GB2119214A - Method of improving the growth of plants - Google Patents

Abstract

The invention is concerned with the problem of how best to infect roots of a plant with the beneficial mycorrhizal fungus. Merely mixing the inoculum of the fungus with soil or other growth medium is time- consuming and not very effective. The invention lies in the idea of positioning across the path of growth of the roots a carrier material, carrying the inoculum. The roots are then allowed to contact and penetrate the carrier material, so that they pick up the inoculum and become infected. Preferably the carrier material is a sheet of cellulosic fibres coated with a layer containing the inoculum, which can also contain cellulosic fibres. The invention is useful in improving the growth of plants, particularly those grown in containers.

Description

SPECIFICATION Method of improving the growth of plants The present invention relates to a method and a material for improving the growth of plants by infecting plants with a beneficial fungus.

The well-being of most plants is highly dependent upon their symbiotic association with certain specialised fungi. This applies to the vast majority of agricultural crops and also to trees and grasses of all kinds. The specialised fungi are those which have the ability to form mycorrhizas on their roots. The plant benefits from this arrangement in that it receives phosphorus, potassium and other essential nutrients accumulated by the fungus from the surrounding soil and in addition, there is evidence that the fungus protects its host from attack by disease organisms by, for example, utilising root carbohydrates and other chemicals which would be attractive to pathogens, providing a physical barrier to pathogens and/or secreting antibiotics which inhibit or kill the pathogens.On the other hand, the fungus benefits in that it is able to receive an abundant supply oF sugars from the plant.

Mycorrhizas have been classified to comprise two main groups:- ecto-mycorrhizas (ecto-M) and endo-mycorrhizas (endo-M). Whereas ecto-M fungi are capable of growth in the absence of the host plant and comprise a very large number of species, endo-M fungi are normally obligate symbionts, i.e. they cannot thrive in the absence of the host. Depsite the large number of species of ecto-M fungi, they are not the most common and their ability to form mycorrhizas is restricted almost entirely to trees in a relatively small number of families. However, these trees do include species of major importance to the timber industry, such as pine, spruce, fir and other conifer species. The endo-M fungi are divided into two sub-groups:- those produced by septate fungi and those produced by nonseptate fungi.Endo-mycorrhizas formed by nonseptate fungi, often referred to as phycomycetous or vesicular-arbuscular (VA) mycorrhizas, occur on more plant species than any other type. The number of species of VA mycorrhizl fungi might be quite limited (approximately 30).

It has been demonstrated experimentally, that endo-M can be induced to develop by growing seedlings in soil and vermiculite containing either infected roots or resting spores of an appropriate fungus. Clearly if this knowledge could be utilised and extended for use on a large scale in agriculture or forestry, while at the same time ensuring that infection of the roots with the fungi occurred, this would be of great value.In particular, in forestry operations in the USA and Canada, only 60% of the young seedlings survive the first year when planted out from nursery stock and in very dry areas it may be necessary to replant every year for 5 to 7 years before even this level of survival is achieved There are many environments where beneficial mycorrhizal fungi are not indigenous and even where they are present it is desirable to be able to ensure that each plant becomes infected to best advantage. The mere dispersal of additional mycorrhizal fungi in the soil or other growth medium will not ensure that the roots necessarily become infected and, in any case, large quantities of the fungus, which will not necessarily remain viable, will be needed.

There has now been found, in accordance with the present invention, a method which ensures infection of the root system wnile requiring only small amounts of the fungus. Accordingly, the present invention provides a method of producing a mycorrhizal condition in a root system of plant, which method comprises growing the plant and positioning across the path of growth of at least part of the root system, root-penetrable, integral carrier material carrying inoculum of a mycorrihzal fungus, so that at least part of the root system of the plant grows into contact with the carrier material and thereby becomes infected. The invention also includes the carrier material, which is preferably a web-like material, carying inoculum of an endo-mycorrhizal fungus.

The potential advantages of producing a mycorrhizal condition in the plant roots include the following: (1) improvement in the establishment of young seedlings, (2) more healthy plants leading to faster growth, (3) shorter overall growth period, (4) a saving in fertilisers.

