Management and Control of Pests and Diseases

The interaction between living organisms and their environment is crucial for a plant‘s health. Plant’s health is more at risk in monocultures and on-farm diversification provide a balanced interaction between different plants and pests and predators. This is why a well-managed ecosystem can be a successful way of reducing the level of pest or disease population. Certain crop varieties have more effective mechanisms than others due to the adaptive nature to the environment and therefore have a lower infection risk. 

The health condition of a plant depends to a large extent on the fertility of the soil. When nutrition and pH is well balanced, the plant becomes stronger and is therefore less vulnerable to infection. Climatic conditions, such as suitable temperatures and sufficient water supply, are further factors which are crucial for a healthy plant. If one of these conditions is not suitable, the plant can become stressed. Stress weakens the defence mechanisms of plants and makes them easy targets for pests and diseases. One of the most important points for an organic farmer is therefore to grow diverse and healthy plants. This avoids many pest and disease problems (Figure 8-2). 

A Prevention Practices and Monitoring

Knowledge about plant health and pest and disease ecology helps the farmer to choose effective preventive crop protection measures. As many factors influence the development of pest and disease, it’s crucial to step in at the most sensitive points. This can be accomplished through the right timing of management practices, a suitable combination of different methods, or the choice of a selective method. Some important preventive crop protection measures are the following ones: 

1) Selection of adapted and resistant varieties: 

• Choose varieties which are well adapted to the local environmental conditions (temperature, nutrient supply, pests and disease pressure), as it allows them to grow healthy and makes them stronger against infections of pests and diseases.

2) Selection of clean seed and planting material: 

• Use safe seeds which have been inspected for pathogens and weeds at all stages of production. 

• Use planting material from safe sources. 

3) Use of suitable cropping systems (see 6. Crop Planning and Management): 

• Mixed cropping systems: can limit pest and disease pressure as the pest has less host plants to feed on and more beneficial insect life in a diverse system. 

• Crop rotation: reduces the chances of soil borne diseases and increases soil fertility. 

• Green manuring and cover crops: increases the biological activity in the soil and can enhance the presence of beneficial organisms (but also of pests; therefore a careful selection of the proper species is needed). 

4) Use of balanced nutrient management: 

• Moderate fertilization: steady growth makes a plant less vulnerable to infection. Too much fertilization may result in salt damage to roots, opening the way for secondary infections. 

• Balanced potassium supply contributes to the prevention of fungi and bacterial infections.

5) Input of organic matter: 

• Increases micro-organism density and activity in the soil, thus decreasing population densities of pathogenic and soil borne fungi. 

• Stabilises soil structure and thus improves aeration and infiltration of water. 

• Supplies substances which strengthen the plant‘s own protection mechanisms. 

6) Application of suitable soil cultivation methods: 

• Facilitates the decomposition of infected plant parts. 

• Regulates weeds which serve as hosts for pests and diseases. 

• Protects the micro-organisms which regulate soil borne diseases. 

7) Use of good water management: 

• No water logging: causes stress to the plant, which encourages pathogens infections. 

• Avoid water on the foliage, as water borne disease spread with droplets and fungal disease germinate in water. 

8) Conservation and promotion of natural enemies: 

• Provide an ideal habitat for natural enemies to grow and reproduce. 

• Avoid using products which harm natural enemies. 

9) Selection of optimum planting time and spacing: 

• Most pests or diseases attack the plant only in a certain life stage; therefore it’s crucial that this vulnerable life stage doesn’t correspond with the period of high pest density and thus that the optimal planting time is chosen. 

• Sufficient distance between the plants reduces the spread of a disease. 

• Good aeration of the plants allows leaves to dry off faster, which hinders pathogen development and infection. 

10) Use of proper sanitation measures: 

• Remove infected plant parts (leaves, fruits) from the ground to prevent the disease from spreading. 

• Eliminate residues of infected plants after harvesting. 

Monitoring

Regular monitoring of pests, diseases and weeds is the basis for effective management. To be able to manage pests, diseases and weeds, information is needed on the specific pests, diseases and weeds present in the region, village or crop fields and the associated damage they cause. 

a) Typical signs of pest attacks on crop plants 

Most crop pests belong to the insects, mites and nematodes. However, in Africa, mammals (like elephants, monkeys or voles), and birds (like sparrows, starlings and crows) can also damage crops. 

