pathogenicity of three isolates each of F. oxysporum and F. solani was studied using tissue-cultured tuber segments. Two isolates of F. solani, and one F. oxysporum caused tuber rot, while the othersappeared to be non-pathogens. We hope that this bioassay will be useful in identifyingFusarium resistance in caladium, and in elucidating the role played by F. oxysporum in caladium tuber rot.

   b.) Fusarium Wilt (F. oxysporum) in Cyclamen. Fungus gnats (Bradysia sp.) and shore flies Scatella sp.) have been suspected vectors of Fusarium wilt of cyclamen, butdata has not been produced that confirms their role in the spread of this disease. Ongoing studies using caged plants have been designed to determine if fungus gnats andshore flies can spread Fusarium wilt of cyclamen. Thus far, 75% of fungus gnats fromthe cage with wilted cyclamens were infested by Fusarium spp. compared to 14% of the insects from the “healthy” controls. These studies will be continued and repeated.

   c.) Fusarium Wilt (F. oxysporum) in Lisianthus. Plant age did not greatly affect thesusceptibility of lisianthus to Fusarium wilt caused F. oxysporum when tested in four experiments. Separate experiments indicated that saturated soil conditions appear to favor Fusarium wilt in lisianthus, and that Florida isolates of the fungus are slightly pathogenic, unlike the highly virulent California isolates used in our other lisianthus research.

   d.) Fusarium Crown and Stem Rot (F. avenaceum) in Lisianthus. Our previous research demonstrated that fungus gnats could transmit F. avenaceum to lisianthus, resulting incrown and stem rot. An additional experiment was carried out to confirm that fungusgnats were carrying the pathogen and not merely causing infection sites. An apparatuswas designed to allow emerging adult fungus gnats to move from a cage with F. avenaceum-infested soil to a separate cage with an exposed petri dishes containing a Fusarium-selective medium. F. avenaceum was isolated from fungus gnats exposed to the fungus but not from controls (exposed plates without fungus gnats). These results confirm that fungus gnats are active vectors of F. avenaceum.

2. Genetic Characterization and Detection of Fusarium

   Although Fusarium is commonly isolated from plant tissue, it may or may not be involved with a specific disease. Genetic characterization of Fusarium through DNA sequencing and determination of vegetative compatibility vegetative compatibility grouping (VCG) will allow for identification and detection of pathogenic isolates and aid in understanding their spread within and between countries. This work will also contribute to the much-needed taxonomic revision of the genus Fusarium.

   a.) Genetic Characterization of Fusarium avenaceum from Lisianthus. We have determined the DNA sequence of a specific gene (beta-tublin elongation factor) in 18 isolates of F.avenaceum from lisianthus, 25 from other hosts, and 10 other related Fusarium species. (This gene has proven useful for genetic analyses in a variety of other fungi). Our findings, along with VCG and pathogenicity studies, suggest that isolates of F. avenaceum represent one lineage, and that most, regardless of original host, may cause Fusarium crown and stem rot inlisianthus.

   b.) Genetic Characterization of Fusarium oxysporum and Fusarium solani from caladium. We have begun analyzing 65 isolates of F. solani and 53 isolates of F. oxysporum from caladium.Techniques employed to examine for host specificity in F. solani include sequencing portions of the ITS region, translation elongation factor, and calmodulin genes. We are looking at regions of he mitochondrial small subunit and translation elongation factor genesto analyze of F. oxysporum.

3. Integrated Management of Fusarium

   Our goal is to develop an integrated approach to management of diseases caused byFusarium spp. in the targeted crops through evaluation of cultivar resistance, induced or acquired resistance, biological and chemical control, and cultural practices.

   a.) Fusarium Tuber Rot in Caladium (F. solani, F. oxysporum)Macroexplant Propagation. We evaluated the combination of fungicide soaks and macroexplant propagation (developed previously) Heat-treated (122^oF/50^oC for ¬? hr.), commercial tubers of ¬ëFrieda Hemple¬í were soaked at ambient temperature for16 hrs. in nine different fungicides. Macroexplants obtained from these tubers and non-treated controls were soaked for 16 hrs. in the same fungicides. Eight of the fungicides, Armicarb 300, Captan, Cleary¬ís 3336 F, Heritage, Mertect 340 F, Prochloraz Mn 50 WP, sodium hypochlorite, Spectro 90 WDG, and hot water alone increased tuber viability compared to the non-treated control. All of these fungicides except for Armicarb 300 and Mertect 340 F also increased macroexplant production compared to no treatment or hot water.

   Fungicide Evaluation. A pot study and a field experiment at a commercial production site evaluated the use of tuber soaks and/or soil drenches in increasing potted plant biomass and tuber yields, and in decreasing tuber rot. Heritage applied as a 1 hr. tuber soak, and follow-up soil drench increased shoot weight in potted ‘Pink Beauty’ and ‘Frieda Hemple’ over non-treated controls, while Hurricane (Medallion and Subdue Maxx) increased shoot weight in ‘Pink Beauty’. Sixteen hr. soaks of commercial seed tubers of caladium ‘Florida Cardinal’ in Heritage, Spectro 90 WDG (thiophanate methyl + chlorothalonil), or Hurricane, and an at-planting spray of Hurricane significantly increased tuber yield in field-grown caladiums by 17-23%. Both application methods of Hurricane decreased tuber rot in harvested tubers.

   b.) Fusarium Wilt in Cyclamen (F. oxysporum). Effects of NaCl and Soil pH. A series of experiments examined the effect of low levels of NaCl (0.25-0.5g/1000cc potting soil) and soil pH (5.5-7.5) on Fusarium wilt in cyclamen. NaCl increased plant growth and reduced the rate of wilt development but not its fi al severity compared with non-treated controls. Varying soil pH (5.5- 7.5) had no effect on disease or the effectiveness of NaCl. On the other hand cyclamens grown in soils with a pH of 7.5 had chlorotic leaves, but the addition of NaCl corrected the symptom.