There is also the prospect of producing genetically engineered fungi capable of fixing nitrogen in association with a plant host.

The carrier material of the present invention is distinguished from, say, infected peat or rockwool, in being a coherent expanse of infected material and the preferred carrier material is further distinguished in having a web-like form. By dispersing infected peat or rockwool in soil there is absolutely no certainty that contact with the root system of a plant will occur. In the present invention, the use of the carrier material gives a very high chance of producing infection as the roots pass through or alongside it. This chance is increased by growing the plant in conditions where root growth is restricted within a confined region, e.g. by a container.While a considerable amount of infection is picked up by the mere contact of the roots with the carrier material, it is envisaged that under all practical circumstances it would be desirable to arrange that the roots penetrate the carrier material. With this in mind, best results are likely to be obtained from use of a carrier material which can be penetrated or pierced by the roots as they grow, although it cannot be ruled out that some measure of infection would be imparted by use of a non-root penetrable carrier, by way of modification of the invention.

The carrier material of the invention preferably comprises a web-like structure (hereinafter "web" for brevity) having thereon a layer (a preformed layer or coating) comprising the inoculum. This layer preferably contains an adhesive for bonding it to the web or binder for cohering particles etc. of finely divided inoculum or both. Alternatively, the web itself can contain the inoculum. Thus the web can be formed from a composition comprising carrier component and inoculum, or a preformed porous web can be impregnated with inoculum.

Referring to the first embodiment of the carrier material, the carrier component preferably has a web-like structure. It can take any elongate form in which one dimension is substantially less than either of the others, and is conveniently a self-supporting sheet. It must be capable of being penetrated by the roots as they grow. It is preferably porous, but non-porous webs capable of being pierced by roots would suffice. It need not be a continuous sheet, but can contain a grid of holes or other discontinuities, provided that it is integral or monolithic to an extent sufficient to achieve the objectives of the invention. It is implicit that the material of which it is made must not damage the plant roots substantially or be harmful to the inoculum.

Preferably it is biodegradable, e.g. a biodegradable synthetic plastics material or a natural polymeric material such as cellulose. Preferably it has a fibrous structure. Cellulose fibres, especially wood pulp, cotton or paper fibres, are preferred because they are of short length and are biodegradable. Thus the web is preferably a wood pulp or paper sheet. Textile materials, foamed plastics and needle-punched plastics are alternatives.

The inoculum must be caused to adhere to the web. One way of doing this is to apply a layer of adhesive, for example magnesium silicate gel, to the web. Inoculum can then merely be roller-coated or sprayed on to the adhesive layer. Alternatively one can apply a mixture of inoculum and adhesive to the web. Another method is to form a film or sheet, not necessarily self-supporting, of particles of inoculum and a binder. This sheet is then caused to adhere to the web. Preferably the binder is chosen to assist in the adhesion. For example, a binder of disintegrated paper or wood pulp is conveniently formulated with the inoculum into an aqueous composition and a layer of this composition is deposited on a cellulosic fibre web and dried.

This binder will normally provide adequate adhesion for the inoculum on the web, especially a paper or wood pulp web. It might be preferable however, to use an additional adhesive or adhesive binder, for example magnesium silicate gel. The binder-containing coating composition can contain any desired concentration of inoculum, e.g. from 5 to 90% by dry weight, but if too much is included the desired binding and adhesive properties of the coating will suffer.

Typically, 50 to 70% by dry weight will frequently be satisfactory.

The coating comprising the inoculum could be sandwiched between two webs. Two or more webs impart added strength to the carrier material. Any laminate structure containing the inoculum within at least one layer thereof is within the present invention.

In the second embodiment of the carrier material, part of all of the inoculum is contained within the web. Conveniently it is formed by dispersing the fungal inoculum in a fibrous aqueous suspension or pulp, forming a web from this suspension or pulp, and drying it. A problem here is to incorporate a high proportion of inoculum in the web, yet also incorporate enough of the carrier component fibres to impart cohesion and strength to the resulting sheet so that it is self-supporting. When, as is preferred, the carrier component is of cellulose fibres, e.g.

of paper, cotton or wood pulp, about 5-40% dry weight of inoculum is usually appropriate.