Insect damage can be categorized by biting and chewing (e.g. caterpillars, weevils), piercing and sucking (e.g. aphids, psyllids) and boring (e.g. borer, leaf miner) species. Some are slow moving (e.g. caterpillars), fast moving (e.g. fruit flies), hidden (e.g. stem borer), or easy to observe (e.g. caterpillars, weevils). 

• Pest damage is often species-specific: leaves with holes or missing parts is an indication of caterpillar or weevil damage; curled leaves is an indication of aphids; damaged or rotten fruits are often caused by larvae of fruit flies; withering plants can also be caused by larvae of noctuids or the stem borer; and branches or trunks with holes may be an attack by lignivorous insects. 

• Mites are very small and cannot be seen with the naked eye. However, some mite species (spider mites) weave a typical tissue on attacked plant parts and can, therefore, easily be detected. If mites are present on plants, leaves and fruits become yellowish. 

• Nematodes are also very small and therefore, they are not easy to observe with the naked eye. They mostly attack plant roots; plants become yellow, wither and die. 

b) Typical signs of disease attacks on crop plants 

Most crop diseases are caused by fungi, bacteria or viruses. 

• Fungi cause the great majority, estimated at two-thirds, of infectious plant diseases. They include all white and true rusts, smuts, needle casts, leaf curls, mildew, sooty moulds and anthracnose. In addition, they are responsible for most leaf, fruit, and flower spots, cankers, blights, wilts, scabs, and root, stem, fruit, wood rots among many others. Parts of plants or the total crop plant can wither and die. 

• Bacteria cause any of the four following main problems. Some bacteria produce enzymes that breakdown the cell walls of plants anywhere in the plant. This causes parts of the plant to start rotting (known as ‘rot’). Some bacteria produce toxins that are generally damaging to plant tissues, usually causing early death of the plant. Others produce large amounts of very sticky sugars; as they travel through the plant, they block the narrow channels preventing water getting from the plant roots up to the shoots and leaves, again causing rapid death of the plant. Finally, other bacteria produce proteins that mimic plant hormones. These lead to overgrowth of plant tissue and form tumours. 

• Viruses mostly cause systemic diseases. Generally, leaves show chlorosis or change in colour of leaves and other green parts. Light green or yellow patches of various shades, shapes and sizes appear in affected leaves. These patches may form characteristic mosaic patterns, resulting in general reduction in growth and vigour of the plant. 

Careful and continuous monitoring of pest and disease levels during critical times of growth of a crop is the key to successful management. This can be done through regular scouting of the field by the farmer. It helps the farmer to intervene early enough before the pest and/or disease cause significant damage. 

Scouting (Figure 8-3) avoids unnecessary use of natural plant extracts. Limited use of these substances (e.g. pyrethrum, derris and tobacco) and oils is important as they also have negative effects on beneficial insects. If the application of these substances is not regulated, many pest predators and parasitoids may be killed as well. Over application of these substances may also lead to pests developing resistance. Therefore, scouting should be planned and done in an organised way. It is important to get a random sample that will be representative of the overall situation in the crop garden. Therefore, the scout (farmer) needs to observe and record any of the findings for better decision making. 

The most common pattern in pest and disease scouting programs involves walking along a predetermined zigzag or M-shaped route through a field. This pattern is commonly used because it is easy to teach, convenient to use, and ensures that all regions of the field are visited. To monitor insect pests, different traps can also be used (Figures 8-4 & 8-5). 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). 

• Fruit flies can be captured using bait traps. For example, PE-bottles with small holes can be half-filled with water, some cattle urine, fruit flesh or a small dead fish and a drop of detergent or soapy water. These bottles are then hung in trees and checked every three days. 

• Yellow plastic cards coated with adhesive are also good for trapping aphids and leafhopper. Yellow-orange plastic boards are appropriate for white flies, while blue cards are appropriate for thrips monitoring. 

• Light traps are especially needed where noctuids (e.g. moths, cutworms, African armyworm, and cotton bollworm) are a problem. Within crops attacked by cutworms, visual checks of caterpillars have to be done by dawn. 

Inducing Plant Resistance

Organic management and control of diseases is based strongly on strengthening the plant with the aim of enhancing its self-defence and thereby preventing the outbreak of the disease. One typical expression of induced resistance is the thickening of cell walls of the plant, which interferes with pathogen entering the cell. Another is the dying of the infested cell walls, which causes the pathogen to die also, and thus reduce its spread. 