   Induced Resistance,Biocontrols,Fungicides. A series of three experiments investigated the ability of salicylic acid, chitosan, and benzothiadiazole (Actiguard) to induce disease resistance to Fusarium wilt in cyclamen. Salicylic acid and chitosan were ineffective while both soil drenches and foliar sprays of benzothiadiazole were effective. However, drench application of benzothiadiazole appears to be phytotoxic. Two experiments indicated that three non-pathogenic isolates of F. oxysporum were ineffective in suppressing Fusarium wilt whether used alone or in combination with the bacterium Pseudomonas fluorescens. An additional experiment evaluated commercially available fungicides and biocontrol products. The biocontrols Actinovate-Fe, Actinovate Plus/M, Companion, Deny, RoootShield + PlantShield, and Mycostop, and the fungicides Banrot, Cleary’s 3336, Medallion, and Terraguard were equally effective in suppressing the rate Fusarium wilt. But Medallion appeared to be phytotoxic to cyclamen. The biocontrols SoilGard and Compete were ineffective.

   c.) Fusarium Wilt (F. oxysporum) and Fusarium Root and Crown Rot (F. solani) in Lisianthus.
   Biocontrols and Fungicides. In five experiments we evaluated the majority of commercially available biocontrol products and fungicides as well as a number of experimental products alone and in combination for reduction of Fusarium wilt in lisianthus. Among the fungicides, Medallion and Terraguard were the most effective in reducing plant mortality, Compass, Cleary’s 3336 F, Heritage, and Systhane were of intermediate efficacy, and Spectro 90 WDG was ineffective. The biocontrol PGA+ significantly reduced the rate of plant death and final mortality, RootShield, and two experimental biocontrols, LS 213 and CS 20 reduced final mortality. Other biocontrol products including Actinovate Plus/Fe, Actinovate Plus/Humate, AtEze, Bac-Pac-Ag, Companion, Deny, Fusaclean, Mycostop, and SoilGard were ineffective, while Actinovate Plus/M and Serenade increased the disease.

   Cultivar Resistance. Fifty-five cut-flower cultivars of Eustoma grandiflorum are being evaluated for susceptibility to F. oxysporum. Seventeen dwarf, bedding plant, cultivars of Eustoma grandiflorum are being evaluated for susceptibility to F. solani. Progress to date indicates a range in percentage dead plants with a low of 0% dead plants for ‘Mermaid Pink’ and ‘Sapphire Blue’ to a high of 32% dead plants for ‘Lisa Blue’.

RATIONALE FOR CONTINUED FUNDING

Fusarium species cause some of the most common, difficult-to-control, and economically important diseases in potted and field-grown ornamental in the US and world-wide. With the exception of our research group currently funded by the AFE, there has been little recent and no organized research on the management of this pathogen in ornamentals. Our interdisciplinary group is developing practical short- and long-term integrated strategies to manage Fusarium.

PROJECTED RESEARCH FOR FY 2000-2001

1. Biology and Epidemiology of Fusarium
   • Determine pathogenicity of all collected Fusarium isolates from caladium
   • Evaluate survival of Fusarium spp. on caladium cutting devices and containers
   • Study potential aerial dissemination of F. oxysporum to cyclamen
   • Evaluate ability of fungus gnats and shore flies to vector F. oxysporum to lisianthus and cyclamen
   • Determine the host ranges of F. oxysporum from lisianthus and F. solani and F. oxysporum from caladium.
2. Genetic Characterization and Detection of Fusarium
   • Complete genetic analysis of F. avenaceum from lisianthus using beta-tubulin sequences and AFLP fingerprints
   • Continue genetic analyses of F. oxysporum and F. solani from caladium to develop specific detection methodologies
3. Integrated Management of Fusarium
   • Confirm effectiveness of fungicides against Fusarium tuber rot in caladium through large-scale studies at commercial sites
   • Evaluate the integration of fungicides and biocontrols, and benzothiodiazole and NaCl for management of Fusarium wilt in cyclamen and chrysanthemum
   • Continue screening for resistance to Fusarium spp. in caladium and lisianthus
   • Optimize fungicide application methodologies for treating caladium tubers

BENEFITS

This work is justified by the lack of organized and recent research on Fusarium species in potted and field-grown ornamentals, and by the national and worldwide importance of this pathogen group. We anticipate that fundamental and practical knowledge on Fusarium will be gained from this project that can be applied to a wide range of ornamentals. The benefits to the industry will include more precise, safe, and effective methods for diagnosing and managing diseases caused by Fusarium species.

PLAN FOR DISSEMINATION OF RESULTS

The findings of our project will be disseminated to the ornamental industry through trade publications such as Grower Talks, Greenhouse Grower, and Greenhouse Product News, through extension publications, websites, and by presentations and written proceedings at grower meetings. Our research results will also be disseminated to colleagues through appropriate journals such as Plant Disease, Plant Health Progress, HortScience, and HortTechnology.

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