PLANT DISEASE MANAGEMENT PRACTICES OF TRADITIONAL FARMERS

Thurston, H. David. 1990. Plant disease management practices of traditional farmers. Plant Disease 74:96-102.

The science of plant pathology has an important role in the future success of programs and policies designed to increase and sustain food production. If plant pathologists are to be effective in addressing the problems of food production in developing countries, the agricultural systems of traditional farmers in those countries must be thoroughly understood so that researchers can appropriately address problems in the context of farmer's systems, and efficient, proven techniques can be disseminated to other farmers. Traditional knowledge can be overvalued or romanticized, but it is a mistake to despise or ignore it. Far too many giant development projects in developing countries have failed dismally, often with serious ecological consequences, because sufficient understanding of traditional agriculture was lacking. Today there are many concerns about modern agriculture because it is highly energy-intensive, has a narrow genetic base, and attainment of increasingly high yields and efficiency may lead to monoculture and overproduction. Sometimes destructive erosion (Fig. 1), pollution, and excessive pesticide residues result. It is time to reexamine the potential for traditional agriculture to contribute to an improved, sustainable system for production agriculture.

Small farms constitute a most important element in the agriculture of developing countries. Although figures vary somewhat, the following are typical. In 1970 holdings of less than 1 ha of land constituted 33% of all holdings in developing countries (13). The average size of agricultural holdings in those developing countries was 6.6 ha. According to the National Research Council (24) "Half of the world's population is engaged in agriculture, the vast majority in the tropics and subtropics." Small farmers till 65% of the world's arable land (17), and 70% of the world's poor live in rural areas and engage primarily in subsistence agriculture (30). Poverty and socio-economic insecurity characterize the lives of many rural people, and are exacerbated among the vast number of small or traditional farmers who often have few resources beyond the labor of their families.

Traditional agriculture usually is associated with primitive agricultural systems or preindustrial peasant agriculture. Traditional farming usually is based on agriculture that has been practiced for many generations. Most small farms in the developing world utilize agricultural

practices that are to some degree traditional, but many small farmers cannot be characterized as traditional. The agricultural activities of traditional farmers are associated closely with their culture, as Schultz (28) explained:

Among primitive and peasant societies, cultural values and attitudes, beliefs and behavior patterns often play an equal or greater role than economic considerations when deciding whether to accept or not new production practices. Kinship obligations, peer group pressure, fatalistic beliefs, negative social sanctions regarding accumulations or surplus, individuality, caste differences and constraints, and the perpetuations of common traditional values through family socialization all represent serious challenges to the foreign change agent.

Why Should Plant Pathologists Study Traditional Agriculture?

Anthropologists, archaeologists, ethnobotanists, and geographers—and, to a lesser degree, ecologists, economists, and sociologists try to understand traditional agriculture. Unfortunately, plant pathologists and others in the "hard agricultural sciences" seldom take courses in these disciplines or read much of their literature, with the occasional exception of ecology and economics. Likewise, professionals in nonagricultural disciplines do not often read agricultural literature or take courses in production sciences. Consequently, each discipline develops a separate language that is often unintelligible to outsiders.

Today the rhetoric in agriculture centers on "sustainability," LISA (low-input sustainable agriculture), and biotechnology. Although these terms are vague and all-encompassing, they strongly affect current funding and research directions. There is debate among economists, some strongly advocating continual growth in the world's economy and others, more ecologically minded, believing that sustainable development should be the goal of mankind. For example, Brown and Shaw (4) stated:

In a world where the economy's environment support systems are deteriorating, supply-side economics—with its overriding emphasis on production and near blind faith in market forces will lead to serious problems.

Rapid economic growth rarely can be achieved without jeopardizing ecological sustainability. Some economists, such as Daly ( 10), argue for a steady-state economy rather than an expanding one. Schultz (29), in his classic study, Transforming Traditional Agriculture, suggested that a country dependent on traditional agriculture is inevitably poor. More recently, Ruttan (27), commented:

Traditional agricultural systems that have met the test of sustainability have not been able to respond to modern rates of growth in demand for agri cultural commodities. A meaningful definition of sustainability must include enhancement of agricultural productivity. At present the concept of sustainability is more adequate as a guide to research than to farming practice.