There are several resistance-inducing substances that can be prepared by the farmers themselves. Some are plant extracts made from efeu (Hedera helix), rhubarb (Rheum rhabarbarum), or giant knotweed (Reynoutria sachalinensis). 

Compost teas and herbal teas are tools that can be made on the farm to enhance crop health and fertility, and to inoculate the leaves and roots with soluble nutrients, beneficial microorganisms, and beneficial metabolites (products that aid in the growth and development of plants). 

Compost extract is a fertilizer, but it also can induce plant resistance. For its preparation, mature compost is mixed with water at a ratio of 1:5 to 1:8 (vol/vol: 1L of compost for every 5 to 8 L of water) and well stirred before it is left to ferment for 3-7 days. One spoonful of molasses can be added per litre of liquid, because this enhances the development of the microorganisms. The fermentation site should be shaded and safe from the rain. After the fermentation period and before the application, the extract is well stirred, then filtered and diluted at a ratio of 1:5 to 1:10. 

Plant extracts can be obtained from stinging nettle, horsetail, comfrey, clover, seaweed and others, alone or mixed with marine by-products such as fish waste or fishmeal. Dilutions of 1:10 or 1:5 are used as foliar spray or soil drench. 

As a general rule it is recommended to apply compost extracts or teas every 7 to 10 days to prevent diseases from developing and as a way to enhance soil microorganisms. 

B. Curative Methods

Promoting and Managing Natural Enemies

The natural enemies of pests are other organisms (fungi, bacteria, viruses, insect predators, and insect parasitoids) which kill pest. Therefore, the organic farmer should try to conserve natural enemies already present in the crop environment and enhance their impact. This can be achieved with the following methods: 

• Minimize the application of natural pesticides (chemical pesticides anyway are not per-mitted in organic farming). 

• Allow some pests to live in the field which will serve as food or host for natural enemies. 

• Establish a diverse cropping system (e.g. mixed cropping). 

• Include host plants providing food or shelter for natural enemies (e.g. flowers which adult beneficial insects feed on). 

There are many possibilities to enhance floral diversity within and along the boundaries of crop fields (Figure 8-6): 

• Hedges - Use indigenous shrubs known to attract pest predators and parasitoids by offering nectar, pollen, alternative hosts and/or preys. Most flowering shrub species have this property. However, care should be taken to not use plant species known to be alternative hosts of pests or diseases. 

• Beetle banks - Strips of grass in the neighbourhood of crop fields harbour different natural pest enemy groups like carabids, staphylinid beetles and spiders. In order to lower the risk of weeds and plants known as host plants of crop pests and diseases, one to three native grass species can be sown in strips of 1 to 3 m. 

• Flower strips - Use indigenous flowering plant species known to attract predators and parasitoids by offering nectar, pollen, alternative hosts and/or preys. Most flowering plant species have this property. However, care should be taken not to use alternative hosts of pests or diseases. Three to five native flowering plant species can be sown in well-prepared seed beds, arranged in strips of 1 to 3 m on the boundary of the crop field. After flowering, seeds can be collected to renew the strip or create new ones. 

• Companion plants - Natural pest enemies can also be attracted by companion plants within a crop. These companion plant species can be the same as used in the flower strips. A few (1 or 2 per 10 m2) flowering companion plants within a crop serve as a ‘service station’ for natural pest enemies. 

Mechanical Control

Mass-trapping of pests is an additional control measure. They often can easily be built with cheap material. Some examples include: 

• Light traps can be used to catch moths such as armyworms, cutworms, stem borers and other night flying insects. Light traps are more efficient when placed soon after the adult moths start to emerge but before they start laying eggs. However, light traps have the disadvantage of attracting a wide range of insect species. Most of the attracted insects are not pests. In addition, many insects that are attracted to the area around the light traps (sometimes from considerable distances) do not actually fly into the trap. Instead, they remain nearby, actually increasing the total number of insects in the immediate area. 

• Colour and water traps can be used to monitor adult thrips. In some cases thrips can even be reduced by mass trapping with coloured (blue, yellow or white) sticky traps or water traps in the nursery or field. The colour spectrum of the boards is important for the efficacy of the sticky traps. Bright colours attract more thrips than darker ones. Sticky traps with cylindrical surfaces are more efficient that flat surfaces. They are best placed within a meter of crop level. Traps should not be placed near the borders of fields or near shelter belts. 

