Rumohra adiantiformis (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Rumohra adiantiformis (G. Forst.) Ching

Protologue: Sinensia 5: 70 (1934).

Family: Dryopteridaceae

Chromosome number: 2n= 82

Synonyms

Polystichum adiantiforme (G. Forst.) J. Sm. (1875), Dryopteris adiantiformis (G. Forst.) Kuntze (1898).

Vernacular names

• Leatherleaf fern, climbing shield fern (En).

Origin and geographic distribution

Originally, R. adiantiformis is a species of the southern hemisphere and is found in Papua New Guinea, Australia, New Zealand, Polynesia, South America, southern Africa, the Comoros and Mascarene Islands and Madagascar. It is cultivated all over the world with the United States and Costa Rica as the main producers.

Uses

R. adiantiformis is one of the most prominent sources of cut foliage. From the 1960s onwards it has been slowly displacing floral arrangements with Asparagus sp. as filler and background material. This change is due primarily to its better keeping quality. It is also sold as an ornamental for gardens and hanging baskets.

Production and international trade

Statistics on global production and trade of R. adiantiformis have not been compiled, but some national statistics include the species. In 1997 the planted area in the United States totalled about 1750 ha, with a production value of 60 million US$, mainly sold on the domestic market. In 1999, the production of the state Florida alone reached a total value of 62.6 million US$, an increase of 7% compared with the previous year. The increase of the sales value is at a lower rate than that of the production since the wholesale price per bunch of leaves has been declining over time. Costa Rica is a major producer with an estimated planted area of about 1000 ha managed by approximately 40 fern growers, dominated by 5 leading companies controlling the largest share of the export market. Many independent growers sell their product to larger companies for export to the main markets in the United States, the Caribbean, Canada, the European Union (The Netherlands, Germany, Norway, Denmark, France, United Kingdom) and Japan. The total Costa Rican export value was about 50 million US$ in 1995, with an annual increase of about 20%. Other Central American exporters include Honduras.

Entire fern plants are sold as landscape ground cover to subtropical areas and for starting cut foliage ferneries. These sales are of far less value than those of the leaves sold to florists.

Properties

Continuous contact with the leaves of R. adiantiformis might induce allergic contact dermatitis.

Description

A terrestrial and epiphytic fern with finely divided, plastic-like leaves. Rhizome long-creeping, (0.5-)1-1.5 cm in diameter, densely scaly; scales attached at the sinus or peltate, cordate, 6-16 mm × 2-5.5 mm, entire, denticulate or erose, acuminate, pale to dark brown. Leaves 3-pinnate-pinnatifid, monomorphous, about 20 mm apart; petiole 14-50 cm long, sulcate, with peltate scales near the base, light brown, decurrent on the rhizome; lamina broadly ovate to deltoid, 10-50 cm × 7-40 cm, the basal pinnae may be basiscopically more produced, the apex acuminate, lustrous bright to dark green above, paler and dull beneath; rachis with an entire central ridge and 2 adaxial grooves; veins free, simple or forked; pinnae approximate, petiolate, acuminate; ultimate segments oblong, crenate to bluntly lobate. Sori circular, 2 mm in diameter, usually one per lobe and black at maturity; indusium circular, peltate, entire, with a dark centre. Spores monolete, ellipsoid, 30-38 μm.

Other botanical information

The taxonomic position of the genus Rumohra Raddi has caused some discussion, chiefly as whether to include it either in the Davalliaceae , on the basis of the similarity of the rhizome and leaves, or in the Dryopteridaceae . The scales and round indusia, however, suggest affinity with Polystichum Roth and Arachnoides Blume, a viewpoint that is supported by the perispore structure. Some examples of cultivars are:

• "Davis": with slightly contracted pinnules, and

• "Underhill": characterized by its upright and outwardly arching plant habit; moderate vigor and rapid growth rate; dark burgundy-coloured rachis that is durable and strong; numerous pinnules per leaf that are medium green, glossy, with finely and deeply serrate margins and acute apices which give a fringed lacy appearance to the leaf; overlapping pinnules which give a full and dense appearance to the leaf that bears no spores.