Alternatively a web can be impregnated with the inoculum which can be applied by passing the web through a suspension of inoculum or by spraying a suspension of inoculum onto the web so that interstices thereof are penetrated by the inoculum. The comments made above in relation to the first (coated web) embodiment apply to the structure of the web and the carrier component thereof in this second embodiment.

If desired inoculum can be present both within the web and as at least one layer thereon, but preferably most of it is present in a layer or layers.

The invention includes a composition suitable for use in preparing the carrier material, either for forming a web or for forming a layer on a preformed web. The composition is an aqueous suspension containing cellulosic fibres, inoculum and optionally other ingredients as desired. It can contain for example a binder, e.g. starch, and/or adhesive e.g. magnesium silicate gel.

Besides cellulosic fibres, other fibres can be present including fibres of a synthetic or man-made polymer.

The term "web" or "web-like" does not imply that the carrier material is necessarily formed as a flat sheet. It can be given any desired configuration, as by rolling, cutting, moulding, heatforming, pressing, spinning etc. as appropriate -so the kind of material of which is is made and the use to which it is to be put. For use in growing container plants the web will frequently be formed into approximately a pot-shape.

Drying can usually be carried out in air at about 25-30"C without adversely affecting the inoculum and higher temperatures might be usable.

The inoculum of the mycorrhizal fungus can take any form known to contain active propagating elements (propagules). In the case of end-M, which do not ordinarily survive in the absence of a host, the inoculum will usually take the form of finely divided root infected with the fungus. Supplies of infected root can be produced in bulk by passaging inoculum in suitable host plants, e.g. lettuce or maize, and harvesting the roots hereof. A proportion of the harvested roots is set aside as inoculum for infecting seedlings of further plants and so on. For growing the plants the nutrient thin film technique (NFT) described in our British Patent Specification No.

2043688 is recommended. The roots can be harvested in bare form or in association with the medium, e.g. peat, in which the host plants were grown. Preferably roots having a high level of sporocarps and/or resting spores of the endo-M are selected for the inoculum, since preliminary evidence suggests that this form of inoculum retains infectivity for longer periods.

Typical examples of methods by which the contact between the infected carrier material and roots of a container-grown plant may be ensured are as follows: a) At a suitable stage after germination, when the seedling is just producing susceptible rootlets, a disc of the mycorrhizally infected carrier material is inserted between the roots and the additional growth medium, e.g. vermiculite/peat mixture and soluble NPK but no sugars.

Infection is more likely to occur, as the roots grow through the disc, under conditions of adequate light intensity and when phosphorous (P) is limiting. This can be achieved by decreasing the proportion of P in the soluble NPK feed or preferably compounds which release soluble P slowly into the growing medium, e.g. rock phosphate or resin-coated controlled release fertilisers.

b) A mycorrhizally infected carrier material in the form of a sheet is cut to the size required to extend across a seedling tray. The sheet can, if desired, be perforated. The tray is divided into compartments in which the seedlings grow and each compartment is also divided into upper and lower compartments by placing a plastics or other impervious sheet at a certain depth across the tray to effect the division. During the initial period of growth, the roots are confined to the upper compartment by the impervious layer. After a period of time, say one to three months, the impervious layer is replaced by the infected paper sheet, the plant illuminated strongly and the phosphate content of the growth medium omitted.

c) The mycorrhizally infected carrier material can be rolled to form a cylinder or formed into the shape of a pot. Either the host plant may be grown initially in this cylinder or pot or it can be grown in a conventional pot and then repotted into the cylinder or pot made of the carrier material. The roots of the plant will then grow out sideways through the cylindrical or potshaped carrier. This is encouraged by placing rock phosphate in the soil outside the cylinder or pot.

Of the three methods, a two-phase method of the type described under b) above is particularly advantageous. By growing plants in containers where the room for initial growth is confined by suitable means, for example an impervious sheet which is later removed and replaced by the infected carrier material of the present invention, rapid and wide-spred infection of the roots, which then grow through into the lower part of the container, can be assured.

