Recent Progress in Postharvest Ethylene and Disease Control for Cut Flowers

Daryl Joyce (A), Dean Beasley (B), Andrew Macnish (B) and Melissa Taylor (B)

A Cranfield University at Silsoe, Silsoe, Bedfordshire MK45 4DT, UK

B The University of Queensland, Gatton College, Queensland 4345, Australia

Introduction:

In production terms, the ornamentals industry in the UK was worth around £655 ($1,060 million in 1997. This value equates to around 30% of UK horticulture production. Flowers and protected crops comprised about half (viz. £319 million) of this value. Among ornamentals, cut flowers in particular, have a large multiplier effect. Consider, for example, the activities of importers and florists.

Quality is a critically important issue in the cut flower trade. Consumers buy cut flowers on the basis of their aesthetic appeal. A sale will not be made if quality is poor. Moreover, in the event of disappointment, consumers may be prompted to buy a competing product, such as chocolates.

Many pre- and post-harvest factors are involved in determining and maintaining cut flower quality. Temperature management is perhaps the most important post-harvest factor. For example, temperature mediates responses to stresses like ethylene exposure or disease development. The closer that a particular cut flower species is kept to its optimum low storage temperature, the slower its rate of deterioration will be. However, it is not always possible to keep cut flowers at low temperature. For example, during air transport and while on display in retail and consumer environments.

This paper will focus on ethylene and disease, two significant problems that afflict cut flowers. Ethylene is a gas that causes rapid fading and wilting and leaf, bud and flower fall in cut flowers. Disease is typically the result of fungal infections that cause unsightly symptoms such as dry lesions and/or superficial mould. Ethylene and disease can act together. For instance, when disease develops on cut waxflower, affected flowers produce ethylene that causes the flowers to drop off.

Ethylene:

Ethylene gas is a remarkably simple but extremely potent regulator of plant growth and development. Each molecule is comprised of just 2 carbon and 4 hydrogen atoms, viz. C2H4. It can cause problems like flower fall at concentrations of <0.1 parts per million. Ethylene comes from non-living sources, such as exhaust fumes from engines, and from living sources, such as decaying plant tissue.

Ethylene gas binds to highly specific sites called receptors in plant tissues. Following binding and recognition, plant cell biochemistry changes. Senescence and abscission are the technical terms for flower fading and flower fall or drop, respectively. Both processes are accelerated by exposure of susceptible cut flowers to ethylene.

Problems associated with ethylene can be reduced or avoided by removing ethylene from the atmosphere, preventing the production ethylene by plant tissue, and/or preventing the binding of ethylene to plant tissue. Ethylene scrubbers are often based on the chemical potassium permanganate (KMnO4). This compound oxidises ethylene to produce water and carbon dioxide. However, scrubbers are not particularly effective if the air surrounding the cut flowers is not circulated through them or if the plant tissue itself is the source of ethylene.

Specific chemical inhibitors of ethylene production plant tissue have been identified, including aminovinyl glycine (AVG) and aminooxyacetic acid (AOA). Such ethylene production inhibitors have been formulated into solutions for treating cut flowers. However, they will not confer protection against ethylene already in the atmosphere.

The most robust strategy to prevent cut flower deterioration mediated by ethylene is to treat them with chemicals that inhibit ethylene binding to the tissue. Silver thiosulfate (STS) was developed to achieve this objective, and has proven to be a highly effective anti-ethylene treatment. STS is, however, falling from favour because the silver ion (Ag+) is regarded to be a heavy metal pollutant.

The quest for an environmentally sound alternative to STS has led to the discovery of 1-methylcyclopropene (1-MCP). Unlike AVG, AOA and STS solutions, 1-MCP is a gas. It has been proven in numerous recent studies that gassing cut flowers with 1-MCP at parts per billion concentration levels renders them completely insensitive to ethylene. Unfortunately, however, sensitivity to ethylene recovers over time. On the other hand, STS treated cut flowers tend to remain insensitive to ethylene indefinitely. Nevertheless, the 1-MCP treatment protocols developed to date can still provide protection for several days. For example, over periods of unrefrigerated air transport.

Disease:

Grey mould is the single-most common disease of cut flowers. It is caused by the fungal pathogen Botrytis cinerea. Spores of this pathogen are virtually everywhere. However, infection of plant tissue is usually only a major problem during wet or very humid conditions. Condensation formed on plants when temperatures fall at night is particularly problematical.

Botrytis cinerea can infect both healthy and dying plant tissues, such as developing rose petals or the spent anthers of waxflower flowers. These infections usually remain inactive until the cut flowers are harvested and start to deteriorate physiologically. The pathogen senses deterioration of the host and the latent or quiescent infection becomes active. Tan coloured lesions and superficial white and gray fungal matting are typical symptoms of grey mould disease.

Effective grey mould control involves the integration of good hygiene and environmental, chemical, physical and biological control measures. Good pre- and post-harvest hygiene, such as diligent removal and destruction of plant debris, reduces the infection pressure.

Possibilities for manipulation of environmental conditions within flower cartons have been explored in recent research. Condensation control packaging and flower cartons with strategically located ventilation holes constitute examples of post-harvest management of water relations with a view to minimise disease.

New fungicides, such as pyrimethanil, are being evaluated as chemical control agents for Botrytis cinerea. Novel chemicals are needed when strains of the pathogen develop resistance to conventional botryticides, such as benomyl. Chemicals that are not biocidal per se but which boost the natural disease resistance mechanisms within plant tissue are also being trialled. Salicylic acid and its derivatives are among these chemicals. Their action may be likened to immunisation of animals and humans against disease.

Physical methods of disease control include hot water dips, gamma irradiation and modified or controlled atmosphere (e.g. high carbon dioxide) packaging. To date these approaches have seen limited application, largely because of a concomitant risk of product damage.

Biological control of Botrytis cinerea involves the application of antagonistic microbes that compete with the pathogen for space and nutrients and/or produce antibiotics. The potential of this approach has been clearly established, although it is not yet practiced widely. Research workers have also investigated the possibility of using bees as vectors to deliver biocontrol agents to cut flowers prior to harvest.

Conclusion:

Novel solutions to ethylene and disease problems afflicting cut flowers are currently being developed and/or optimised. In view of an increasing public concern about heath and environment, sustainability of any new control measures is an important consideration. The use of 1-MCP for control of ethylene problems is likely to gain popular acceptance. It is considered both safe and effective. 1-MCP has already received Environmental Protection Agency approval for commercial use in the USA. Adoption of socio-environmentally acceptable disease control measures both pre- and post-harvest should achieve effective gray mold control on cut flowers. Combinations of such measures, as opposed to reliance on any one measure, is expected to control disease as effectively as use of traditional fungicides.