PATHOGENICITY OF THE LEATHERLEAF FERN ANTHRACNOSE

FUNGUS, COLLETOTRICHUM ACUTATUM, ON OTHER

WILD AND CULTIVATED FERNS

J. O. Strandberg

Plant Pathology Research Report 2002-2E

(Originally published as Research Report SAN 96-05)

 

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SUMMARY -

The leatherleaf fern anthracnose pathogen, Colletotrichum acutatum, was found to infect and produce varying levels of disease damage on many, but not all, wild and cultivated ferns that were tested. These results indicate potential disease hazards for other cultivated fern species and identify possible sources of inoculum and opportunities for pathogen survival in wild ferns.

 

PURPOSE -

To help determine possible origins of disease, inoculum sources, means of persistence, and the potential impact of the leatherleaf anthracnose pathogen on other ferns grown for cut foliage, ornamental ferns, and wild fern species growing in central Florida.

 

METHODS AND MATERIALS -

Collection of fern plants - Various species of wild and cultivated ferns were collected and tested for susceptibility to the Anthracnose fungus in a controlled environment.

The following wild fern species (representative of species occurring in leatherleaf fern production regions) were collected in the Ocala National Forest near Altoona, FL, in July of 1995, and maintained in a warm greenhouse: resurrection fern (Polypodium polypodioides), netted chain fern (Woodwardia areolata), bracken fern (Pteridium aquilinum), royal fern (Osmunda regalis), and cinnamon fern (O. cinnamomea) were collected and tested.

The resurrection fern was collected and maintained on a small, decaying oak log. Bracken fern was transplanted into field soil (sand); the other species were transplanted into Fafard #2 peat/vermiculite horticultural growth medium contained in 6- or 10-inch plastic pots. Specimens of other fern species that are commonly cultivated as ornamental plants or for cut foliage were obtained from growers.

The following cultivated fern species were tested for susceptibility: button fern (Pellaea spp.), holly fern (Cyrtomium falcatum), rabbit's-foot fern (Davallia spp.), bird's-nest fern (Asplenium nidus), staghorn fern (Platycerium spp.), variegated East Indian holly fern (Arachniodes aristata cv `variegatum') Australian tree fern - (Alosophila copperi), Boston sword fern (Nephrolepis exaltata), and tuber sword fern (N. cordifolia). Cultivated ferns were re-potted into Fafard #2 growth media and maintained in the same greenhouse as the wild ferns.

Inoculation methods - Spores of C. acutatum were rinsed from a single 4-day-old PDA (potato glucose agar) lawn plate with 100 ml of distilled water dispensed from a wash bottle. The suspension was then diluted to provide 1 liter of spore suspension containing approximately 10⁶ spore/ml and applied to fern plants. For the first inoculation, the spore suspension was applied only to young fern fronds with a small artist's brush on August 15. Of the 5 wild species, only bracken and resurrection ferns had both old and young leaves when inoculated for the first time.

On August 25, all of the wild species had produced new leaves in various stages of development so they were inoculated again. However, this inoculation was carried out using a spore suspension washed from 7-day-old PDA lawn plates. The spore suspension was applied to all fern leaves with a spray bottle; fern plants were sprayed to wetness. Plants were maintained in the same greenhouse and periodically examined for disease damage.

Cultivated ferns were inoculated using methods similar to those used on wild ferns with the following modifications: In order to assure inoculation of young leaves (assuming that stage is most susceptible to the pathogen), spore suspensions were applied twice, on September 26 and again on October 3, using a spray bottle and a spore suspension prepared from a 4-day-old PDA lawn plate as described above. Since these plants were produced under commercial greenhouse conditions and growing rapidly, new leaves in various stages of development were present at inoculation.

 

RESULTS AND DISCUSSION -

Wild Fern Species -

Two days after the first inoculation, only the bracken fern showed leaf discoloration, water soaking, and browning on the tips of all but the oldest leaves. After 3 days, leaf tip lesions were numerous and were sporulating profusely. For resurrection fern, only the youngest leaves developed characteristic lesions on the leaf tips at 8 days after inoculation. These lesions also produced abundant spores, but leaf damage was minimal.