It is also possible to apply the method of the invention to seedlings grown in the open soil.

Long strips of the carrier material can be placed in furrows which are then covered with soil and sown with seeds. This aspect of the invention is of interest for improving crop yields on poor soil, e.g. reclaimed gravel pits, coal mine spoil tips or slag heaps.

The host plant should preferably be exposed to infection at an early stage of growth preferably before it has established any natural mycorrhizal association. In the case of conifers (e.g. pines, spruce and Douglas firs) this is preferably about the time when the first tertiary roots appear.

The effectiveness of the association will depend also on many factors other than the nature of the plant and its fungal partner, particularly pH of soil, temperature and humidity.

It is clearly important to be selective in the matter of which mycorrhizal fungus to use in promoting the mycorrhizal condition. The value of the individual partnerships will vary widely depending on the suitability of the fungal associate. Both endo- and ecto-M fungi are useful in forestry, ecto-M fungi being particularly suitable for nursery infection of pine, spruce and fir cedar, hemlock and eucalypts. Endo-M fungi are considered suitable for similar application to most hardwoods and most tropical species. Ornamental trees and shrubs, both outdoors and indoors, are suitably treated with endo-M fungi, as are citrus tree and orchard tree (e.g. walnut or almond tree) root stock.Endo-M fungi are also considered of value in agriculture, particularly in the case of crops which require a transplantation stage, e.g. coffee and tobacco, which are in an equivalent position to the growth of tree seedlings which are outplanted, or where soil sterilisation is routinely practised as in the case of soft fruits or vegetables.

The most useful ecto-M fungi are the VA mycorrhizas, for example Pisolithus tintorius, Rhizopogon vulgaris (particularly for sugar pine), Laccaris lacata (particularly for ponderosa pine), Cenococcum graniforme and Suillus granulatus. Examples of useful endo-M fungi are Glomus mosseae, Glomus macrocarpus, Gigaspora margarita and Gigaspora gigantea. In some cases it will be advantageous to infect the plant with more than one mycorrhizal fungus and this may be achieved either by using a single carrier material infected with two or more fungi or, more preferably, operating with separate carrier materials for each fungus. For example, by growing them through two sheets of paper carrying different mycorrhizal fungi the roots would become infected with both.

The carrier material can act as a carrier for other plant growth-stimulating agents, e.g.

hormones or agents which will prevent indigenous pathogens from becoming established in the plant before the mycorrhize becomes well established. These may be anti-bacterial or anti-fungal agents. It can also carry beneficial organisms, e.g. rhizobia.

The present invention is especially important in that it imparts a mycorrhizal condition to plants grown, at least initially, in containers whereby rapid infection is ensured and can be achieved with relatively low quantities of mycorrhizal fungus, thus providing a valuable method for use on a commercial scale.

The following examples illustrate the invention.

EXAMPLES Methods are described in which chopped roots infected by VA mycorrizal fungi are incorporated within or attached to the surface of paper sheet. This can then be cut to fit plant containers of various sizes or applied as long strips in field situations.

1. Types of paper used Three types of paper supplied by Whatman Paper Division have been used.

a) 100% cotton linter-Whatman Grade No.1.

b) 100% wood pulp-Whatman Grade No. 6, c) Creped wood pulp containing ureaformaldehyde (wet strength resin)-Whatman Grade No.

91.

2. Inoculum used The endophytes used were Glomus mosseae. YV strain and 'E3' thought to be Glomus fasciculatus (supplied initially by Rothamsted Experimental Station, Harpenden).

Infected bare maize root was produced by the nutrient thin film technique (NFT) described in British Patent Specification No. 2043688. The maize plants were raised in 5 cm lattice pots containing peat with lime and trace elements. After three weeks propagation the plants were positioned at 20 cm intervals along a channel 30 cm wide. Nutrient solution containing bone meal 309/100 litres and 50 mg N per plant per week was supplied. Plants were harvested 73 days after planting out. They had 65-85% of their root length infected and the root mat filled the 30 cm wide channel to a depth of 5-7 mm. The majority of root length (72% on average) was composed of roots with a diameter between 0.2 and 0.4 mm.

