Plant Diseases and Their Management in Organic Agriculture

Chapter 2.1

General Principles of Organic Plant Production Ariena H. C. van Bruggen University of Florida, Gainesville, U.S.A.

Maria R. Finckh University of Kassel Witzenhausen, Germany

2.1.1. R ules and Regulations Related

to Crop Production

2.1.2. L ong-­Term Management Decisions 2.1.3. S hort-­Term Management Decisions 2.1.4. R eferences

Plant production practices have a profound effect on plant disease development. These practices can be very different on organic farms compared with those on conventional farms. To understand the ecology of plant pathogens on organic farms, we need to understand the fundamental ways in which organic and conventional production practices differ. These practices are determined partially by the philosophy behind organic farming and partially by the rules and regulations imposed by certification bodies. In this chapter, we first describe the most common rules and regulations that apply to organic plant production. Within the context of these regulations, farmers still need to make many management decisions that influence the productivity and long-­term sustainability of their production systems. We distinguish between long-­term strategic management decisions that influence the farm operation for many years and short-­term management decisions that have an immediate impact on the current crop (Ohlander et al., 1999). In organic crop production, the long-­ term strategic management decisions are relatively more important than in conventional crop production, because quickly acting remedies such as nitrogen fertilizer side dressing or synthetic pesticide applications are not available. In this chapter, we present an overview of various management decisions. The three following chapters deal with the practices commonly used in annual (Chapter 2.2), perennial (Chapter 2.3), and greenhouse (Chapter 2.4) crop production.

At the end of each of these chapters, a brief overview is given of the main pests, diseases, and weeds and their control. Organic production of ornamental crops is still in its infancy and is not covered in this book.

2.1.1. R ules and Regulations

Related to Crop Production

As mentioned in Chapter 1, the rules and regulations for organic agriculture vary from country to country and even among certification agencies in one country. However, these rules and regulations have been derived from the principles associated with organic ideals. These principles are clearly expressed in the definition of organic agriculture given in the FAO/WHO Codex (italics added by authors): “Organic agriculture is a holistic production management system which promotes and enhances agro-­ecosystem health, including biodiversity, biological cycles, and soil biological activity. It emphasizes the use of management practices in preference to the use of off-­farm inputs, taking into account that regional conditions require locally adapted systems.

This is accomplished by using, where possible, agronomic, biological, and mechanical methods, as opposed to using synthetic materials, to fulfill any specific function within the system.”

The Codex Alimentarius guidelines specify that “an organic production system is designed to 1) enhance biological diversity within the whole system, 2) increase soil biological activity, 3) maintain long-­term soil fertility, 4) recycle wastes of plant and animal origin in order to return nutrients to the land, thus minimizing the use of non-­renewable resources, 5) rely on renewable resources in locally organized agricultural systems, 6) promote the healthy use of soil, water and air as well as minimize all forms of pollution thereto that may result from agricultural practices, 7) handle agricultural products with emphasis on careful processing methods in order to maintain the organic integrity and vital qualities of the product at all stages, and 8) become established on any existing farm through a period of conversion, the appropriate length of which is determined by site-­specific factors such as the history of the land, and type of crops and livestock to be produced” (Codex Alimentarius Commission, 1999). The European Union (EU) issued its first organic regulation, EU Council Regulation (EEC) No. 2092/91, in 1991. It has been updated several times since then, and the original regulation was replaced by EU Council Regulation (EC) No. 834/2007 in 2007 (European Commission, 2007). In addition, each European country can have its own regulations as long as they are at least as strict as the EU regulations. In the United States, the National Organic Program (NOP) Rule was issued by the U.S. Department of Agriculture (USDA) in 2000 (U.S. Department of Agriculture, Agricultural Marketing Service, 2000).

• 15

16 • Chapter 2.1

Organic regulations are meant to protect the integrity of organic agriculture, protect consumers from fraud, and prevent environmental problems that would violate organic ideals. The process of organic production is certified rather than the products at the end of the pipeline. What does this mean in practice?

