Introduction
Organic plant pest and disease management is based on:
a. Planting into a pest- and disease-free field
Proper land preparation is required to create a suitable soil environment for growing crops, while removing any weeds or crop remnants that could harbour crop pests and diseases. This, together with a good crop rotation reduces the chances of pests and disease-causing organisms incubating in the crop field and building up their populations. An interval of 1 to 3 years between crops of the same family grown on the same field breaks the life cycle of most pathogens. The minimal duration of the interval depends on the disease or pest persistence in the soil. Some diseases stay dormant as spores in a field for many years (e.g. bacterial wilt stays infectious for at least 2 years, late blight up to 4 years, and Fusarium wilt up to 6 years). Many pests like bean aphids or plant-parasitic nematodes will easily die from starvation if host crops and or other host plants are not available in the following year. If soil-borne pests and diseases are a problem, improved fallows for at least one season can also be applied if land is not limiting. Biofumigation with plants such as marigolds and rapeseed oil before planting crops can control soil borne pests like nematodes. Burning of any pest or disease infested materials (only mulch with pest and disease-free materials), and solarisation of seedbeds or plots, where feasible, to kill any soil borne pathogens are recommended practices.
b. Timely planting
Planting should be scheduled so that the most susceptible time of plant growth does not coincide with the life stage of pest or disease inoculum that damage the plant. Timing of seeding and planting is used to avoid invasion by migrant pests, or the oviposition period of particular pests, and the introduction of disease in the crop by insect vectors (like aphids and psyllids). In addition, it is used to synchronise the pest or disease attack with its natural enemies, with weather conditions that are adverse for the pest, disease pathogens, or with the abundance of alternative hosts. Although timing needs specific knowledge of the pest or disease pathogen and when it is likely to attack, the commonly promoted practice is to plant as early as possible during the season.
c. Selection of tolerant cultivars, and healthy seeds and planting materials
The use of pest- and disease-free seeds, and planting materials is a very effective tool to limit the introduction and spread of pests and diseases into the crop field. Certified seeds are normally clean, but if such seeds are not available to the farmers, un-certified seeds should be treated before use to eliminate seed-borne diseases (for example using hot water treatment at the recommended temperatures to avoid damaging the seed if the water is too hot).
In addition, the use of crops and varieties tolerant or even resistant against common pests and diseases is another effective measure to lower risks of pest and disease damage. There are more commercial crop varieties with disease resistance than are known for pest resistance. Therefore, for pest resistance, partial resistance, or field tolerance to pests by locally adapted varieties is more practical. Traditional and locally adapted crop varieties should be prioritised – even ‘resistant’ varieties need to be adapted to local climatic conditions for effective resistance and usefulness to the farmer.
d. Promotion of natural enemies for biological control
Natural enemies play a critical role in the regulation of pest and disease populations. By encouraging, for example, a diversity of flowering plants, the availability of nectar, pollen, fruits, insects, and other organisms increase and help to keep a check on the pest populations keeping them at low levels that do not threaten the crops. The main natural enemies include a wide variety of spiders, ladybird beetles, long horned grasshoppers, earwigs, wasps, etc. Some of the strategies for promoting the proliferation of natural enemies include:
Certain flowering plants, especially perennial ones, along the border of the crop fields can attract and promote natural enemies. The flowering plants can also be sown or planted along internal bunds inside the field or as companion crops within the field. These plants act as a source of food in the form of pollen and nectar for adult natural enemies, source of shelter or alternative hosts when primary hosts are not present.
Promotion of soil-borne natural enemies through regular addition of organic matter to the soil in the form of compost, mulches, farmyard manure, or crop residues. Green manure cover, and careful soil cultivation also enhances below ground biodiversity which reduces soil-borne pests and diseases. Practicing reduced tillage also minimises the destruction of hibernating natural enemies, or their hibernating homes or shelters.
e. Intercropping (or mixed cropping)
Growing different crop and plant species together creates a less favourable habitat that visually or chemically interferes with pest activity. Mixed intercropping systems where two or often more species are grown intermingled without distinct rows are very commonly used in the tropics. Row intercropping (growing two or more crops together in rows) and strip cropping (cropping by growing two or more species in strips) must be sufficiently wide to allow separate management regimes, but sufficiently close to influence each other. These types of mixed cropping systems have been widely investigated as they have great potential for reducing pest attacks. The mixture of plants needs to be carefully chosen. Anise, chives, garlic, onions, radish, parsley and many other species are reported as good partners for intercropping. Mexican marigold has also shown the advantage of repelling pest insects like aphids and root nematodes when mixed with crops, although it can attract slugs which is a pest for vegetables, if not well monitored.