After harvest, root mats were stored in a cold store (0-2"C) in moist condition from two to twenty-five weeks before use. Several methods of chopping roots were employed but the most successful involved using sections of root mat usually approximately 30 cm2 and 0.2-0.5 g and added to 1 50 ml of tap water in an "Atomix" homogenizer operated at full speed for 1 min.

The suspension was then passed through a sieve to remove thick or tough roots which had not been adequately chopped. In some cases thick 'primary' roots were removed before chopping to improve the efficiency of the process. A "Silverson" blender, operated for up to 1 min, was also used when small root pieces were required. After chopping, the majority of intact root cylinders, from a typical sample of root mat, were estimated to be 0.5 to 2 mm in length. Often about 30% of chopped root fagments were no longer recognizable as complete cylinders.

Successful formulation experiments have also been conducted using larger portions of root, ranging from 1-4 mm.

Mycorrhizally infected root from plants grown in peat has also been used as a source of inoculum. A mixture of peat with mycorrhizal maize roots was produced by NFT. The plants were grown in "Jiffy" pots, and the inoculum was prepared by cutting the pot walls away from the root mat, breaking them open and removing all coarse primary root and stem. The remaining peat and heavily infected root pieces (92-98% of root length infected) were chopped in an "Atomix" blender for 1 min. A 1:1 by volume mixture of moist peat/root to tap water was generally used. Root fragments of between 1 and 3 mm predominated after this treatment.

The principal propagul in both the bare root and peat admixture types of inoculum was considered to be a small cylinder of root containing fungal hyphae and possibly vesicles, (thought, primarily, to be storage organs). Although extra-matrical mycelium was present, this was less likely to survive severe drying and storage than hyphae within roosts. The function of secondary spores which often occur on extra-maiicall hyphae is not fully understood, but they are unlikely to be infective. Material with few sporocarps ('YV' strain) structures containing several resting spores, was deliberately selected for much of this work in an attempt to assess the potential of chopped root pieces as infective propagules.

Another inoculum used was a mixture of peat with mycorrhizally infected lettuce roots obtained by NFT, stored for several weeks in moist condition, and then milled to a dry powder (800 micron size). This material was supplied by Rothamsted Experimental Station, Harpenden.

3. Production of paper sheets carrying mycorrhizal propagules Method 1 Various proportions ranging from 5 to 40% (by dry weight) of fresh chopped maize roots infected with Glomus mosseae YV strain were mixed with a suspension of cotton linter fibres obtained by homogenizing Whatman No. 1 paper. The suspended mixture was brought down on to a filter paper disc in a Buchner funnel apparatus. The resulting reconstituted paper layer was peeled from its backing sheet and allowed to dry.

This method is considered possibly disadvantageous for scaling up. It was felt that the physical presence of "contraries" i.e. foreign matter, could reduce the tensile strength of the paper. There was also a possibility of introducing cellulose-degrading organisms as contaminants into the paper-making machinery.

Method 2a Infected root pieces were coated on the surface of preformed paper sheets. Method 1 was modified as follows. The proportion of chopped root in the mixture was increased to between 50 and 70% (by dry weight). A thin film of paper fibres and chopped root was then brought down on to discs of Whatman No. 1 or No. 91 (creped) paper. The layer thus desposited adhered well to the disc. Providing that the discs were not roughly handled, the deposited layer remained attached to the backing sheet after drying. The irregular surface of the creped paper assisted in the formation of a stronger bond between the two layers.

Method 2b Another procedure recently adopted for attaching chopped root or chopped root + peat to paper involved using "Laponite 550" (Laporte industries plc) a magnesium silicate gel. Finely chopped root suspension (with or without peat) was mixed with "Laponite 550" powder (2.0 to 2.3 9/100 ml) in a Silverson blender until the gel had formed. Inoculum was then sprayed on to paper by passing a jet of compressed air over the top of the vertical 3.5 mm diameter tube held in the gel.

This process could readily be adapted for industrial use, perhaps by the inclusion of a peristaltic pump to assist in the accurate delivery of gel to the compressed air spray.