Do such conclusions by eminent economists suggest that nothing is to be gained by a study of traditional agriculture? I think not, and I doubt if such a conclusion is intended. If modern scientific agriculture is to help alleviate world hunger and starvation, caused in part by population pressure that results in environmental degradation, sustainable agricultural practices of traditional farmers in developing countries must be thoroughly understood and compared with alternative, new practices. If changes in traditional systems are necessary or needed, a thorough understanding of these systems is imperative as a first step before changes are initiated.

Often, traditional farming practices provide effective and sustainable means of disease control. Traditional practices and cultivars (landraces) have had a profound effect on modern agriculture, and most of our present practices and cultivars evolved from these ancient techniques and plant materials. The agricultural systems of traditional farmers, including their disease control practices, are in danger of being lost as agriculture modernizes. Those practices should be studied carefully and con served before they disappear.

Wilken (34) suggested several additional reasons to study agricultural activities of traditional farmers. First, some traditional farming systems have excellent records in resource management and conservation. Systems that have lasted for thousands of years surely justify serious study, although practices and systems developed by traditional farmers are not always successful. Eckholm (12) wrote: "The littered ruins and barren landscapes left by dozens of former civilizations remind us that humans have been undercutting their own welfare for thousands of years." Perhaps we can learn from their mistakes. The study of successful systems is especially important as petroleum, water, and other resources become scarce.

Second, although many traditional practices are labor-intensive, this aspect may be important and attractive in societies having an abundance of labor and chronic unemployment. Although traditional technology may be of little interest to scientists and Western businessmen, it represents the labor of millions of humans and the management of millions of hectares, and even small improvements would be significant for the world as a whole. For planners in developing countries, traditional methods have some advantages over modern agricultural techniques. For example, capital and technological skill requirements of traditional technologies are generally low, and adoptions often re quire little restructuring of traditional societies.

Finally, Wilken (34) suggested that because modern agriculture has developed primarily in temperate regions, these practices may have unexpected and undesirable impacts in developing countries, especially those found in the tropics.

Traditional Farmers' Knowledge

The knowledge of traditional farmers is often impressively broad and comprehensive. A few examples illustrate this point. The agricultural knowledge of the Hanunoo, a mountain tribe of Mindoro in the Philippines, is amazingly wide, accurate, and practical (9). They distinguish 10 basic and 30 derivative soil and mineral categories and understand the suitability of each for various crops, as well as the effects of erosion, exposure, and overfarming. Their more than 1,500 useful plant types include 430 cultigens, and they distinguish minute differences in vegetative structure.

Mayan Indians in Mexico have their own comprehensive plant classification system. Berlin et al (2), describing the Mayan (Tzeltal) taxonomic system, stated that 471 mutually exclusive generic taxa were established as legitimate Tzeltal plant groupings.

Much of the literature on traditional agriculture is anecdotal rather than experimental, to the distress of scientists who believe that only information obtained by scientific methods is of real value. Also, traditional agriculture often includes a mixture of superstitious, religious, and magical beliefs. Some beliefs are of no practical value, but others are linked to sound agricultural practices. Ayamara Indians near Lake Titicaca in Peru were interviewed regarding their knowledge of plant diseases (19). They believe that plant diseases are caused by halos around the sun, certain phases of the moon, drought, hail, lightning, excessive humidity, fog, frost, dew, and the use of horse or cattle manure. Entrance into a field of animals in heat, pregnant or menstruating women, drunk men, or people or animals when dew is on the ground also is thought to cause disease. The Ayamara dust their crops with ashes, spray them with fish water, place branches of muna (Minthostachys sp., a traditional insect repellent) between plants, and rogue diseased plants. To control diseases they practice careful seed selection and crop rotation and do not plant when the moon is full or the sun has a halo. Several of these practices would reduce disease incidence and severity, but clearly such activities are a mixture of the useful and the useless.

Traditional Farmers Practices for Managing Plant Diseases

Archeologists believe that humans began crop production perhaps 10,000 years ago. Some ancient farmers developed sustainable agriculture practices that allowed them to produce food and fiber for thousands of years with few outside inputs, but other traditional systems were not so successful. Many of their successful practices have been forgotten or abandoned in developed countries, but still are used by many traditional, subsistence, or partially subsistence farmers in developing countries. Although considerable evidence shows that traditional farmers experiment and innovate, most useful traditional methods of agriculture probably were developed empirically through millennia of trial and error, natural selection, and keen observation. These practices often conserve energy and maintain natural resources. Traditional farming systems, especially in the tropics, frequently resemble natural ecosystems. This, and their high level of diversity, appear to give them a high degree of stability, resilience, and efficiency. Traditional farmers are not always interested in the highest yields, but are concerned more with attaining stable, reliable yields. They minimize risks and seldom take chances that may lead to hunger, starvation, or loss of their land.