Trap cropping ensures that the trap crop is more attractive to the pest as either a food source or egg laying site than the main crop. A particularly successful example is the push-pull trap cropping used in East African maize production. Push-pull strategy is a simple cropping strategy, whereby Napier grass (Pennisetum purpureum) or bracharia grass and desmodium legume (Desmodium uncinatum) are used as intercrops in maize. Desmodium is planted in between the rows of maize. It produces an odour that stem borer moths do not like and, therefore, ‘pushes’ away the stem borer moths from the maize crop. Napier grass is planted around the maize crop as a trap plant. Napier grass is more attractive to stem borer moths than maize and it ‘pulls’ the moths to lay their eggs on it. But Napier grass does not allow stem borer larvae to develop on it. When the eggs hatch and the small larvae bore into Napier grass stems, the plant produces a sticky, glue-like substance which traps them, and they die. So, very few stem borer larvae survive and the maize is saved because of the ‘push-pull’ strategy. In addition, the ground cover of desmodium within the maize field reduces witch weed (Striga spp.). Desmodium suppresses the witch weed by herbicidal compounds produced in the roots. Being a legume, desmodium also fixes nitrogen in the soil and thus enriches the soil.
f. Proper soil fertility management
The enhancement of a healthy, fertile soil and good growing conditions promotes healthy plants, improving pest and disease control. A fertile soil enhances plant health and even triggers the immune reaction of plants. Crop rotations especially with leguminous plants which fix nitrogen into the soil, helps to reduce nutrient stress to the plants and enhances crop vigour - vigorous plants are less susceptible to pests and disease damage.
Soil fertility can be improved through the following approaches:
Planting cover crops and green manures, which besides feeding the soil also improves its organic matter content and reduces pests by interrupting their movements across the field and diseases through biofumigation. Pest species like aphids, root flies or weevils will not find the crop plants due to the intercrop causing smell-related confusion as the cover crops or green manures emit different odours.
Application of compost can help control plant pests and diseases through (i) successful amendment of the soil with insect pathogenic microorganisms (ii) antibiotic production by beneficial microorganisms and (iii) activation of pest-tolerance or disease resistant genes in plants by essential nutrients from compost. Some of the microorganisms, for example Trichoderma fungus, exhibit antagonistic characteristics against diseases causing microorganisms and prevent them from multiplying. Just like compost, application of disease and pest free organic mulches also can, in special cases, reduce pests like root flies, cutworms or aphids, by altering the immediate microenvironment around the crop or by reducing raindrop splash-dispersal of some soilborne diseases. However, organic mulches might also enhance some fungal diseases by moderating soil moisture.
g. Regular scouting
Regular monitoring of the presence of pests in the field should begin as soon as the crop emerges after planting and should continue throughout the growing cycle. Randomly locate specific sites (about 5) in the field along a Z-path, visit them following a defined schedule (e. g. weekly), recording counts of insects as per procedure finalised for individual insects. For disease scouting, the cause of the symptom may not be obvious. What may appear as disease symptoms of fungal, bacterial, viral pathogens, are sometimes effects of abiotic factors such as weather, fertilisers, nutrient deficiencies, damage by pesticides, or soil related problems and it is important that farmers know these differences for appropriate action. Generally abiotic problems cause regular, uniform colouring symptoms (except viruses), as compared to fungal diseases which cause irregular growth & spotting colour patterns, cankering, blighting; wilting, scabbing and rotting on different plant parts, and withering or dying of plants. Viruses cause light green/yellow patches on leaves, and stunted growth, while bacteria cause rotting plant parts, and early dying of the plant or tumours.
To monitor insect pest populations, different traps can also be used. The simple idea is to know more about the presence of the insect pests in the field, especially the fast moving (mobile) insect pests (e.g. fruit flies, lepidopteran pests) as described in section i below (direct control of insects).
h. Field hygiene and sanitation
Maintaining sanitation practices in the field prevents the introduction and spread of pests and disease-causing organisms.