Alternatively the gel could be applied to paper by means of "lick-up" rollers-a technique commonly used for applying coatings to paper.

One advantage of the use of a gel is that the inoculum remains evenly distributed in the medium for a long period and that relatively small amounts of moisture are used in the application process. This in turn requires less severe (and thus cheaper) drying procedures which could reduce inoculum viability.

4. Drying and storage In most cases drying was carried out with paper sheets spread on a wire tray placed in a laminar flow bench with air at about 25-30"C passing over it. After air drying for various times (10-60 min) residual moisture levels were determined by taking sub-samples of paper and weighing before and after drying at 100" for 6-8 h. Moisture levels of between 5% and 38% have been achieved.

After drying to various moisture levels, the treated paper sheets were packed in polythene bags which were sealed and stored at 0-4"C. Some undried sheets, to serve as controls, were also stored in polythene bags.

3. Infectivity tests Bioassays using red or white clover have been used to assess infectivity of inoculum after the formulation and storage treatments. Several similar methods have been tried in the past but in the one currently used, gamma-irradiated (0.8 Megarad) soil, known to give suitable infection conditions for the endophyte under test, was mixed with the same volume of horticultural perlite or sand. Plastic tubes (9 cm high x 2.5 cm diameter) with drainage holes were half filled with the mixture. Single sub-sample discs of inoculum paper were placed in each tube and buried under about 2 cm of soil mixture. Four chitted red clover seed were sown per tube and lightly covered. The test plants were raised under standard conditions in a growth room at 22"C.

As roots developed they readily passed through the paper discs. The number of primary infection points was scored after 21 days growth on one set of three tubes per treatment and again on another set after 28 days.

Mycorrhizal infection in test plants was induced by this method. The amount of infections was compared with that induced by the wet control sheets, to determine the effect of drying.

Preliminary results showed that formulated bare-root inoculum (YV) can be stored for at least three months in a dry condition and still retain some infectivity (Table 1). However, despite considerable variation in bioassay tests, there was a noticeable decline in infectivity during storage (Tables 1 and 2). Part of this residual infectivity could have been due primarily to the presence of a few resting spores accidentaily mixed with the root inoculum. Where chopped roots associated with high levels of sporocarps and resting spores (YV strain) have been deliberately applied to the paper, the inoculum has retained a high level of infectivity for 28 weeks (Table 3).

Results shown in Tables 4 and 5 indicate that the presence of peat in the inoculum seems to enable propagules of the YV strain to retain viability for longer.

Evaluation of the method in which "Laponite 550" is used as an adhesive is not complete.

Initial results using a bare-root inoculum of the 'E3' endophyte indicate that the paper sheet formulation will survive at least two weeks dry storage with only slight loss of infectivity.

Table 1 Bare maize root (YV infected) stored wet for 23 wks before formulation in reconstituted paper sheet (Whatman No. 1) (Method 1) and subsequent storage for 1-23 wks.

Mean no. of primary infection points arising from each disc after 28 days Length of storage after formulation 1 wk 6 wk 1 2 wk 25 wk moisture in disc 23% by wt. 7.7 2.0 1.3 - 32% ,, ,, 7.7 3.3 3.3 0.7 control (stored wet) 8.7 11.0 5.7 6.7 Table 2 Bare maize root (YV infected) stored wet for 5 wks before formulation in a thin paper layer on preformed sheets (Whatman No. 1 and No. 91) (Method 2a) and subsequent storage for 2-28 wks.

Mean no. of primary infection points after 28 days (mean of two types of paper) Length of storage after formulation 2 wk 4 wk 103 wok 28 wk moisture in disc 10% by wt. 4.3 2.2 1.5 21% ,, ,, 6.5 2.3 2.3 0 control (stored wet) 9.3 10.2 8.5 5.0 Table 3 Bare root + sporocarps/individual resting spores of YV stored wet for 5 wks before formulation (Whatman No. 1) (Method 2a) and subsequent storage for 4Q-28 wks.