Most practices for disease management practices used by traditional farmers in developing countries consist of cultural controls, yet little information is available in an easily accessible or understandable form on the cultural practices of traditional farmers. The book by Palti (1981) - Cultural Practices and Infectious Crop Diseases - is an excellent source of information on cultural practices for the management of plant diseases, but emphasizes primarily "modern" agriculture. Some of the practices traditional farmers use include the following: alterations of plant and crop architecture, biological control, burning, crop density, depth of planting, diversity, fallow, flooding, mulches, multiple cropping, no-till, organic amendments, raised beds, ridges, rotation, sanitation, shade, tillage, time of planting, and other cultural practices for the management of plant diseases. Most, but not all of these practices are sustainable. The disease resistance of traditional cultivars selected for millennia is also highly important.

The quantity of pesticides used by traditional farmers is very small, but their expectations for pesticides are often unrealistically high. For example, Rosado and Garcia (1986) interviewed 59 farmers in Tabasco, Mexico relative to their control methods for web blight of beans. Although they used several cultural methods of control, 100% of the farmers interviewed said they were expecting a chemical solution to the problem.

Examples of a few of the practices used by traditional farmers are given below.

Examples of Cultural Practices Used by Traditional Farmers to Manage Plant Disease

1. Mulching (Tapado) for Management of Web Blight of Beans

Web blight of beans (Phaseolus vulgaris L.) is caused by the fungus Thanatephorus cucumeris (Frank) Donk (anamorph - Rhizoctonia solani Kühn). In the warm and humid lowlands of the tropics, web blight is possibly the single most destructive disease of beans and can cause rapid defoliation and sometimes complete crop failure. In 1980, an epidemic of web blight that occurred in the Guanacaste region in northern Costa Rica, resulted in a 90% reduction in bean yields (Galindo 1982). This loss occurred on beans planted using clean cultivation.

The main sources of inocula which can initiate infection in the hot, humid tropics of Costa Rica are mycelial fragments and sclerotia in the soil (Galindo et al. 1983a). Inoculation of beans occurs mainly by splashing of rain drops containing infested soil. Traditional farmers in many areas of Costa Rica grow beans using a system called in Spanish "frijol tapado" which in English means "covered beans". The frijol tapado procedure consists of broadcasting bean seeds into carefully selected weeds, then cutting and chopping the weeds with a machete so the broadcast bean seeds are covered with a mulch of weeds. The fields selected for tapado are generally occupied by broadleaf weeds and certain grasses that will not regrow after they are cut. A semi-determinate type of bean, between a bush and a climbing bean, is used. The beans grow through the mulch and eventually cover it. This combination of mulch and bean plants effectively prevents weed growth and appears to conserve soil moisture. In addition, the mulch prevents soil splashing, which was found in a Costa Rican study by Galindo et. al (1983b) to be the most important source of inoculum causing web blight.

In the absence of web blight, yields in fields under the frijol tapado system are generally lower than those in fields planted in drilled rows with clean cultivation. For this reason, some in Central America oppose continuation of the frijol tapado system; however, on small farms in Costa Rica, most of the beans currently produced are grown by the frijol tapado system. Small farmers persist in using the system because its risk is low, investment in labor is small (primarily to cut weeds), and there is always some yield even when prolonged periods of rain produce conditions which allow T. cucumeris to destroy bean yields under the clean cultivation system. Covered beans can be planted on steep hillsides without erosion problems. Furthermore, once planted tapado fields require little if any maintenance, so farmers may safely leave a planting while they harvest coffee or engage in other off-farm activities. Tapado fields require less labor and, although they give a low productivity per land unit, they give a higher return to labor per work day than clean-cultivated beans.

