This briefing paper is based on the findings of a literature review carried out as part of a MAFF funded project titled "Development Of Disease Control Strategies For Organically Grown Field Vegetables" carried out by ADAS in collaboration with Henry Double Day Research Association, Elm Farm Research Centre and the Soil Association .

Disease development is dependent on the interaction between the pathogen, its host (crop plant) and the environment. In agriculture the key components are crop, cultivar, soil, plant nutrition, cropping pattern, agronomic practices, weather factors and disease presence. Changes to one component can have positive or negative repercussions on the others ultimately affecting overall crop production. The objective for organic farmers is to manage the components of the farm system to favour crop plants over disease organisms. To achieve this organic growers apply a diverse approach to suppressing disease incidence. This approach relies, above all, on the creation of diverse farm habitats, cultural practices and host resistance. This paper details the cycle of infection of diseases and considers the specific mechanisms and processes present that serve to control diseases in organic crops.

In general, disease is seen to arise when crop plants are growing under "stress" and therefore emphasis must be given to preventing crop stress, principally through appropriate care for the soil in which the crop is grown. A number of practices used in agriculture can heighten the crops susceptibility to disease. For example artificial fertilisers can cause nutrient imbalances either by luxury uptake and storage or by blocking uptake and release of other nutrients. Disease increases have also been seen after soil sterilisation or heavy irrigation where the effect is to partially clean or sterilise the soil (Huber and Watson 1970).

The infection cycle of a disease can be broken down into a number of stages. Organic farming practices adopted, will have a bearing on the success of the disease causing pathogen at each of these stages.

1. Recognition
The first stage in the infection cycle involves the pathogen identifying the host in order to colonise it and later propagate itself. Recognition depends on the presence of the pathogen and infective inoculum and on the activation of any protective mechanisms which the host may have which act to block recognition or inhibit germination.

2. Penetration
After recognition has occurred the pathogen will attempt infection of the host, which will be influenced by the following factors :

* virulence of the pathogen
* abundance of inoculum
* interactions with environmental factors which can either inhibit or promote germination
the susceptibility of the host plant.

As we have little control over the first three factors, control methods will centre around avoidance of infective propagules and on reducing host susceptibility. The susceptibility of the host will depend on its inherent genetic resistance to the pathogen, the degree of maturity of the plant and on the presence and degree of its defence mechanisms. Host defence will depend on the host physiology and also on any beneficial interactions that the host has with non-pathogenic microorganisms

Host physiology
Plant physiology is affected by the host nutritional status, therefore development of defence mechanisms such as the production of toxins, will be influenced by the adequacy of its nutrition. Adequate nutrition allows the plant to invest in structural strengthening and allows increased resistance to fungal penetration and increased vigour therefore plants can overcome attack or produce defence and attack mechanisms.

Plant defence mechanisms include the strength of the waxy cuticle and morphology of the stomata, presence of trichomes which may act as a physical barrier (e.g. on tomatoes and courgettes) and the production of defence compounds (e.g. the anti fungal phytoalexin called Rishitin produced by potato plants in response to blight infection and the compounds called glucosinolates which are involved in the resistance of brassicas to downy mildew).

Non-pathogenic microbial protection
The presence of other non-pathogenic organisms or avirulent strains will also effect the ability of the pathogen to colonise the host due to the following interactions:

* Competition - for nutrients or production of toxins.
* Antagonism - including mycoparasitism, predation or hyperparasitism and fungistatis (temporary or permanent inactivation of fungal germination and growth usually through contact with inhibitory or suppressive chemicals or enzymes). Antibiosis and lysis can also reduce disease propagule germination in many instances through production of enzymes which degrade the walls of resting spores or mycelium.
These interactions can take place in the soil or other environments between micro-organisms in the absence of the host plant. Organic systems aim to encourage these interactions.

3. Colonisation
Once the pathogen has infected the host plant it will then establish itself on or within host tissues and grow to maturity. Colonisation is influenced by many of the same factors as above. At this stage systemic acquired resistance can be stimulated through the use of elicitors (Often in plant extracts) and maintenance of nutrition and nitrogen levels will facilitate the plant to operate its full defences to attack the invading organism. Hypersensitivity can prevent disease spread after infection.

4. Reproduction
At this stage the pathogen is trying to maximise its reproductive capacity by developing as many reproductive propagules (Such as spores) for dispersal as possible. If the pathogen manages to take hold and develop to this stage, it is important that its reproductive success is reduced as far as possible. Reproduction of the pathogen can be controlled by manipulation the environment around the host to make conditions less favourable to the pathogen.