1. Organic seeds and vegetative propagation materials need to be used (if available); this is true in the United States as well as in Europe. Some exceptions can be made, for example, if organically grown seed of a particular cultivar is not available, when planting stock has been grown under an organic system for at least 1 year, or when a prohibited substance is used to treat an organic crop, but the treatment is mandated by federal or state phytosanitary regulations (http://frwebgate.access.gpo.gov/cgi-­bin/get-­ cfr.cgi?TITLE=7&PART=205&SUBPART=C&TYPE =TEXT).

2. Genetically modified seed and propagation material is prohibited to prevent potential ecological problems and to be true to the principle of “naturalness” (see section 2.1.3 and Chapter 4.6).

3. Depending on the country, there are minimum standards for crop rotation to prevent the buildup of insect pests, diseases, and weeds adapted to particular crops.

4. There are standards for quality of the manure and compost that can be used in organic systems. For example, to minimize the risks of resistance to antibiotics, manure from intensive production units cannot be used. Further, only quality-­controlled composts from off-­farm sources are allowed and at no more than 5 t of dry matter ha–1 year –1 to minimize the risk of heavy metal accumulation in the soil. There are standards for maximum fresh or composted manure application in the EU (170 kg N/ha); a maximum is not mentioned in the USDA NOP regulations, but standards for the use of fresh manure are stricter in the United States than in the EU. 5. There are standards in the USDA NOP regulations to minimize the risk of human pathogens; such standards are not included in the EU regulations.

6. The use of synthetic fertilizers and pesticides is prohibited to protect the ecosystem and human health.

In general, regulations on fertilizers (including organic fertilizers) and pesticides are formulated in terms of what is allowed or prohibited and what is regulated or restricted; in the latter case, certain practices may be allowed under certain circumstances or only if they are specifically listed. An example of regulations relative to manure management in the United States and EU countries is as follows: Allowed. United States: composted manure; EU: no requirement for composting manure.

Prohibited. United States: any fertilizer or manure that contains a synthetic substance that is not on the National List of Substances, which are allowable in organic agriculture and sewage sludge; EU: sewage sludge or manure from industrial conventional farms.

Regulated or restricted. Raw manure is allowed only under the following conditions: • United States: plant and animal material that does not contain any synthetic substance other than what is allowable in the National List of Substances; EU: manure from conventional sources is not allowed, and fertilizers can contain only substances that are listed in the annex of the organic regulations.

• United States: no regulation about seepage or run-­off; EU: manure storage facilities must be designed to prevent con-

tamination of water through direct discharge, or by run-­ off, or by infiltration of the soil.

• United States: no regulation about application rates; EU: application rates must be moderate to preclude the contamination of surface or groundwater; the total amount of manure added must not exceed 170 kg of nitrogen ha–1 year –1 of agricultural area used.

• United States: no regulation about time of year when manure can be applied on fields not used for human food crops; in some EU countries: application must occur only when soil temperatures and soil moisture are favorable for microbial digestion and unfavorable for seepage.

• United States: for fields where human food crops are grown, raw manure can be applied up to 120 days prior to harvest if the edible portion of a crop touches the soil.

Raw manure can be applied up to 90 days prior to harvest if the edible portion of a crop does not touch the soil; EU: raw manure can be used, but it is recommended (not mandatory) that it be applied 4 months before a crop is planted (http://www.ams.usda.gov/AMSv1.0/getfile?dDocName= STELPRDC5087165) (http://eur-­lex.europa.eu/LexUriServ/ LexUriServ.do?uri= OJ:L:1999:222:0001:0028:EN: PDF) Similarly, the use of pesticides is prohibited, allowed, or restricted. All synthetic pesticides derived from fossil fuels are prohibited. The allowed and restricted pesticides are all specifically listed. Natural pesticides such as plant extracts or toxins produced by bacteria are generally allowed, provided that no synthetic materials are used in their formulations. Mined products are usually also allowed, e.g., silicate, a residual product from the phosphate fertilizer industry, in the United States. In some countries copper fungicides are considered “mined, natural products” and are allowed, but the number of countries with restricted use of copper fungicides (when a special derogation must be granted) is increasing, especially in Northern Europe and the United States (see Chapter 4.8).