2. The Use of Raised Fields, Raised Beds, Ridges, and Mounds for Plant Disease Management

Raised fields, raised beds, ridges, and mounds were used widely for millennia by traditional farmers in geographically separated areas of tropical America, Asia, and Africa. Raised bed systems of agriculture with striking similarities evolved in these widely separated areas. Drainage, fertilization, frost control, and irrigation were among important considerations in these systems, but planting in soil raised above the surrounding area is also an important disease management practice, especially for soil pathogens. Specialized traditional raised bed systems such as chinampas, tablones, and waru waru, are still in use today. How much the management of plant diseases and other pests entered into the evolution of these systems is unknown. Raised beds are used extensively today in Asia, often after a rice crop. Flooding for rice culture destroys many soilborne pathogens, and subsequently vegetables and other crops can be grown on raised beds with fewer disease problems. Similar practices are widely used in tropical Africa after rice. Most vegetables in Asia are grown on raised beds. Mounds, ridges, and raised beds are used worldwide today by indigenous farmers for root and tuber crops, and their use often reduces root rot problems. The widespread use of raised beds in both modern and traditional agriculture today testify to their value.

While searching for practices traditional farmers use to control plant diseases, I have read innumerable articles and books by anthropologists, many of which are in-depth, excellent, fascinating studies of traditional peoples and their agriculture. Anthropologists occasionally mention insects in their studies, but they almost never mention plant diseases. An example is the study by Waddell (1972) who describes the "mound builders" of New Guinea, a group of traditional farmers who have worked out a sustainable system of agriculture by cultivating sweet potatoes on mounds, producing high yields for long periods of time, with no apparent disease problems. The mounds permit continuous cultivation without fallow. Sites in the study area are known to have been in continuous cultivation since 1938 (when Europeans first made contact with these people). When a new mound is prepared, approximately 20 kg of old sweet potato vines, sugar cane leaves, and other sources of vegetation are placed in the center of the mounds. When this material begins to decompose, the mound is closed with soil and subsequently planted with sweet potato cuttings. According to Waddell (1972) the two to three harvests obtained per year total 19 tons/ha of sweet potato roots. The only reference to disease I could find in this excellent, detailed treatise is the following:

It [sweet potato] is also less susceptible to disease than taro (Colocasia esculenta), which has suffered greatly in recent years from the depredation of the taro beetle (Papuana spp.) and the virus Phytophthora colocasiae in various parts of the Pacific.

The above error regarding the nature of Phytophthora (an oömycete - not a virus) perhaps illustrates the level of knowledge and interest that most anthropologists, archaeologists, economists, geographers, and sociologists have achieved regarding disease problems. Rarely are diseases even mentioned in their published studies of indigenous or traditional agriculture. On the other hand, few plant pathologists study or reference work in the above disciplines.

Probably the best known raised field system is the chinampas or "floating gardens" of the Valley of Mexico which the Spanish conquistadors erroneously thought floated. When the Spanish arrived in Mexico in 1521 and entered the capitol of the Aztec civilization located on an island in Lake Texcoco, they were amazed by the immense areas in chinampas. They are still farmed near Mexico City at Xochimilco. The high productivity of chinampas has been cited as a major factor that allowed the Aztecs to grow from a small tribe to a powerful group that essentially dominated most of Mexico when the Spanish arrived. Armillas (1971) has estimated that the Aztec chinampas may have fed 100,000 people.

The chinampas currently found at Xochimilco constructed in the shallow Lake Texcoco are generally rectangular in shape and separated by canals (Coe 1964). The surface of the chinampas is usually several feet above the water level in the canals. Two operations build up the chinampas. First, mud rich in nutrients from the bottom of the canals is dredged up using a hand tool and spread on the chinampa surface. This maintains the canals and enriches the chinampas. In addition, aquatic weeds, animal manure, (and in the time of the Aztecs, human waste) is also spread on top. A wide variety of crops were grown by the Aztecs on the chinampas and many diverse crops are still seen today. Most crops are first planted in seedbeds prepared by spreading a layer of mud over vegetation, cutting it into small rectangular blocks called chapines, and planting a seed in each chapín. The chapines are subsequently transplanted to the soil of the chinampas, thus giving the crops a good start. Cropping can continue year round, even through the dry season. The chinampa system, in sum, allows continuous cropping made possible by sophisticated water control, multiple cropping, high levels of organic material and nutrients periodically added to the system, and transplanting of healthy, selected seedlings (chapines) with strong root systems. The diversity of crops grown on traditional chinampas also may have contributed to the success of the system by inhibiting the spread of disease.