• Water traps should be at least 6 cm deep with a surface area of 250 to 500 cm2, and preferably round, with the water level about 2 cm below the rim. A few drops of detergent added to the water ensure that thrips sink and do not drift to the edges and escape. Replace or add water regularly. 

• Yellow sticky traps can be used to control whiteflies, aphids and leaf mining flies. Yellow plastic gallon containers mounted upside down on sticks coated with transparent car grease or used motor oil, is one such trap. These should be placed in and around the field at about 10 cm above the foliage. Clean and re-oil when traps are covered with flies. Yellow sticky boards have a similar effect. To use, place 2 to 5 yellow sticky cards per 500 m2 field area. Replace traps at least once a week. To make your own sticky trap, spread petroleum jelly or used motor oil on yellow painted plywood (size 30 cm x 30 cm). Place traps near the plants but faraway enough to prevent the leaves from sticking to the board. Note that the yellow colour attracts many insect. Note that the yellow colour attracts many insect species, including beneficial insects, so use yellow traps only when necessary. 

• Fruit bagging prevents fruit flies from laying eggs on the fruits (Figure 8-7). In addition, the bag provides physical protection from mechanical injuries (scars and scratches). Although laborious, it is cheap, safe and gives a more reliable estimate of the projected harvest. Bagging works well with melon, bitter gourd, mango, guava, star fruit, avocadoes and banana (plastic bags used). 

Recommendations to farmers regarding fruit bagging: Cut old newspapers to fruit size and double the layers, as single layers break apart easily. Fold and sew or staple the sides and bottom of the sheets to make a rectangular bag. Blow in the bag to inflate it. Insert one fruit per bag then close the bag and firmly tie the top end of the bag with sisal string, wire and banana fibre or coconut midrib. Push the bottom of the bag upwards to prevent fruit from touching the bag. For example, start bagging the mango fruit 55 to 60 days from flower bloom or when the fruits are about the size of a chicken egg. When using plastic bags (e.g. with bananas), open the bottom or cut a few small holes to allow moisture to dry up. Moisture trapped in the plastic bags damages and/or promotes fungal and bacterial growth that causes diseased fruits. Plastic also overheats the fruit. Bags made of dried plant leaves are good alternatives to plastic. 

Biological Control

Biological control is the use of natural enemies to manage populations of pests (such as ladybird beetles, predatory gallmidges, hoverfly larvae against aphids and psyllids) and diseases (Figure 8-8). This implies that we are dealing with living systems, which are complex and vary from place to place and from time to time. 

If populations of natural enemies present in the field are too small to sufficiently control pests, they can be reared in a laboratory or rearing unit. The reared natural enemies are released in the crop to boost field populations and keep pest populations down. There are two approaches to biological control through the release of natural enemies: 

• Preventive release of the natural enemies at the beginning of each season. This is used when the natural enemies could not persist from one cropping season to another due to unfavourable climate or the absence of the pest. Populations of the natural enemy then establish and grow during the season. 

• Releasing natural enemies when pest populations start to cause damage to crops (Figure 8-9). Pathogens are usually used in that way, because they cannot persist and spread in the crop environment without the presence of a host (“pest”). They are also often inexpensive to produce. 

Natural enemies that kill or suppress pests or diseases are often fungi or bacteria. They are called antagonists or referred to as microbial insecticides or bio-pesticides. Some commonly used antagonistic microbes are: 

• Bacteria such as Bacillus thuringiensis (Bt). Bt has been available as a commercial microbial insecticide since the 1960s. Different types of Bt are available for the control of caterpillars and beetles in vegetables and other agricultural crops, and for mosquito and black fly control. The best-known biocontrol agent used in field crops is the bacteria Bacillus thuringiensis var. kurstaki and Bacillus thuringiensis. var. aizawai against diverse lepidopteran pests, and the Bacillus thuringiensis var israeliensis against mosquitoes. Bacillus thuringiensis var kurstaki is produced in local factories in different African countries (e.g. South Africa, Kenya and Mozambique) and can be used against different pests (African armyworm, African bollworm, bean armyworm, beet armyworm, cabbage webworm, cabbage moth, cabbage looper, cotton leafworm, diamondback moth, giant looper, green looper, spiny bollworm, spotted bollworm, pod borers, tomato looper). 