Ecology

R. adiantiformis is particularly common in cooler, temperate areas. Depending on the latitude, the preferred altitude ranges from near sea-level to well over 2000 m. It grows on shaded mountain forest floors, in forest margins, scrub forest, or more rarely on more open stony sites. Usually it grows as an epiphyte, but it is also found as a terrestrial and, for example, on boulders, scree and humus-filled pockets in limestone ledges. In moist forests it can be found as an epiphyte on tree ferns, rotting trunks and logs. Commercially R. adiantiformis is grown under shade, predominantly on well-drained sandy soils with a low water- and nutrient-retaining capacity. It is tolerant of a wide range of soil pH and rarely visibly suffers from micro-element deficiencies. The maximum levels of photosynthetic active radiation (PAR) range from 470-670 μE/m².s or approximately 73% shade.

Propagation and planting

The propagation of R. adiantiformis is predominantly vegetative by rhizome cuttings. Site preparation prior to planting is simple: the land is cleared and the soil is rototilled. Terminal rhizome pieces 10-15 cm long are planted in 3 or 4 rows in a bed 1.2 m wide.

Husbandry

When grown commercially R. adiantiformis is shaded by trees or by artificial shading with polypropylene shade fabric. Weeds are a nuisance requiring extensive hand labour. Growth and productivity are reduced by water stress and therefore irrigation is often applied using overhead sprinklers. NPK fertilizer is applied, the amount and frequency depend on the type of soil. Typical application rates per ha in commercial cultivation in Florida (United States) are for N: 112-392 kg, P as P₂O₅: 134-168 kg and K as K₂O 112-392 kg. Lower nutrient application rates should be used for newly planted leatherleaf fern. Applications should not start until feeder roots start developing on the transplanted rhizomes. Nutrients should be applied in small amounts or in controlled or slow release forms to minimize leaching and other losses.

Diseases and pests

In the United States and Central America the most dangerous fungus in R. adiantiformis cultivation is Colletotrichum sp., which causes severe anthracnose. Symptoms consist of necrosis of the outermost portions of unfurling croziers. When the infected leaf grows and expands, it appears severely burned or scorched and cannot develop normally. Lesions may appear at or near the base of petioles. Under natural conditions the pathogen is incapable of rhizome infection and mature fern foliage does not appear to be susceptible either. The pathogen apparently spreads easily and the disease is very difficult to control once it becomes established. The spread of this pathogen into uninfected ferneries should be prevented because the yield of marketable leaves from infected areas may drop to nothing. The best strategy is to prevent movement of this pathogen into uninfected ferneries by limiting access and implementing strict decontamination procedures to delay or prevent infection, especially in isolated ferneries. Uninfected areas should not be visited after infected areas. Activity in the latter should be scheduled for the end of the work day. Infected areas should be well marked and avoided by personnel and vehicles except when applying fungicides or other disease management practices. It is most important to decontaminate personnel, equipment and vehicles when traveling between ferneries. Quaternary ammonium detergent-disinfectants mixed with water are recommended to inactivate anthracnose inoculum on tools and equipment, cloth and footwear. Products such as Galloway GX 1027 hand soap-disinfectant can be used for skin decontamination.

Other fungal pathogens are Cylindrocladium sp. and Rhizoctonia sp. which require preventive fungicide applications to control the fungi in the field. Cylindrocladium leaf spot causes reddish to greyish-brown spots that vary from pinpoint to 2.5 cm long. They can be water soaked and coalesce to encompass much of the leaf. The disease is most severe in periods of high temperatures. Rhizoctonia aerial blight causes dark brown to greyish spots all over the plants and sometimes covering entire leaves. The mycelium of the fungus frequently spreads up the petioles onto the leaf blades, especially in the moister centre of the plants. This disease is also correlated with higher temperatures. Many other cut foliage crops are hosts of Rhizoctonia spp. and the disease can readily spread from one crop to another. Plants treated year after year with a fungicide may develop optimum soil conditions around the roots for growth of certain bacteria that produce phytotoxic chemicals, resulting in distorted and off-colour fern leaves.