Lumsden et al. (1987) studied chinampa soils relative to disease. They compared relative levels of damping-off disease caused by Pythium spp. on seedlings grown in soils from the chinampas versus those grown in soils from modern systems of cultivation near Chapingo, Mexico. Disease levels were lower in the chinampa soils, and when inoculum of Pythium aphanidermatum (Edson) Fitzpatrick was introduced, the fungus was suppressed by chinampa soils. They concluded:

In the chinampa agroecosystem, apparently a dynamic biological equilibrium exists in which intense management, especially of copious quantities of organic matter, maintains an elevated supply of organic nutrients and calcium, potassium and other mineral nutrients which stimulate biological activity in the soil. The elevated biological activity , especially of known antagonists such as Trichoderma spp., Pseudomonas spp., and Fusarium spp., can suppress the activity of P. aphanidermatum, other Pythium spp. and perhaps other soilborne plant pathogens".

More recently Zuckerman et al. (in press), in a cooperative study between scientists from Mexico and the U.S.A., also studied suppression in chinampa soils, but of plant parasitic nematodes rather than fungi. The authors pointed out that the high organic content of the soil is probably responsible in part for the relatively few nematodes in chinampa soils, but they also found nine organisms which had antinematodal activity. Their results were summarized as follows:

Soil from the Chinampa agricultural system in the Valley of Mexico suppressed damage by plant parasitic nematodes in greenhouse and growth chamber trials. Sterilization of the chinampa soils resulted in a loss of the suppressive effect, thereby indicating that one or more biotic factors were responsible for the low incidence of nematode damage. Nine organisms were isolated from chinampa soil which showed antinematodal properties in culture. Naturally occurring populations of plant- parasitic nematodes were of lower incidence in chinampa soils than in Chapingo soil."

Little is reported on the utility of raised fields in general for plant disease control; but there is little doubt that, in addition to their obvious irrigation, drainage, and agronomic value, disease management is often an additional benefit.

3. Cultural Practices in Mexico

The cultivation of maize gives an excellent illustration of the knowledge of traditional farmers and the complexity of their cropping systems. Mexican farmers near Puebla have been growing maize for perhaps 7,000 years, and thus their accumulated experience with the crop is considerable. First, most maize cultivars they grow are native landraces, as they are best adapted to the area and perform better than those available from CIMMYT (International Maize and Wheat Improvement Center) or the government (CIMMYT 1974). Here, maize is not grown in a monoculture, but together with squash and climbing beans. Economic yields and nutritional benefits are often superior with this multiple cropping system. Other benefits, according to Van Rheenen et al. (l981), include cultural control of the major bean diseases effected by growing beans in association with maize.

Imagine a maize field near harvest time close to Puebla, Mexico. An observer from the temperate corn belt might not approve of the appearance of the Mexican maize field, because near harvest time the field is choked with weeds. Mexican scientists have observed that farmers often weed their fields for about 90 days and then let the weeds grow as, under their conditions, little additional yield results from additional weedings. Furthermore, the weeds are used as fodder for animals in the dry season. Farmers have noted that there is far less wind and water erosion when weeds are present in a field. Ing. Efraim Hernandez X. (personal communication) noted that about 40 of the weed species found in Mexico's corn fields are also eaten as pot herbs by small farmers. In fact, some are allowed to go to seed in order to encourage a good seed set. Traditional farmers in Tabasco, Mexico do not have a word for "weed" in their vocabulary, but use a concept of "good" and "bad" plants (mal y buen monte), and the same plant may be either good or bad depending on where and when it is found (Chacón and Gliessman 1982). Thus, the weeds in their fields are not necessarily there because of poor farming practices.

In addition, the ears in the maize field have been bent downwards (doblando la mazorca), so the tip of the ear hangs down. Farmers have found that by using this practice the grain is protected from rain, it dries better on the plant in the sun than in storage, is less accessible to rats and birds, and reaches such a low moisture content that storage deterioration is greatly reduced. Montoya and Schieber (1970) sampled maize in Guatemala which had been bent down and found only 1.0% of the grain damaged by fungi compared to a mean of 14.5% of the grain damaged from similar maize plants whose ears had not been bent down.

