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Vurdering af muligheder for forebyggelse og alternativ bekæmpelse i planteskoler

Summary

Nursery production

Nurseries produce plants for forests, landscapes, shelter belts, parks, fruit growing and gardens. Nursery stock production in Denmark is highly diversified and covers a large number of species (about 300) and varieties. Besides the many species and varieties, nursery production is characterised as being specialised and relatively long-term. It includes either field-grown or container-grown crops, and in some cases both. According to Statistics Denmark, nursery stock production in 1999 covered an area of 2,789 ha distributed among 206 nurseries (nurseries under 2 ha were not included in the survey). This area comprises 195 ha of container-grown plants and a greenhouse area of 20.8 ha. Greenhouses are mainly used for propagation, climate protection, and storage of containerised plants during winter. The area of container-grown plants in Denmark is probably underestimated, as many small nurseries producing containerised plants were not included in the survey. The area used for container-grown plants has therefore been increased to 210 ha in the table below.
The area classification shown in the table below is based on a qualified estimate made by consultant Bent Leonhard, DPF (Danish Nursery Association).

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Danish seed sources and varieties suited to the Danish climate are used to a large extent, as hardiness and climate adaptation are important elements in nursery production. Phytosanitary measures are also crucial for the production and export of nursery stock as regulations stipulate zero-tolerance of any insect pests for some products and acceptance of only a small number of insect pests for others.

In 1998, DPF and DIAS, Aarslev, jointly prepared a basic concept for the integrated production (IP) of nursery stock. However, specific regulations for IP were not drawn up as another environmental control system (MPS) was launched at the same time, providing Danish nurseries with the opportunity of a trial arrangement. As a result, a small number of nurseries have joined MPS (nine in 2001). Only a very small part of the nursery area is cultivated in accordance with EU regulations for organic production.

Pesticide consumption and environmental effects

Herbicide consumption in nurseries during the period 1996-99 was 4,400-5,100 kg active ingredient annually, corresponding to an average treatment frequency of 0.7-1.4.
Insecticide consumption in nurseries during the period 1996-99 was 535-985 kg active ingredient annually, corresponding to an average treatment frequency of 0.4-1.3.
In the same period, annual fungicide consumption was 5,200-10,000 kg active ingredient, corresponding to an average treatment frequency of 1.1-2.2. In particularly demanding crops, treatment frequency can be as high as 8–10. In other crops, pesticide consumption is very low. The use of soil disinfectants was permitted during part of the period in question.
Crops that require much pesticide treatment pose a risk of percolation or run-off to ground and surface water. Containerised crops in which the ground is covered with plastic sheeting or the like pose a particular risk when pesticides are applied broadly over the entire area rather than being applied to the containers.
It is difficult to estimate the effect of pesticides on the flora and fauna, as it is hard to assess the natural value of nurseries because of their "alienness" and location. Areas used for the cultivation of containerised plants are not expected to have any natural value. No literature describing the natural flora and fauna of nurseries has been found. We have assumed that effects on flora and fauna are comparable with those found in row-grown vegetable crops where natural life forms are very restricted by the use of pesticides.

Alternative methods

The present review of alternative methods to prevent and control weeds, diseases, and insect pests examines field production and container production (including greenhouse production) separately. Descriptions and assessments of alternative methods are based mainly on published material, but are supplemented by practical experience. Only the alternative methods with the greatest potential based on present knowledge are described. Methods have not been prioritised, as several of the methods cannot be used alone but must be integrated with one another and with current cultivation techniques. Finally, nursery production is so variable that only few methods can be used universally - methods must be adapted to suit specific purposes.

Alternative methods of weed, disease and insect-pest control, forecasting/warning systems, and pesticide application techniques are reviewed below (A-F).

A1. Weed control in field-grown crops

Weeds are an enormous problem in nurseries, where they compete with the crop plants for water, nutrients and light. Most woody crop plants grow slowly and therefore compete extremely poorly with weeds. In addition, weeds impede crop lifting. Furthermore, the long-term crop rotation, where plants remain in the same spot for several years, makes weed control more difficult.
The diversified production in nurseries makes it impossible to assess alternative methods generally, but the use of specific methods in specific areas will be evaluated. In practice, weed control must integrate several methods to achieve the required effect.

