Epidemiology and Management of Fusarium Root, Crown, and Stem of Lisianthus (Eustoma grandiflorum)

R. J. McGovern¹, B. K. Harbaugh¹, and R. D. Berger², University of Florida, ¹Gulf Coast Research and Education Center, Bradenton, FL , ²Plant Pathology Department, Gainesville, FL

Industry Needs Addressed and Objectives

A destructive crown and stem rot caused by the fungus Fusarium avenaceum has become widespread in production areas devoted to both cut-flower and pot-flower production of lisianthus in Florida and California. In 1995-1996 losses of up to 30% occurred among Florida cut flower producers. During 1996-1997 we found that plant mortality varied by cv., planting date and transplant source, and ranged up to 70% in both California and Florida. (We estimated that the three Florida lisianthus growers surveyed sustained combined losses of about $150,000). Significant losses due to Fusarium crown and stem rot continued into the 1997-98 season at two of the cut flower sites in Florida but were generally lower than in the previous season. Our objective with this project initiated in 1997 was to develop an effective management strategy for Fusarium crown and stem rot based on obtaining data in the following areas:

1. Potential of F. avenaceum to survive in soil following the cropping season
2. Determination of contamination/infection of lisianthus seed by the pathogen
3. Identification of alternate hosts of F. avenaceum
4. Identification of sources of resistance among commercial lisianthus cvs.
5. Examination of primary and secondary spread of the pathogen at production sites
6. Evaluation of interactions between F. avenaceum, F. solani, and F. oxysporum
7. Evaluation of the efficacy of chemical and biological controls

Progress to Date

Progress on our research objectives has been made as follows:

1. Pathogen Survival.

   Using a bioassay of lisianthus ‘Maurine Blue’ we found that survival of F. avenaceum was very low in soil removed from two lisianthus production sites in Florida in 1997 with high incidences (>50% plant mortality) of crown and stem rot. We, therefore, feel that over-seasoning of the pathogen in soil plays a minimal role in new outbreaks of the disease in Florida. On the other hand, we detected the fungus in styrofoam and plastic transplant trays, and such contaminated trays may provide a source of infection for new transplants.

2. Seed Transmission.

   Fusarium avenaceum was not detected by culturing 1,000 seeds from each of 25 common commercial cvs. of lisianthus on a Fusarium-selective medium in 1997. Although a polymerase chain reaction (PCR)-based test specific for F. avenaceum gave a positive result when 100-200 seeds of lisianthus ‘Echo Blue’ were assayed, viable F. avenaceum could not be recovered when 16,000 seeds of the same seed lot were placed on the selective medium. In addition, 1000-2000 seeds of ‘Heidi Blue Rim’ and ‘Dwarf Mermaid White’ 500 tested negative by selective culturing and PCR for F. avenaceum. Likewise, c. 1000 seeds of ‘Heidi Rose Pink’ were negative for the fungus by PCR. Therefore, infection or contamination of lisianthus seeds by F. avenaceum does not provide an explanation for the prevalence of crown and stem rot during the past several years.

3. Alternate Hosts.

   We detected F. avenaceum following surface disinfestation, by isolation and PCR in the symptomless root systems of a broad range of unrelated ornamentals including wax begonia, carnation, exacum, gerbera, Madagascar periwinkle, marigold, pansy, petunia, rudbeckia, salvia, and verbena one month following inoculation with the fungus. The survival of the F. avenaceum in the root systems of cultivated plants and weeds, especially those prevalent near transplant houses, could be an important link in the epidemiology of crown and stem rot. (F. avenaceum has a broad host range including both dicots and monocots).

4. Sources of Resistance.

   Forty-seven cultivars (16 each of blue and pink and 15 white commercial cultivars) of lisianthus (Eustoma grandiflorum) were evaluated for their susceptibility to F. avenaceum in order to identify possible sources of resistance. The percentage of diseased plants 56 days after inoculation ranged from highs of 100% in ‘Mermaid Blue’, ‘Tiara White’, ‘Maurine White’, ‘Yodel White’, and ‘Malibu White’, and 98% in ‘Maurine Pink’ to lows of 22% in ‘Bridal Pink’, 25% in ‘Ventura Deep Blue’ and ‘Hallelujah Purple’, and 52% in ‘Heidi White’. While no cultivars were found that were totally resistant, these results indicated that breeding efforts might provide a useful strategy in the control of F. avenaceum.

5. Primary and Secondary Pathogen Spread Transplants.

   Fusarium avenaceum was consistently detected in lisianthus transplants from two California production sites during the 1997-98 growing season, and this appears to be the primary source of the fungus for recent outbreaks of crown and stem rot in Florida.

