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

R. J. McGovern, Wade H. Elmer, David M. Geiser, and B. K. Harbaugh University of Florida, Gulf Coast REC, Bradenton, FL Connecticut Agric. Exp. Sta., New Haven, CT; Penn State University, University Park, PA

Industry Needs Addressed and Objectives

Fusarium species can cause common, persistent, and damaging diseases in most major potted ornamentals. Recently, severe outbreaks in lisianthus 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 resulted in significant losses in the US. The decline in caladium production over the past decade with the concomitant shortage of tubers for potted plants is strongly linked to a high incidence (>90%) of F. solani and possibly F. oxysporum in seed tubers. Fusarium wilt of cyclamen caused by 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 challenge for the worldwide production of florists’, chrysanthemum. Despite their importance, there has only been sporadic research on diseases caused by Fusarium spp. in potted ornamentals over the past three decades.

The objective of this long-term, interdisciplinary project is to generate useful information on the biology, epidemiology, genetics, detection, and integrated management of diseases of potted ornamentals caused by Fusarium. Model patho-systems to be examined include cyclamen (F. oxysporum), caladium and chrysanthemum (F. solani and F. oxysporum), and lisianthus (F. avenaceum, F. solani, and F. oxysporum).

Progress Report for FY 1999-2000

Progress on our research objectives has been made as follows:

1. Biology and Epidemiology of Fusarium

   Knowledge of pathogen survival, spread, and the conditions which favor infection is the starting point for effective disease management. We have gained such useful data on the following diseases:

   a.) Fusarium Wilt (F. oxysporum) and Fusarium Root and Crown Rot (F. solani) in Lisianthus. A series of experiments studied the effects of temperature and inoculum concentration (spore number) on infection and symptom production in lisianthus using both California and Florida isolates of F. oxysporum. Root rot and wilt symptoms occurred over a range of temperatures and inoculum concentrations but were most rapid after inoculation of plants with 1.0 x 10⁶ microconidia/ml at 29^o C (84.2^o F). A number of the Florida isolates of F. oxysporum from lisianthus were associated with fungus gnats (Bradysia sp.) suggesting the existence of a vector relationship similar to that which we previously observed between F. avenaceum and fungus gnats. Florida isolates of F. solani caused the most severe root and crown rot symptoms in lisianthus at 34^o C (93.2^o C).

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.) Morphological Identification and Genetic Characterization of Fusarium from Caladium and Lisianthus. One hundred and seventeen isolates of F. solani and F. oxysporum were obtained from caladium, and 38 isolates of F. avenaceum were obtained from lisianthus. Microscopic examinations were performed to provide a correct species identification, and notes were made about unusual morphological characteristics. Most strains were fairly typical of the morpho-species F. solani and oxysporum (caladium), and F. avenaceum (lisianthus) but many were found that had unusual characteristics. A culture of each strain has been lyophilized and stored at -40C at the Fusarium Research Center. Genomic DNA was extracted from each isolate, and stored at –20C for future analysis.

   b.) F. avenaceum Vegetative Compatibility Studies. To date, 46 isolates of F. avenaceum have been collected from diseased lisianthus plants from diverse area in the US. Of these 20 have been placed into six vegetative compatibility groups. The remaining isolates have not been assigned to any group and may be single members of their own VCG. Given the high degree of variability in vegetative compatibility, additional molecularly-based assays are warranted to assess the genetic diversity.

3. Integrated Management of Fusarium

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

   a.) Fusarium Wilt in Cyclamen. Chloride and soil pH. Chloride nutrition has proven useful in the suppression of Fusarium diseases on many crop that have tolerance to Cl. A single application 0.25-0.5g of NaCl/L soil applied to cyclamen plugs grown in soil infested with Fusarium oxysporum decreased mortality and increased fresh weight and leaf area. The most noticeable effect was its ability to postpone the onset of wilt symptoms and delay disease severity. Plant tissue analyses revealed elevated levels of Na, Cl, and Mn . Since Mn is associated with defense mechanism in plant tissue, this may be one mechanism by which NaCl suppresses Fusarium wilt. Adding lime to a potting mix to raise to pH from 6.5 to 7.5 did not result in any significant effect on disease, plant weight, or flower number. Also, no significant differences in growth or disease were noted when the pH of medium was lowered to 5.5 with sulfuric acid. When NaCl was combined with the different pH treatment, NaCl improved growth, but the greatest benefit was seen at a pH of 7.5 pH. This may be due to the NaCl increasing Mn and other trace element at the higher pH.