• Viruses such as NPV (nuclearpolyhedrosis virus), effective for control of several cater-pillar pest species. Every insect species, however, requires a specific NPV-species. An example: The armyworm Spodoptera exigua is a major problem in shallot production in Indonesia. Since experiments showed that SeNPV (NPV specific for S. exigua) provided better control than insecticides, farmers have adopted this control method. Many farmers in West-Sumatra are now producing NPV on-farm. 

• Fungi that kill insects, such as Beauveria bassiana. Different strains of this fungus are commercially available. For example: strain Bb 147 is used for control of corn borers (Ostrinia nubilalis and O. furnacaiis) in maize, strain GHA is used against whitefly, thrips, aphids and mealybugs in vegetables and ornamentals. Several species of fungi can occur naturally in ecosystems. For example, aphids can be killed by a green or white coloured fungus during humid weather. 

• Fungi that work against plant-pathogens. Some examples include: Trichoderma sp., widely used in Asia for prevention of soil-borne diseases such as damping-off and root rots in vegetables (Figure 8-10). Some Trichogramma species against the African bollworm are bred in some laboratories in Africa against lepidopteran pests and aphids. A successful introduction of the neotropical parasitoid Apoanagyrus lopezi against the cassava mealybug (Phenacoccus manihoti) caused a satisfactory reduction of P. manihoti in most farmers’ fields in Africa. This is one of the success stories of classical biocontrol. 

• Entomopathogenic nematodes against different weevil species (e.g. Steinernema carpocapsae, Heterorhabditis bacteriophora) and to control soil insects like cutworms (Agrotis spp.) in vegetables. 

Natural Pesticides

Some plants contain components that are toxic to insects. When extracted from the plants and applied on infested crops, these components are called botanical pesticides or botanicals. The use of plant extracts to control pests is not new. Rotenone (Derris sp.), nicotine (tobacco), and pyrethrins (Chrysanthemum sp.) have been used widely both in small-scale subsistence farming as well as in commercial agriculture. 

Most botanical pesticides are contact, respiratory, or stomach poisons. Therefore, they are not very selective, but target a broad range of insects. This means that even beneficial organisms can be affected. Yet the toxicity of botanical pesticides is usually not very high and their negative effects on beneficial organisms can be significantly reduced by selective application. Furthermore, botanical pesticides are generally highly bio-degradable, so that they become inactive within hours or a few days. This reduces again the negative impact on beneficial organisms and they are relatively environmentally safe compared to chemical pesticides (Figure 8-11). 

The preparation and use of botanicals requires some know-how, but not much material and infrastructures. It’s a common practice under many traditional agricultural systems. Some commonly used botanicals are: 

• NEEM: Neem derived from the neem tree (Azadiracta indica) of arid tropical regions, contains several insecticidal compounds. The main active ingredient is azadiractin, which both deters and kills many species of caterpillars, thrips and whitefly. Both seeds and leaves can be used to prepare the neem solution. Neem seeds contain a higher amount of neem oil, but leaves are available all year. A neem solution loses its effectiveness within about 8 hours after preparation, and when exposed to direct sunlight. It is most effective to apply neem in the evening, directly after preparation, under humid conditions or when the plants and insects are damp. There exist different recipes for the preparation of a neem solution. 

Recommendation to farmers about preparation of neem pesticides: In Ghana, Africa, neem seed kernel extract was tested on cabbage in Farmer trainings and had a very good repelling effect on diamondback moth (Plutella xylostella). Here is their recipe: Pound 30 g neem kernels (that is the seed of which the seed coat has been removed) and mix it in 1 L of water. Leave it overnight. The next morning, filter the solution through a fine cloth and use it immediately for spraying. It should not be further diluted. 

Neem cake (ground neem seed or neem kernel powder) has also a considerable potential as a fertilizer and at the same time it will hinder nematode attacks of the crop roots (e.g. tomato). Put neem cake in the planting pit (200g per m2) and mix it with substrate. The neem cake will repel and even kill nematodes and other root pests. Insecticidal agents (azadirachtin) will be translocated to above-ground parts of the plant and help to get rid of pests there (Figure 8-12). 