5. Dispersal
Once the pathogen has successfully reproduced it will aim to distribute its propagules widely as soon as conditions are suitable. Some diseases will be wind dispersed others may require water for dispersal. Ease of dispersal of the pathogen will be affected by the area and uniformity of the cropping and again can be controlled through environmental manipulation. An understanding of pathogen life-cycles and timings of dispersal can be used in forecasting systems which allow the grower either to protect the crop using physical methods or to avoid sowing a crop until the danger has passed. Cultural techniques will also effect pathogen dispersal and good hygiene is an essential part of good farm management.

6. Maintenance
After dispersal, the pathogen will only have successfully perpetuated itself if it can ensure survival of its spores or propagules through its resting stage before infecting its next host. At this maintenance stage the pathogen cycle can be broken and potential host crops protected, through cultural controls which promote propagule destruction and fungistasis. The use of long and varied rotations is fundamental in organic systems for breaking infection cycles and preventing the build up of soil-borne pathogens as well as for building fertility and providing adequate nutrition for the crop.

Strategies for disease control and management in organic growing systems:
There are a number of key practices that are used within organic farming systems to ensure that levels of disease incidence are maintained below economically damaging levels. The following practices are encouraged in order to reduce disease impacts.

* Promoting Soil Health and Crop Vigour.
Optimum soil health is essential in order to give the crop the range and quantity of nutrition required. Availability and balance of minerals is important for production and stimulation of good plant defence systems which can reduce the success of the pathogen at the penetration and colonisation stages. Healthy soil systems will also be suppressive to pathogens (especially at the maintenance stage) while encouraging interactions between the crop and beneficial organisms. Interactions between crops and beneficial micorrhiza, rhizobacteria and VAM can lead to increased nutrient uptake, increased drought tolerance and increased production for the crop. Optimum soil health will be facilitated through regular input of organic residues in the form of composted manures and plant remains as part of appropriate fertility building methods and suitable management of pH, irrigation and tillage.

* Rotation
The development and implementation of well designed crop rotation is central to organic production systems. Balanced rotations which ensure that plants with similar disease susceptibility are separated by appropriate time intervals, will prevent unnecessary exposure of the crop to disease. Rotations will break the disease cycle at the maintenance stage and therefore irradicate or reduce the amount of disease inoculum available which in turn will reduce the success of the recognition and penetration stages. Soil Association standards prohibit the cultivation of alliums, brassicas and potatoes on the same land more than one year in four for this reason.

* Varietal Selection
Careful choice of varieties appropriate to the location, which may be fast maturing or resistant to known farm disease problems, is essential for reducing the success of the penetration and colonisation stages. If there is a particularly high disease risk in your area it may be sensible to consider not growing certain crops. Grafting onto resistant root stocks can be particularly useful for crops such as cucumbers.

* Planting dates and cropping
Strategic use of sowing dates is important for avoiding having young and vulnerable crops at times of high disease risk. It is useful to make disease risk assessment part of the management plan. Forecasting systems are available to help make decision on sowing dates outside of pathogen dispersal phases and therefore reducing the chances of recognition and penetration. These may be based around occurrence of various weather or environmental factors. Mixed cropping, variety mixtures and separating sowings in time and space can all help to reduce disease success at the dispersal and recognition stages. Crop spacing will effect the micro-climate around the plants so can be used to make conditions less favourable to disease especially at the reproduction and dispersal stages.

* Avoidance of Pesticides
The use of pesticides will affect the leaf and soil microbial balance. In situations where control becomes necessary (e.g to break the disease cycle at the colonisation stage) make use of knowledge about the benefits of cultural techniques, avirulent strains, biological control (natural is preferable to imported) and elicitors before turning to permitted sprays and inputs.

* Diversity
It is important to encourage the creation and establishment of a diverse and balanced ecosystem within and around the crop so that no one species/organism gets out of hand.This helps to keep pathogen levels down at all stages of the lifecycle but especially at the maintenance stage. There are many possible techniques for achieving community stability including through direct manipulation of the farming system (e.g. cropping patterns such as rotation, patch size, undersowing, intercropping, companion planting and trap cropping) to complement habitat management (e.g. trees hedgerows, field margins, water and wetland). These methods can also be used to encourage environments suitable for natural biological control organisms that control the pathogen through antagonistic effects.
Soil augmentation with organic materials may also have detrimental effects on soil-borne plant pathogens at the maintenance stage, due to increased antagonism and competition with other micro-organisms.

* Cultural Measures
Along with the other methods mentioned above direct intervention against disease can be made by creating physical barriers to disease dispersal through the use of mulches.
Diseased plants and volunteers should always be removed and residues destroyed, in order to reduce disease reservoirs and prevent spread at the dispersal phase.
Proper composting, where a heap is turned inducing a temperature rise to 60 degrees centigrade will facilitate the destruction of disease propagules and spores. Green manuring, tilage, understories and use of rotations can also aid destruction of spores and prevent build up in the reproduction and maintenance stages.

These principles and strategies need to be applied throughout the whole crop production cycle, from seed through propagation and growing of the crop to harvesting and storage.