   Airborne Spread at Transplant Site.

   In order to determine if aerial dissemination of F. avenaceum occurs, a fungal spore trap was operated during the Fall of 1998 through the Spring of 1999 in a lisianthus transplant facility in California with a history of crown and stem rot. Fusarium avenaceum was recovered from the spore trap on only 3 days during the sampling period, indicating that a high level airborne contamination of lisianthus transplants was unlikely.

   Plant-Plant Spread at Cut Production Sites.

   During the 1997-98 growing season secondary spread of F. avenaceum was monitored biweekly in plantings of lisianthus ‘Echo Blue’ and ‘Mariachi Blue’ at two cut-flower production sites in West Central and Southwest Florida. Crown and stem rot was also monitored in ‘Flamenco Purple’ at the West Central site. Two methods of spatial analysis indicated that the pathogen appears to spread only over short distances to adjacent plants; disease foci increased in size but not in number.

   Fungus Gnat and Mechanical Transmission.

   In order to assess the potential of fungusgnats (Bradysia spp.), to transmit F. avenaceum, fungus gnat larvae were allowed to feed on weed seed infested with the fungus maintained in screened cages. Within 8 weeks fungus gnat adults had transmitted F. avenaceum to 34% of the seedlings of lisianthus ‘MaurinePink’; seedlings maintained in cages with either infested wheat seed or the fungus gnats alone remained disease-free. Since lisianthus are very susceptible to infestations of fungus gnats, we believe that control of the insect is essential for reducing outbreaks of F. avenaceum in lisianthus transplants. We also established that the fungus may be transmitted on pruning tools; a mode of spread that may detrimentally impact multiple harvests of cut-lisianthus.

6. Interactions Between Fusarium spp.

   Thus far all F. oxysporum isolates detected in association with F. avenaceum in lisianthus have not caused disease when this host was inoculated with the F. oxysporum isolates alone. However, in collaboration with Dr. Dick Morrison of Sakata Seed Co., we have identified a number of highly virulent isolates of F. oxysporum from lisianthus which will be used in mixed inoculations with F. avenaceum. We are also assembling a collection of Nectria haematococca (F. solani) for future testing in mixed inoculations with F. avenaceum.

7. Chemical and Biological Control.

   Fungicides tested against Fusarium crown and stem rot included ammonium bicarbonate, azoxystrobin (Heritage), copper sulfate pentahydrate (Phyton 27), fluazinam, fludioxonil (Medallion), iprodione (Chipco 26019), myclobutanil (Systhane), propiconazole (Banner), quaternary ammonium salts (Timsen), thiophanate methyl (Cleary’s 3336, Domain), thiophanate methyl + ethazole (Banrot), and a combination of thiophanate methyl with iprodione (Domain + Chipco 26019). Biological controls evaluated included Bacillus subtilis (Kodiak), Gliocladium virens (SoilGuard), Pseudomonas chlororaphis (AtEze), Streptomyces lydicus (Actinovate), and Trichoderma harzianum (Root Shield and T-22G). Heritage, Medallion, thiophanate alone (Domain, Cleary’s 3336), and the Domain + Chipco combination significantly increased plant survival, Systhane and Banner were generally less effective or inconsistent, and the biocontrols and the rest of the fungicides were ineffective. Heritage, Medallion, and a thiophanate methyl product (Cleary’s 3336, Domain) when used in rotation should provide useful components in the integrated management of Fusarium crown and stem rot in lisianthus.

Future Research

1. Insect Transmission.

   Research evaluating fungus gnat transmission of F. avenaceum to lisianthus will be continued to include spread of the fungus from infected to healthy plants. We will also study the potential of shore flies to transmit the fungus.

2. Interactions Between Fusarium spp.

   The potential synergistic effects of mixed infections of F. avenaceum, F. oxysporum, and Nectria haematococca (F. solani) will be studied.

   Benefits

   This work was justified by the lack of fundamental data on Fusarium crown and stem rot in lisianthus and the crop’s growing importance to the U.S. and worldwide florists’ crop market. During the course of this research we have worked with seed companies, transplant growers, and cut- and pot-flower producers. This research has enabled lisianthus growers to implement an integrated approach to managing crown and stem rot which is both effective and environmentally sound. As a result of this project there has been a dramatic decrease in the incidence of Fusarium crown and stem rot of lisianthus among California transplant producers and Florida and California cut-flower growers. In addition, the management techniques developed we have developed may have applications for the control of diseases of other florists’ crops caused by Fusarium spp.

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