Weatherwax (1954) found references describing the agriculture of the Aztecs in Mexico using the "doblando la mazorca" practice in the 16th Century. He cites Friar Sahagún (who went to Mexico in 1529) as listing the following work for an Aztec maize farmer :

The duties of the farmer are: to fill up the holes where maize is planted, to heap the earth around the young plants, to eradicate the grass, to thin out the plants and remove the small ears and ear suckers and tillers so that the plants will grow well, to take off the green ears at the proper time, to break over the stalks at maturity and harvest the corn when it is dry, to husk the ears and knot the husks together or fasten the ears together in strings, to carry the harvest home and store it, to break up the stalks which have no ears and to shell the grain and clean it in the wind.

Although Mexican maize fields may look haphazard and poorly attended to observers from the temperate zones, the Mexican traditional farmers have sound reasons for their practices, and perhaps agricultural scientists can learn from them.

In 1980 I made a trip to Mexico with a group of students. One of our visits was to the farm of a traditional farmer near Puebla, Mexico. As the students were talking to him I noticed a basket containing beans near where I was sitting. I separated out 17 different types of beans from the pot and later found out that the collection included common beans (Phaseolus vulgaris L.), lima beans (P. lunatus L.), and scarlet runner beans (P. coccineus L.). The farmer said he grew all of them on his 1.5 ha farm. When asked why he grew so many varieties, he noted that some years it was wet and some years it was dry. Some varieties did better in wet years and some better in dry years. Some years insects attacked, and some varieties survived while others did poorly. In common with anthropologists and geographers he mentioned nothing about diseases. His wife preferred certain varieties for specific cooking purposes. The diversity of his many varieties probably gave him some protection against various insects, diseases, other biological stresses, and the vagaries of climate.

4. Rotation/Fallow for Management of Potato Cyst Nematodes by the Incas of Peru

In Peru, before the arrival of the Spanish, farmers of the Inca empire used fallow and rotations for potatoes according to Garcilaso de la Vega (de la Vega 1966). Today, long rotations for potatoes of 6 to 8 years are used by isolated communities in the Andes. Brush (1977) describes a typical rotation/fallow in an isolated mountain valley of Peru as follows:

A third stratagem used by Uchucmarcan peasants to assure a potato harvest is to cultivate fields for only one to three years before returning them to a long fallow of eight or more years. Farmers usually sow potatoes in the first year and other Andean tubers - oca (Oxalis tuberosa), mashua (Tropaeolum tuberosum), and ullucu (Ullucus tuberosum) - for one or two subsequent years. The long fallow period lowers subsistence risk in two ways: by reducing the amount of erosion and soil loss and by killing disease vectors such as nematodes and fungi, which remain in the soil and depend on the continued potato planting to survive.

Brodie (1984) indicated that nonhosts play an important role in management of the potato cyst nematode, and stated that nematode densities in the soil decline 30-50% annually when a nonhost crop is grown. Both mashua (Tropaeolum tuberosum R. & P.) and tarwi (Lupinus mutabilis Sweet) are common in rotations with potatoes and in addition both contain nematicidal compounds. Thus, the strategy of the Peruvian farmers to rotate with nonhosts of the potato cyst nematode is a sound nematode management practice.

Through centuries of trial and error the Incas and their predecessors must have learned that long rotations/fallows gave the best potato crops. We now know that the destructive potato cyst nematodes (Globodera pallida (Stone) Behrens; G. rostochiensis (Wollenweber) Behrens) are present in extremely high population levels in most potato-growing areas of the Peruvian Andes since in many areas the traditional long rotation/fallow period is not used. Studies in Rothamsted, England demonstrated (Jones 1970, Jones 1972) that a 7-year fallow reduces potato cyst nematode populations below their economic threshold so that a profitable crop can be grown. To the Spanish the Inca fallow/rotation seemed to be a senseless custom. Long fallow/rotation periods were abandoned, and serious losses due to the potato cyst nematodes have occurred in Peru ever since their abandonment. Thus, the Inca fallow/rotation had a sound practical basis and was an effective disease management practice.

5. Manipulations of Architecture - Multi-story Cropping in Household Gardens

Michon et al. (1983) discusses the village gardens in West Java which were first described in the tenth century. Small in size (often less than 0.1 ha) they nevertheless are important in feeding the dense populations of Java. Such gardens may constitute from 15 to 50% of a village's land available for cultivation. Over 70 plant species are grown in the gardens, including plants for food, timber, firewood, medicinal plants, and ornamentals.