• PYRETHRUM: Pyrethrum is a daisy-like Chrysanthemum. In the tropics, pyrethrum is grown in mountain areas because it needs cool temperatures to develop its flowers. Pyrethrins are insecticidal chemicals extracted from the dried pyrethrum flower. The flower heads are processed into a powder to make a dust. This dust can be used directly or infused into water to make a spray. Pyrethrins cause immediate paralysis to most insects. Low doses do not kill but have a “knock down” effect. Stronger doses kill. Pyrethrins break down very quickly in sunlight so they should be stored in darkness. Both highly alkaline and highly acid conditions speed up degradation so pyrethrins should not be mixed with lime or soap solutions. Liquid formulations are stable in storage but powders may lose up to 20% of their effectiveness in one year. 

Recommendation to farmers about preparation of Pyrethrum pesticides: Pyrethrum powder is made with dried ground flowers. Use pure or mix with a carrier such as talc, lime or diatomaceous earth and sprinkle over infested plants. To make liquid pyrethrum extract (mix 20 g pyrethrum powder with 10 L of water), add soap to make the substance more effective. Strain and apply immediately as a spray. For best effects this should be applied in the evening. Pyrethrum can also be extracted by alcohol. 

• CHILLIPEPPER: Chillies and capsicum pepper have both repellent and insecticidal effects. 

Recommendations to farmers on preparation of chilli pesticides: To make the chilli extract grind 200 g of chillies into a fine dust, boil it in 4 L water, add another 4 L of water and a few drops of liquid soap. This mixture can be sprayed against aphids, ants, small caterpillars and snails. 

• GARLIC: Garlic has antifeedant (insect stop feeding), insecticidal, nematicidal and repellent properties. Garlic is reportedly effective against a wide range of insects at different stages in their life cycle (egg, larvae, adult) (Figure 8-13). This includes ants, aphids, armyworms, diamondback moth, whitefly, wireworm and termites. Garlic is non-selective, has a broad-spectrum effect and can kill beneficial insects as well. Therefore, it should be used with caution. 

Recommendations to farmers on preparation of garlic pesticides: To make the garlic extract, grind or chop 100 g garlic into 0,5 L of water. Allow mixture to stand for 24 hours, add 0,5 L of water and stir in liquid soap. Dilute at 1:20 with water and spray in the evening. To improve efficacy, chilli extract can be added. 

There are many other extracts of plants known to have insecticidal effects like tobacco (Nicotiana tabacum), yellow root (Xanthorhiza simplicissima), fish bean (Tephrosia vogelii), violet tree (Securidaca longepedunculata), and nasturtium (Nasturtium trapaeolum) which are traditionally used to control pests in Africa. 

Anise, chillies, chives, garlic, coriander, nasturtium, spearmint and marigold are plants known to have a repellent effect on different pest insects (aphids, moths, root flies, etc.) and can be grown as intercrop or at the border of crop fields (Figure 8-14). Marigold is especially known to deter root nematodes, while neem cake is known to deter mice. 

Precautions to Farmers Regarding Use of Plant Extracts

-Despite being “natural” and widely used in agricultural systems, some botanicals may be dangerous for humans and they can be very toxic to natural enemies. Nicotine for example, derived from the tobacco plant, is one of the most toxic organic poisons for humans and other warm-blooded animals. Pyrethrins are not poisonous for humans and warm-blooded animals. However, human allergic reactions are common. It can cause rash, and breathing the dust can cause headaches and sickness. 

- Before a new botanical pesticide is applied in a large scale, its effect on the ecosystem should be tested in a small field experiment. Do not just use botanical pesticides as a default option! First understand the ecosystem and how botanicals influence it! 

- Do not have direct skin contact with the crude extract during the process of preparation and application. 

- Contact with plant extracts should be avoided in the eyes. 

- Make sure that you place the plant extract out of reach of children during storage. 

- Wear protective clothing (eyes, mouth, nose and skin) while applying the extract. 

- Wash your hands after handling the plant extract. 

Besides extractions of plants, there are some other natural pesticides, which are allowed in organic farming. Although some of these products have limited selectivity and are not fully biodegradable, there are situations, when their use is justified. However, in most cases, the desired effect is best reached in combination with preventive crop protection methods. Some examples are: 

• Soft soap solutions: against aphids and other sucking insects. 

• Light mineral oil: against various insect pests (harms natural enemies!). 

• Sulphur: against spider mites (harms natural enemies!). The acaricidal effect of sulphur is best at temperatures above 12° C. However, sulphur has the potential to cause plant injury in dry hot weather (above 32° C). It’s also incompatible with other pesticides. Sulphur should not be used together or after treatments with oil to avoid phytotoxicity. 