When the fernery has poor drainage and excessive rains or other excessive applications of water, Pythium root rot may affect the plants, which become greyish-green or chlorotic and may wilt. Roots are brown, mushy and stunted. Several fungicides are labelled for this disease on R. adiantiformis , but providing adequate drainage is a better way of preventing root rot.

The Florida fern caterpillar ( Callopistria floridensis ), the leatherleaf fern borer ( Undulambia polystichalis ) and leaf hoppers ( Eupterix spp.) are the most important insect pests. Currently, in addition to rotation to control the Florida fern caterpillar, growers depend heavily on diflubenzuron and Bacillus thuringiensis ( B t)-based products.

High temperatures may cause the frond curl syndrome (FCS), resulting in leaves wilting rapidly after harvesting through desiccation.

Harvesting

The leaves of R. adiantiformis are harvested with clippers and tied with rubber bands into bunches of 20-25 leaves.

Yield

In Florida (United States) the annual yield of R. adiantiformis averages over a million leaves per ha.

Handling after harvest

The leaf bunches of R. adiantiformis are dipped in or sprayed with water, packed in corrugated fibreboard cartons and stored or shipped at 4°C. Packed this way, the leaves may be stored for a month and still have a vase life of 1-3 weeks. Preventive application of fungicides may be beneficial prior to storage to control post-harvest decay. The leaves can be preserved with glycerine and may also be dyed various colours. No effective means of extending the vase life of good quality leaves has been found yet, though the durability of leaves with a short vase life may be increased by up to 75% using commercial dip treatments.

Genetic resources and breeding

Almost all R. adiantiformis grown for cut foliage is propagated vegetatively, but the origin of current cultured plant material is unclear. Germplasm collections are not known to exist, but may become increasingly important to select disease-resistant material. Relatively few cultivars are offered for sale.

Prospects

Opportunities exist for setting up ferneries for the production of cut foliage of R. adiantiformis for local and export markets, especially at higher altitudes since the frond curl syndrome might cause serious problems in warm tropical areas. Research is required to develop wilt and pest-resistant cultivars.

Literature

• Jones, D.L., 1998. Dryopteridaceae. In: Jones, D.L. & Clemesha, S.C. (Editors): Flora of Australia 48: Australian ferns and fern allies. 2nd Edition. Reed, Sydney, Australia. pp. 393-418.
• Leahze, R., Schubert, T., Strandberg, J., Stamps, R. & Norman, D., 1995. Anthracnose of leatherleaf fern. Plant Pathology Circular No 372. Florida Department of Agriculture and Consumer Services, Division of Plant Industry, United States.
• Milton, S.J. & Moll, E.J., 1988. Effects of harvesting on frond production of Rumohra adiantiformis (Pteridophyta: Aspidiaceae) in South Africa. Journal of Applied Ecology 25: 725-743.
• Stamps, R.H., 1992. Commercial leatherleaf fern culture in the United States of America. In: Fern horticulture: past, present and future perspectives. Proceedings of the international symposium on cultivation and propagation of pteridophytes, London, 7-11 July 1991. The British Pteridological Society, Intercept, Andover, United Kingdom. pp. 243-249.
• Stamps, R.H., 2001. Effects of postharvest dip treatments on leatherleaf fern (Rumohra adiantiformis) frond vase life. Acta Horticultura (ISHS) 543:299-303.
• Stamps, R.H., 2001. Irrigation and nutrient management practices for commercial leatherleaf fern production in Florida. Bulletin 300, first published 1995, revised June 2001. Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, United States.
• Stamps, R.H., Nell, T.A. & Cantliffe, D.J., 1989. Production temperature affects leatherleaf postharvest desiccation. HortScience 22: 261-264.
• Strandberg, J.O., Stamps, R.H. & Norman, D.J., 1997. Fern anthracnose - a guide for effective disease management. Florida Agricultural Experiment Station Technical Bulletin No 900.

Authors

W.P. de Winter