a) Field production of small plants grown in rows - e.g. seedbeds and transplant beds of forest, hedge and shelter-belt plants, fruit bushes and rootstocks
Mechanical weed control is already used to a great extent in nursery crops as nurseries have traditionally welcomed new methods of weed control and have been willing to invest in new equipment. Weed control methods and equipment used in field-grown vegetable crops are thought to have potential for weed control in nursery crops, but further development and implement adaptation are necessary for full technology transfer. Mechanical harrows connected to hi-tech sensors or vision technology for the selective detection of crop and weed plants are assessed as having considerable potential in crops with well-defined crop plant spacing. Soil disinfection by thermal treatment has potential in intensively cultivated seed and cutting beds, but the method requires further development in order to be made economical and to have sufficient capacity to be practical. Among others things, a method to treat limited bands is required.
Mulching of seedbeds and cutting beds with organic material is assessed as having potential in weed control in certain crops. A lack of knowledge concerning crop plant germination and growth, and economic and practical aspects of laying the mulch make further development of the method necessary. Mulching with biodegradable plastic, paper or similar material is also assessed as being of interest in specific crops. The development of laying techniques, the price of plastic or paper material and the determination of possible effects on crop plant quality are of vital importance for the potential of the method.
Pricking out instead of direct drilling also has potential, but the use of the method will depend on economic and product quality considerations.

b) Field production of well-spaced row crops - e.g. ornamental trees and shrubs, roses and root-balled plants
In the production of nursery stock at relatively wide row spacing, implement carriers or portal tractors are used to some extent for weed control by a combination of mechanical inter-row hoeing and mechanical sensor-controlled hoeing within rows.
Intercropping is to some extent used in ornamental trees, but for the method to be extended to other nursery crops more knowledge is required about crop competitiveness to weeds, and about establishment strategies.
Cultivation practices, such as fertiliser application in bands, are currently used in ornamental trees, and the method possibly has potential in other crops, but method documentation and development are required.
Moreover, as described in the report, several methods, including the use of lasers, UV-light and electricity, require considerable development before they can be used in practice.

A2. Weed control in container-grown crops

Typically, only about half the area used to produce container-grown plants or used for greenhouses is cultivated. The rest is used for roads, alleys that crisscross the beds, and shelter belts. Containerised plants are usually grown on a layer of gravel or similar material on top of plastic or MyPex® ground cover. The alleys generally consist of the soil on which the container area is located. Roads are typically established with road metal, gravel or similar material. Weeds have therefore ample opportunity to establish themselves on these well-irrigated and well-fertilised areas. Weeds only compete with containerised crop plants when weed pressure is extremely high, or when the crop plants are very small. However, weeds in the container reduce product quality and should be eliminated before sale.

Mulching of the production area with plastic or similar material is estimated to have potential for weed control. Cost-benefit analyses (including energy consumption) should be carried out for different mulching methods.
Thermal treatment is estimated to be of interest for solving massive weed problems, and problems with root diseases. The potential of the method will depend on its efficiency in relation to economic considerations, resources and manageability.
Growing containerised plants on moveable benches or the like can minimise weed problems. It is, however, unclear which crops could provide a suitable return on the necessary investment. Mulching of individual containers is estimated as having potential for large plants that are to be grown in the same container for several years.

B1. Disease control in field-grown crops

Root diseases can cause serious problems in intensive seed, transplant and cutting beds. Danish nurseries have been granted exemption to apply the soil disinfectant dazomet (Basamid) to such beds in 2001.
Experience with alternative methods to control root diseases in nurseries is lacking.
Thermal treatment is estimated as having potential for disease control in intensive seed, transplant and cutting beds where the potential will depend on efficiency and economic and practical conditions as described for weed control. Thermal treatment in bands is an uncertain method to control root diseases, as the duration of the disease-controlling effect is unknown.
Cultivation practices, such as transplanting instead of direct drilling, have potential that will depend on economic considerations and on the possibility of maintaining plant quality.
There are only few cases in which the biological control of root and leaf pathogenic fungi has been demonstrated under field conditions. Applying specific antagonists will require a thorough understanding of their ecology in order to find the optimum time, dosage and formulation for their application. The use ofmicrobiological pesticides will probably not be able to totally replace chemical control, but it will be an important factor in a diversified strategy for the control of disease in horticulture in general. Microbiological pesticides must be approved in accordance with the same regulations that apply to chemical pesticides. One of several options available in nurseries is the stimulation of antagonists through the addition of organic material to the soil. However, there is a lack of knowledge of methods to promote antagonist populations in the soil so that they can be used in the best possible way to control disease.
Seeds for propagation are in some cases harvested under relatively primitive conditions in nature, and disease problems on or in the seed can occur. Alternative methods for the control of seed diseases are poorly investigated, but heat treatment of seeds is assessed as having potential - a potential that should be investigated.