• Plant ashes: wood ashes from fire places can be efficient against ants, leaf miners, stem borers, termites and potato moths. Ash should be dusted directly on pest colonies and infested plant parts. The ash will dehydrate the soft bodied pests. Wood ashes are often used when storing grains to deter storage pests such as weevils. In addition, ashes are used against soil borne diseases. 

Other Practices for Disease Control Include the Use of:

• Sulphur is mostly used against plant diseases like powdery mildew, downy mildew and other diseases. The key to its efficacy is that it prevents spore germination. For this reason, it must be applied prior to disease development for effective results. Sulphur can be applied as a dust or in liquid form. It is not compatible with other pesticides. Lime-sulphur is formed when lime is added to sulphur to help it penetrate plant tissue. It is more effective than elemental sulphur at lower concentrations. However, the odour of rotten eggs usually discourages its use over extensive fields. 

• Bordeaux mixture (Copper sulphate and lime) has been successfully used for over 150 years, on fruits, vegetables and ornamentals. Unlike sulphur, Bordeaux mixture is both fungicidal and bactericidal. As such, it can be effectively used against diseases such as leaf spots caused by bacteria or fungi, powdery mildew, downy mildew and various anthracnose pathogens. The ability of Bordeaux mixture to persist through rains and to adhere to plants is one reason it has been so effective. Bordeaux mixture contains copper sulphate, which is acidic, and neutralized by lime (calcium hydroxide), which is alkaline. 

Recommendations to farmers on preparation of Bordeaux mixture: Bordeaux mixture comes in several formulations. One of the most popular, effective and least phytotoxic formulations for general use is the following formulation: Mix 90 g of blue copper sulphate with 4,5 L of water (in a non-metallic container). In another non-metallic container, mix 125 grams of slaked lime with 4.5 litres of water. Stir both, mix both solutions, and stir again. This formulation was developed in recognition of the fact that copper, like sulphur, is phytotoxic and that the level of toxicity is related to the age of plant tissue being treated. Application of Bordeaux during hot weather (above 85° F or 30° C) may cause yellowing and leaf drop. Additionally, leaf burn can occur if it rains soon after a Bordeaux application. Care should be taken when applying this fungicide to young, tender leaves of fruit trees. Do not apply Bordeaux mixture to corn or sorghum, which are described as copper-sensitive plants. There are other, very common and cheap copper formulations available: copper hydroxide and copper oxychloride. They are accepted in organic farming provided that the number of applications is strictly followed and a proper soil amendment is observed to prevent copper accumulation in the soil. 

• Acidic clays have a fungicidal effect due to aluminium oxide or aluminium sulphate as active agents. They are used as an alternative to copper products but, are often less efficient. 

• Milk has also been used against blights, mildew, mosaic viruses and other fungal and viral diseases. Spraying every 10 days with a mixture of 1 L of milk to 10 to 15 L of water is effective. 

• Baking soda has been used to control mildew and rust diseases on plants. Spray with a mixture of 100 g of baking or washing soda with 50 g of soft soap. Dilute with 2 L of water. Spray only once and leave as long gaps as possible (several months). Do not use during hot weather and test the mixture on a few leaves because of possible phytotoxic effects. 

Many plant extracts are known to have fungicidal effects. Onion and garlic are effective against many diseases such as mildew and fungal and bacterial diseases. Mexican and African marigold act as a crop “strengthener” to help potatoes, beans, tomatoes and peas resist fungal diseases such as mildew. The leaves of pawpaw (Carica papaya) and sweet basil have a general fungicidal effect. Many other plant species are known to have fungicidal effects. Traditional knowledge might be of help to amend the range of plant extracts in each region. 

REFERENCES 

FiBL. 2011. African Organic Agriculture Training Manual – Pest, Disease and Weeds. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick 

IFOAM. 2003. Pest and disease in organic management. A Latin American perspective. Compiled and edited by Dr. Dina Foguelman. 

IFOAM. 2003. Training Manual for Organic Agriculture in the Tropics. Edited by Frank Eyhorn, Marlene Heeb, Gilles Weidmann, p 156-183, http://www.ifoam.bio/ 

SOURCES 

FiBL / IFOAM 

ON TECA 

Pest and Disease Management in Organic Agriculture: http://teca.fao.org/read/8372