Three days after the second inoculation, all of the young bracken fern leaves showed extensive disease damage typical of leatherleaf fern anthracnose. To confirm pathogenicity, the pathogen was re-isolated from diseased tissue and used to inoculate leatherleaf fern (methods described above). All inoculated leatherleaf fern plants became infected within one week after inoculation using inoculation procedures already described.

On September 1, new foliage began to appear in the netted chain fern. These plants were inoculated again using the spray bottle method. The new leaves developed severe disease damage and the pathogen sporulated profusely on these lesions as confirmed by examination under the microscope on September 5.

When the royal fern, produced new leaves, similar spray inoculation methods were used to apply a spore suspension. These leaves also developed severe disease damage within 5 days after inoculation, and abundant spores were produced on these lesions. Following similar inoculations of young leaves, the cinnamon fern, Osmunda cinnomomea, developed only mild disease damage which appeared as a slight leaf tip-burn. However, these small lesions also produced abundant spores within 3-5 days after inoculation.

From these results, it was concluded that several wild fern species are susceptible to infection by the anthracnose fungus. Moreover, as for leatherleaf fern, infection of wild fern species occurs only on the young emerging fronds except on bracken fern where lesions do form on older, mature leaves. Providing that the youngest leaves are inoculated, infection and subsequent sporulation readily occurred. From this point, disease progress seemed to be related to growth habit. The pathogen devastates those species which produce a steady supply of new leaves underneath a canopy of older leaves. On fern species which slowly or sporadically produce new fronds, and which do not produce a dense canopy, disease did not spread further and seemed to eventually disappear.

Cultivated Fern Species -

On October 3, the cultivated ferns were checked for infection prior to the second inoculation. Holly, rabbit's-foot, bird's-nest, and Australian tree fern all showed severe disease damage on the youngest leaves and sporulation was occurring on diseased tissues. On October 10, all ferns were re-evaluated for damage and sporulation. A summary of the observations follows: button fern, minimal lesion formation on tips of youngest leaves; holly fern, youngest leaves sustained 40% damage; rabbit's-foot fern, youngest crosiers destroyed, acervuli production heavy, and extending down the petioles; bird's-nest fern, entire crown area severely damaged, young fronds infected as they emerge; Australian tree fern, youngest leaves and petioles wilted and collapsed, severe damage on tips of older leaves and eventually, the plants died,; staghorn, Boston sword, tuber sword, and East Indian holly fern were not susceptible (Table 1).

From these results, it was concluded that many fern species are likely to be susceptible to the leatherleaf fern anthracnose fungus. However, many other fern species have yet to be tested. Recently, the occurrence of serious anthracnose damage in two commercial plantings of holly fern grown for cut foliage has been documented. The fungus was isolated from holly fern and found to be identical to the leatherleaf fern pathogen. These occurrences were in the same region where leatherleaf fern production has been seriously damaged by leatherleaf fern anthracnose (Volusia County). However, in very limited surveys, the pathogen has not been found on any wild fern species or on cultivated fern species other than leatherleaf and holly fern. Disease damage symptoms that are similar to leatherleaf fern anthracnose were commonly observed on bracken fern in the wild. These symptoms were determined to be caused by disease incited by another Colletotrichum species, which was not pathogenic to leatherleaf or other ferns. The identification and importance of the bracken fern pathogen has not yet been determined.

REFERENCES

Strandberg, J. O. 1999. Pathogenicity of the fern anthracnose fungus, Colletotrichum acutatum, on wild and cultivated ferns in Florida. Proceedings of the Florida State Horticultural Society. 112:274-277

Strandberg, J. O., Stamps, R. H., and Norman, D. J. 1997.  Fern anthracnose; A guide for effective disease management. University of Florida, Agricultural Experiment Station, Technical Bulletin No. 900. Gainesville, FL. 78 p.

Revised 02/05/02