B2. Disease control in container-grown crops

Root and leaf fungi may cause problems in containerised crops. Non-optimal conditions for the crop plants, such as low temperatures, temperature fluctuations, fluctuating air and water content in the growing medium, may increase the risk of fungus attack. There are very few pesticides available for the control of root fungi. The possibility of stimulating antagonists by using biologically active growing media in the cultivation of containerised crops should be examined. The method is used to some extent in the USA.

C. Control of insect pests

Many species of insect pests are found in nurseries. They can have a direct growth-limiting effect and a quality-deteriorating effect, cf. regulations for pests. There is only limited knowledge of the use and biological effect of alternative methods for the control of insect pests in field crops, mainly because of the relatively large number of crops within a relatively small area.
Biological control is estimated as having potential in relation to specific insect pests, but considerable development of the methods is needed, including the investigation of population dynamics for specific insect pests. Integrated cropping with insect-repelling flowering plants may have potential for the control of specific insect pests, but the method requires development as its biological effect has not been proven in practice. The selection of pest-resistant varieties is assessed as being of limited potential as complete resistance against insect pests is seldom found.
The biological control of insect pests in nursery greenhouses is currently used to a limited extent, corresponding to that of pot plant production (cf. the report on greenhouse crops). However, biological control in nursery greenhouses is more expensive as the greenhouses are relatively open most of the year to control the climate and to harden the plants. This means that the strategy for biological control must be adapted to these conditions.

D. Forecasting and warning

Forecasting and warning systems have not previously been used in nurseries, mainly because of the high number of crops and the relatively limited area. It is estimated that the use of forecasting and warning systems has possibilities in nurseries. Their implementation will, however, require further development and adaptation of the systems to specific diseases and pests in nursery crops, and investment in equipment and software in individual nurseries. Covering the plants with polyethylene may be a possibility in some crops where specific problems occur, but the effect has not been demonstrated for nursery stock.

E. Selection and plant breeding as alternative methods

The breeding of nursery stock world-wide is very small-scale, with the exception of a few plant groups (in particular roses, fruit trees and bushes, and herbaceous perennials). On the other hand, gathering plant material in nature, botanical gardens and parks in Denmark or areas of other countries with similar climate is widespread, especially for garden plants.
After gathering, selection must be made. The process is therefore long-drawn-out and consequently expensive. Selection is estimated as having the potential to restrict some fungal diseases in nursery stock, especially in plants used for gardens and landscaping. The method is already in use today (the Dafo^® system), although the great number of species grown in nurseries can make it economically prohibitive. Another method is plant breeding, where resistance genes that have been developed through genetic adaptation over many generations are introduced from wild relatives of the crop plants. In some cases it will be possible to incorporate resistance genes by cross-pollinating the crop plant with the wild relative. In many cases, however, there will be barriers to pollination - barriers that can possibly be overcome in the laboratory by somatic hybridisation for example. Such breeding programmes are estimated to be a possibility for specific diseases in certain high-value crops in which selection is deemed insufficient.

F. Pesticide application techniques and integrated systems

It is estimated that there are possibilities for reducing the use of pesticides by adapting PC crop protection and decision support systems to the special conditions found in nurseries. System development to suit specific nursery use is however required.
Crops are generally sprayed using field sprayers that cover the entire area. Band spraying is estimated to have potential in a limited number of wide-row crops, but the application technique requires development and the use of the method will depend on economic and practical conditions. Site-specific spraying is estimated to have potential only on relatively large areas with the same crop. Areas used for the production of containerised plants are usually sprayed using backpack sprayers (when the area to be sprayed is small, or when the crop plants themselves must not be sprayed), a spray boom mounted on the irrigation system covering the area, or a tractor-mounted sprayer. Generally, pesticide application techniques could be optimised further for areas used for container-grown plants by developing and adapting equipment for specific purposes.

Environmental evaluation of alternative methods

Alternatives to herbicides include mulching with different materials. Such weed control makes the cultivated areas uninteresting for the natural flora and large parts of the fauna.
Another alternative is to increase the use of mechanical or thermal weed control. The environmental advantage of these methods is obviously that the risk of herbicide percolation and run-off is reduced or eliminated. However, weed harrowing, flame weeding and other alternative methods also have environmental costs. The consumption of fuel per ha is increased considerably. However, a comparison of energy consumption in sprayed and non-sprayed fields also requires calculation of the energy consumed in the production of herbicides, including industrial works, and the energy consumed in the manufacture of tractors and implements. The present report does not include detailed calculations of energy consumption and CO₂ emission. There are no suitable tools for comparing, for example, the environmental risk of ground water pollution with the risk of CO₂-emission.

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