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

FINAL REPORT

Summary

Research funded by the American Floral Endowment was conducted in 1997 through 1999 on the etiology, epidemiology, and management of Fusarium crown and stem rot. We found that outbreaks of crown rot were strongly linked to infected transplants and worked closely with production facilities to remedy the situation. Our research indicated that F. avenaceum had poor survivability in Florida soils, did not appear to be seed-borne, but did persist for a number of months on plastic and Styrofoam transplant trays, was readily transmitted by fungus gnats, and aerially dispersed to a limited extent. We developed an integrated strategy for transplant growers to manage crown and stem rot, which includes routine use of effective fungicides (Heritage and Medallion), sanitation (avoidance of transplant tray reuse, and disinfestation of surfaces), and fungus gnat control. Outbreaks of Fusarium crown and stem rot were not reported during 1998-1999 or the current growing season.

Industry Needs Addressed and Objectives

A destructive crown and stem rot caused by the fungus Fusarium avenaceum became widespread in production areas devoted to both cut-flower and pot-flower production of lisianthus in Florida and California beginning in 1995. 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. (The three Florida lisianthus growers surveyed estimated combined losses of about $150,000 from Fusarium crown and stem rot). Significant losses due to the disease 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

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. (The Styrofoam trays were being used as a bench-top surface for replacement of diseased/weak lisianthus transplants at one transplant facility, and the plastic trays were being reused).

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. On the other hand, we were able to infect seed pods of lisianthus ‘Maurine Blue’ by applying spores of F. avenaceum to pistils of open flowers. Infected seed pods rotted and the necrosis expanded into stem below. However, no viable seeds were produced and the fungus could not be isolated from rotted seeds. 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 outbreaks of crown and stem rot in Florida and California.

   Airborne Spread at Transplant Site

   In order to determine if aerial dissemination of F. avenaceum occurs, a fungal spore trap was operated during November 1997 through March 1999 in a lisianthus transplant facility in California with a history of crown and stem rot. Fusarium avenaceum causing crown and stem rot in lisianthus was recovered from the spore trap on only 3 days during the initial two weeks of the sampling period. An explanation for the low recovery of F. avenaceum may be provided by the rigorous integrated management program instituted shortly after spore trapping began. Disease management measures consisted of disinfestation of transplant facilities, avoidance of reusing transplant trays, reduction of fungus gnat populations through more rigorous cultural and chemical control, and preventative application of effective fungicides. Our findings indicate that airborne spread of F. avenaceum may occur but plays a minor role in epidemics of crown and stem rot in lisianthus transplant facilities.

   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 fungus gnats (Bradysia spp.), to transmit F. avenaceum, fungus gnat larvae were allowed to feed on wheat 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 ‘Maurine pink’; 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 could 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 causing wilt in lisianthus. In addition, outbreaks of Fusarium wilt occurred in 1999 in two cut-flower facilities in California, and pathogenic F. oxysporum has been recovered. We have also assembled a collection of Nectria haematococca (F. solani) from Florida and elsewhere which causes root and crown rot in lisianthus. Pathogenic isolates of F. oxysporum and F. solani will be tested in mixed inoculations with F. avenaceum within the context of our current AFE project.

7. Chemical and Biological Control

   Fungicides tested against Fusarium crown and stem rot included azoxystrobin (Heritage), copper sulfate entahydrate (Phyton 27), fluazinam, fludioxonil (Medallion), iprodione (Chipco 26019), myclobutanil (Systhane), propiconazole (Banner), 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 applied every 3-4 weeks and used in rotation should provide useful components in the integrated management of Fusarium crown and stem rot in lisianthus.

Future Research

1. Interactions Between Fusarium spp. in lisianthus

   The potential synergistic effects of mixed infections of lisianthus by F. avenaceum, F. oxysporum, and Nectria haematococca (F. solani) will be studied within the context of our current AFE grant.

2. Integrated Management of Diseases of Potted Ornamentals Caused by Fusarium Species

   We are conducting research under a new grant funded by the AFE in conjunction with Dr. David Geiser at the Fusarium Research Center, Penn State University, and Dr. Wade Elmer at the Connecticut Agricultural Experiment Station. This four-year project has targeted the integrated management of disease caused by Fusarium species in caladium, chrysanthemum, cyclamen, and lisianthus.

   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 was 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. Outbreaks of the disease were not reported during the 1998-1999 season or the current season. In addition, the management techniques we have developed may have applications for the control of diseases of other florists’ crops caused by Fusarium spp.

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