The striking diversity of species used (some villages are reported to use up to 250 crop species) has important implications for the importance and severity of diseases in the gardens. Pesticides seldom are used or needed. Animals of various kinds are also important constituents of the gardens, and they graze, feed, or are fenced within the garden and fed with products of the gardens. Their waste contributes to nutrient cycling in the gardens. Fish found in ponds in some gardens are fed vegetable and human waste.

Michon et al. point out that in these small gardens each plant receives individual care. The gardens imitate the tropical forest ecosystems of Java. Each plant has its "place" in the garden and the physical arrangement (horizontally and vertically) is sophisticated, taking advantage of the available solar energy and the tolerance of individual species to shade. The upper layer or top crop canopy utilizes sunlight tolerant species; and, as the gradient of light and humidity changes vertically, different species are grown in their proper "niches". The authors report that traditional gardeners have reliable ecological knowledge that allows them to fit plants into sites fitting their various requirements. Many other descriptions of the household gardens of Indonesia are found in the literature.

The Chagga, a Bantu group living on Mt.Kilimanjaro in Tanzania are skilled traditional farmers who make use of multi-story gardens to support their dense populations on about 1200 square km (Fernandes et al. 1984). Their gardens contain both animals and food and cash crops. More than 100 different plant species have been recorded in Chagga home gardens. This number includes fifteen different types of bananas that are grown for food, brewing beer, and fodder. Vertically, five relatively distinct zones or layers can be distinguished in the Chagga homegardens. The lowest (0-1 m) contains various food crops, herbs, and grasses. The second zone (1-2.5 m) is comprised of coffee and various small trees and shrubs. The third zone (2.5-5 m) Fernandes et al. call the banana zone. This canopy also includes some fruit and fodder trees. Next comes a

5-20 m zone or canopy consisting of fuel and fodder trees. Finally the fifth zone (15-30 m) consists of a canopy of valuable timber, fuel, and fodder trees. There is considerable overlap among these zones.

Little information was given on disease occurrence in these systems; however, an analysis helps to explain why these systems can often exist for centuries without apparent major plant disease problems. The systems resemble in some respects the stable natural ecosystems of the region. Traditional farmers have selected landraces for centuries that can thrive under their conditions, and that may represent an optimum for the conditions under which they are grown. A great diversity of crops is grown, and this provides a degree of protection because pests are less able to build up to destructive proportions on the few isolated plants of each species found in household gardens. The use of intercropping reduces losses by most plant pathogens. The architecture of the the entire system and the individual plant is manipulated by traditional farmers, especially in intercropping situations. Shade can have important effects on humidity, dew deposition, and temperature which often reduce the severity of some pathogens.

Conclusions

Cultural controls are often forgotten or barely mentioned in the modern literature on plant diseases; however, traditional farmers have successfully controlled plant diseases for millennia primarily with cultural practices. Many of these practices are sustainable, although some are highly labor-intensive. It is important to integrate traditional cultural controls into pest management systems for developing countries, especially those for control of plant diseases, to a greater degree than has been done in the past. Efforts must be made to thoroughly understand the agricultural systems and practices of traditional farmers in developing countries if the serious errors and failed projects of agricultural development efforts in recent decades are to be avoided. At the very least, these traditional practices are a point of departure and will contribute to the development of appropriate and acceptable improved practices.

Traditional agricultural practices deserve more respect than they receive. The knowledge of traditional farmers is often broad, detailed, and comprehensive. Although traditional farmers may not know what fungi, bacteria, or viruses are, in many cases they have effective, time-tested practices for managing pathogens. Traditional agricultural practices must be understood and conserved before they are lost with the rapid advance of modern agriculture in developing countries. Plant pathologists can learn much from traditional farmers to elucidate principles and practices useful in the future management of plant diseases.

The remarks of Haskell et al. (l979) summarize the complexity and challenge of traditional agriculture:

It is now becoming recognized that any attempt to import technological change in ignorance of, even in defiance of, the socio-cultural background of small farmer practice is a recipe for disaster. The basic reason is simple; traditional peasant systems of agriculture are not primitive leftovers from the past, but are, on the contrary, systems finely tuned and adapted, both biologically and socially, to counter the pressures of what are often harsh and inimical environments, and often represent hundreds, sometimes thousands, of years of adaptive evolution in which the vagaries of climate, the availability of land and water, the basic needs of the people and their animals for food, shelter, and health, have been amalgamated in a system which has allowed society to exist and develop in the face of tremendous odds.

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