Sanitation of existing crops, especially perennial crops should be done regularly. Poorly managed or abandoned perennial crops can result in buildup of pest and disease problems. All damaged plant materials and rotten fruits from the ground must be either burned or deeply buried at least 50 cm deep.
Pruning is an important operation especially in plantation or tree crops. It eliminates inoculum in perennial crops. All infected branches or shoots should be cut at least 20 cm below the visible damage and burnt. Pruning also improves aeration and light exposure to the crown, exposing the hiding places of pests and disease-causing organisms.
Regular cleaning of all the tools used for pruning infected plants or gardens is important, especially in the case of bacterial and viral diseases. If possible, alcohol (>70 %) can be used to disinfect the tools or heating the metal parts of tools over a fire.
i. Natural direct control measures
Direct control measures that respect and protect the natural resources - soil, water, air, and biodiversity are encouraged. Negative impact on the environment is also avoided by not using synthetic and potentially harmful pesticides.
Examples of organic direct control measures;
Direct biological control of pests involves the release of mass-reared live agents such as bacteria, viruses, fungi, insect predators and insect parasitoids into crop fields to control pests for a brief or extended period. However, not all commercially available biocontrol agents can be used in organic agriculture; for instance, genetically modified organisms are prohibited. Direct biological control is widely used in greenhouse crops.
Insecticides of vegetal or mineral origin. There are commercial organic insecticides available in many countries of Africa (e. g., neem, rotenone, pyrethrum). Most of them can be produced by a farmer on the farm.
Pheromone traps are mostly used for monitoring pest insect populations in the crop field, but can also be used for mass-trapping of pests. Pheromone dispensers release a sex hormone similar to that produced naturally by the female insect, which attract the male insects into the trap in which they get stuck. Several types of pheromone traps have been developed for monitoring African bollworms, cutworms, fruit flies, etc. and are widely used. However, in most parts of Africa, pheromone traps are not locally available and imported ones are not affordable to small-holder farmers. Sex pheromones can also be used for mating disruption. Here, a high number of dispensers are installed in the crop (fruit orchards, or vineyard) to get a pheromone cloud within or over the crop. Male insects will then not be able to find the female and mating is, therefore, disrupted. Consequently, the pest offsprings will be reduced, hence lower damage to the crop.
Mass-traps often can easily be built with cheap material. For example, light traps can be used to catch moths such as armyworms, cutworms, stem borers and other night flying insects. Coloured sticky traps (blue, yellow or white) can be used to monitor and mass trap adult thrips, house flies and fruit flies in the crop nursery or field. Yellow sticky traps can specifically be used to mass-trap whiteflies, aphids and leaf mining flies. Alternatively, water traps (with few drops of a detergent) with a surface area of 250 to 500 cm2, and preferably round, with the water level about 2 cm below the rim can also be used.
Physical barriers against pests like fruit bagging also help to prevent fruit flies or other pests from laying eggs inside or on the fruits.
Direct measures should be used as a last option for the control of pests and diseases when all methods used in preceding steps fail. For certified organic production, only agents of non-synthetic origin are permitted. One allowed exception is the use of synthetic pheromones because they are contained in dispensers and, therefore, do not come into contact with crops. It is therefore important for certified organic farmers to check with their organic certifiers before applying any commercial products.
The intensity of these direct measures will depend on the pest or disease being targeted as well as the type of crop being considered (annual or perennial crops). They should therefore be taken in combination, not in isolation. For example, when armies are preparing for combat warfare, they take time to study the opponent in terms of their numbers, type of weapons they have, from where they are likely to attack and potentially when, in order to equip themselves well if they are to win the fight. For the farmer, the pest or disease-causing organism is the opponent that he or she needs to know the type of the pest or disease-causing organism, how it is transmitted, what stage of the plant it attacks and the damage it causes.
Monitoring improves the knowledge on pest or disease development and helps the farmer to manage them more effectively.
The following questions may help one to better understand pest behaviour;
At what stage of its lifecycle is the insect a pest: When it is a larva/ a caterpillar, a nymph or an adult?
Which stage of the plant does it attack: the seedling, the growing plant or the mature plant?
Which part of the plant does it attack: leaves, roots, the stem, fruits, seeds or the entire plant?
What kind of damage does it cause: chewing, sucking or wilting?
When does it attack: in the dry season or in the wet season?
At what time of day is the pest most active – night or day?
The same process can be repeated for any disease.