Mean no. of primary infection points after 28 days Length of storage 4+ wks 11 wks 28 wks moisture level in disc 20% by wt. 8.3 10.0 10.7 control (stored wet) 14.3 11.0 12.7 Table 4 Maize root (YV infected) in peat stored for 5 wks before formulation in a thin paper layer (Whatman No. 1) (Method 2a) and subsequent storage for 2-28 wks.

Mean no. of primary infection points after 28 days Length of storage after formulation 2 wks 42 wks 11 wks 28 wks moisture level in disc 28% by wt. 13.0 8.7 7.7 7.0 control (stored wet) 15.7 11.7 10.7 Table 5 Miled lettuce root (YV infected) in peat supplied by Rothamsted, stored for several weeks before formulation (Whatman No. 1 and No. 6) (Method 2a) and subsequent storage for 2-18 wks.

Mean no. of primary infection points after 28 days Length of storage after formulation 2 wks 10 wks 1 8 wks Moisture level in disc 24% by wt. 8.2 5.2 4.0 control (stored wet) 7.2 9.7 3.8

Claims (24)

1. A method of producing a mycorrhizal condition in a root system of the plant, which method comprises growing the plant and positioning across the path of growth of at least part of the root system, a root-penetrable, integral carrier material, carrying inoculum of a mycorrhizal fungus, so that at least part of the root system of the plant grows into contact with the carrier material and thereby becomes infected.

2. A method according to Claim 1 wherein the carrier material comprises a web-like structure haivng thereon a layer comprising the inoculum.

3. A method according to Claim 2 wherein an adhesive for bonding the inoculum to the web-like structure is provided as a separate layer between the inoculum layer and the web-like structure or as a component of the inoculum layer.

4. A method according to Claim 2 or 3 wherein the inoculum is in a finely divided form cohered with the aid of a binder.

5. A method according to Claim 2, 3 or 4 wherein the web-like structure is a sheet of cellulosic fibres.

6. A method according to Claim 5 wherein the fibres comprise cotton, wood pulp or paper fibres.

7. A method according to Claim 5 or 6 wherein the sheet of cellulosic fibres is coated with a layer comprising celluiosic fibres and the inoculum.

8. A method according to Claim 7 wherein the cellulosic fibres comprise cotton, wood pulp or paper fibres.

9. A method according to Claim 8 wherein the layer comprises from 50 to 90% by dry weight of inoculum.

1 0. A method according to Claim 1 wherein part of all of the inocolum is contained within the carrier material.

11. A material according to Claim 10 wherein the carrier material is formed from an aqueous suspension containing inoculum and cellulosic fibres.

1 2. A method according to Claim 1, 2, 3 or 4 wherein the carrier material is of a biodegradable synthetic plastics material.

1 3. A method according to any one of the preceding claims wherein the fungus is an endomycorrhizal fungus.

1 4. A method according to Claim 1 3 wherein the fungus is a vesicular-arbuscular mycorrhizal fungus.

1 5. A method according to any one of the preceding claims wherein the plant is grown in a container.

1 6. A method according to Claim 1 5 wherein the carrier material has substantially the shape of a plant pot suitable for growing the plant therein or for enclosing a smaller pot in which the plant is grown.

1 7. A method according to Claim 1 5 or 1 6 wherein the plant infected is a tree seedling which is subsequently outplanted.

1 8. A method according to Claim 15, 1 6 or 1 7 wherein the growth of the root system in the container takes place in two phases, a first phase where the growth is confined by means restricting room for growth, and a second phase, in which the room-restricting means is replaced by the infected carrier material and the roots are allowed to grow-through the carrier material.

1 9. A web-like carrier material carrying inoculum of a mycorrhizal fungus.

20. A carrier material according to Claim 1 9 as further defined in any one of Claims 2 to 14.

21. A carrier material according to Claim 19 comprising a web of paper, cotton, wook pulp or adhered to a layer of wook pulp or paper containing the inoculum.

22. A carrier material according to Claim 21 in substantially the shape of a plant pot.

23. A composition suitable for use in preparing a carrier material according to Claim 19, which composition comprises an aqueous suspension containing cellulosic fibres and inoculum of a mycorrhizal fungus.

24. A composition according to Claim 23 wherein the cellulosic fibres are of cotton, wood pulp or paper.