   Induced resistance. In the last decade, a few products have been discovered that induce a type of plant resistance which has been termed systemic acquired resistance (SAR). The effects of different rates of two SAR products, salicylic acid, and its synthetic analog, benzothiadiazole (Actigard®) were tested on cyclamen grown in soil with and without F. oxysporum. Healthy cyclamen plugs were soaked in solutions of benzothiadiazole at levels of 1, 5, or 10 µg/ml or salicylic acid at 10, 50, or 100 µg/ml prior inoculation with the pathogen. In the absence of the pathogen, all treatments slightly reduced plant growth when compared to the controls. However, in the presence of F. oxysporum application of benzothiadiazole at 5 µg/g significantly increased growth and flower number. Disease severity and vascular discoloration in the corm was also reduced on plants treated with benzothiadiazole when compared to untreated plants. Salicyclic acid had only marginal affects on growth and disease when compared to controls.

   Biological Control. Greenhouse experiments were conducted to examine biocontrols including three different strains of nonpathogenic F. oxysporum (FO47 from France, 618-12 from the Netherlands, and CS-20 from the US) combined with and without Pseudomonas fluoresens for their ability to suppress Fusarium wilt. Healthy cyclamens were exposed to the nonpathogenic Fusarium strains combined with and without P. fluorescens and planted in soil infested with the pathogen. All three strains, but not P. fluorescens reduced disease severity, with FO47 giving the best suppression.

   Fusarium Tuber Rot in Caladium Seed Tuber Soaks

   Three experiments were conducted to evaluate the effect of fungicide soaks on Fusarium infection in caladium seed tubers. Fungicides that were tested included azoxystrobin (Heritage), fludioxonil (Medallion), thiophanate methyl (3336 F), thiophanate methyl+chlorothalonil (Spectro 90 WDG), an ammonium bicarbonate formulation (Armicarb 300), ammonium bicarbonate, and ammonium sulfate. All were used in combination with a standard hot water treatment (50⁰C/30 min). Fusarium incidence in hot water-treated and nontreated tubers exceeded 90%. Fusarium incidence was significantly reduced compared to both the nontreated control and hot water alone by thiophanate methyl+chlorothalonil and ammonium sulfate. The severity of Fusarium infection was significantly reduced by 3336 F, azoxystrobin, thiophanate methyl+chlorothalonil, ammonium bicarbonate, and ammonium sulfate.

   Macroexplant Soaks. An experiment was conducted combining a rapid propagationtechnique for caladiums developed by J. E. Polston at the Gulf Coast REC called macroexplant propagation with prolonged (16 hour) soaks in thiophanate methyl, thiophanate methyl+chlorothalonil, or azoxystrobin. All fungicides significantly increased survival by 35-55% compared to the nontreated controls.

Projected Research for FY 2000-2001

The following research is planned for FY 2000-2001:

1. Biology and Epidemiology of Fusarium
   • Inoculation methodologies will be developed for F. solani in lisianthus.
   • The infection process by F. solani and F. oxysporum in caladium will be studied using tissue cultured plants.
   • Experiments will be conducted on the affect of temperature on Fusarium wilt in cyclamen.
2. Genetic Characterization and Detection of Fusarium
   • Genetic analyses of isolates of F. avenaceum from lisianthus will study correlations between DNA sequences, VCG grouping, and pathogenicity, and will examine their relatedness to F. avenaceum from other hosts.
   • Genetic analyses including VCG typing of F. oxysporum from caladium and lisianthus will enable the development of molecular identification systems and epidemiological studies.
3. Integrated Management of Fusarium
   • Fusarium Wilt in Cyclamen. Additional studies will be conducted to optimize application methodologies and rates for benzothiadiazole.
   • Fusarium Wilt in Lisianthus. Experiments will be conducted to evaluate the effectiveness of biocontrols and fungicides in suppressing the disease. Evaluation of cultivar resistance to F. oxysporum will be initiated.
   • Fusarium Tuber Rot in Caladium. Tissue cultured caladium plants donated by Twyford Laboratories will be used to establish motherblocks for future experiments. Fifteen common caladium cvs. will be screened for resistance to Fusarium using tissue-cultured plants. Additional experiments will be conducted which combine macroexplant propagation and fungicide soaks.

Benefits

This work is justified by the lack of organized and recent research on Fusarium spp. 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 spp.

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