In addition to restrictive regulations, there are management practices that are encouraged (although not strictly enforced as yet). A farm biodiversity and conservation plan is often required for certification. Lengthy crop rotations are suggested, and a crop rotation plan is required. A plan for building up soil fertility is also required. And finally, a farm waste-­management plan will be requested by the certification agency.

For conversion from conventional to certified organic farming, there is a minimum transition period during which the products cannot be sold as organic. The length of this period varies among production sectors and is also dependent on the certification agency. For example, the transition period for annual crops is 3 years in the United States and only 2 years in the EU. For perennial crops, the conversion period is 3 years in both the United States and the EU. (In exceptional cases, a shorter period can be granted.) The reason for a transition period is to allow decomposition of residual pesticides and antibiotics. The time needed to improve soil quality is often much longer than the official transition period, and this is the main reason that crop yields are reduced in the first years after conversion (Lotter et al., 2003). Another reason is that natural enemy populations have not yet been built up. Moreover, there are considerable managerial challenges during the transition period when cultural practices and inputs need to be changed, new insight into ecosystem management needs to be gained, new techniques need to be learned, and more extensive production and sales records need to be kept.

General Principles of Organic Plant Production • 17

2.1.2. Long-­Term Management Decisions Under long-­term strategic management decisions for crop production, we can distinguish decisions in relation to 1) capital infrastructure, 2) integration of arable land and natural areas, 3) integration of crop and animal production, 4) crop rotation and sequence, 5) soil health management, and 6) general tillage decisions.

2.1.2.1. C apital

infrastructure

Capital infrastructure includes buildings and machinery.

The most basic decision in plant production with respect to buildings is the choice between greenhouse and open-­air production. Some open-­air vegetable producers grow their own transplants in small greenhouses on their own land, but most large-­scale vegetable growers obtain their transplants from specialized organic transplant producers with commercial greenhouses. Greenhouses also enable the production of (sub)tropical crops in temperate zones. Because of the demand for these crops by consumers in the winter, many farmers in the Mediterranean climates opt for the construction of plastic houses, while farmers in temperate climates choose to build glass or plastic-­ covered houses. Despite the high capital inputs and risks associated with greenhouse production, many operations are highly profitable. The ecological sustainability is sometimes debated in view of the high energy consumption and potential nutrient losses to the environment (see Chapter 2.4), but personal and economic motives play an important role in the choice to invest in greenhouse infrastructure.

Storage facilities also fall in the building category of capital infrastructure. Since organic products have to remain strictly separate from conventional products, storage and other facilities need to be dedicated to organic products. Many large growers (or growers’ cooperatives) of arable, vegetable, and fruit crops have drying, cooling, and storage facilities. For example, large grain growers usually have silos to store products until they can fetch a good price. Many grain and nut producers have drying facilities, and fruit and vegetable growers have cooling facilities. It is a challenge to keep pests and diseases out of organic storage facilities, because there are very few options to

control any infestation. Thus, having such facilities is generally more risky for organic than for conventional growers.

Farm machinery can be very specific for particular crops, for example, potato harvesting machines or combines. These machines are generally very expensive, so they need to be used on a regular basis to pay for themselves. Expensive, specialized equipment has promoted specialization in agricultural production with short rotations for economic reasons. Sharing equipment or hiring contract companies to perform certain operations has allowed diversification of the crop production system. A recent development in farm machinery is the use of GPS-­controlled tractors with caterpillar tracks to reduce the pressure on the soil. These tractors are expensive but allow the return to exactly the same tracks year after year, so compaction is limited to those tracks only, leaving well-­structured and aerated soil in between. In the Netherlands, the first farmers who used such a GPS system were organic farmers, whose yields increased by 30% after this transition, partially as a result of a reduction in root diseases (Vermeulen and Mosquera, 2009).

2.1.2.2. I ntegration

of arable land and natural areas to enhance biocontrol

Variation in the landscape where agricultural and natural areas are interwoven (Fig. 2.1.1) can be beneficial in many ways. Natural areas include hedgerows and trees, strips with flowering plants, and natural or manmade waterways with diverse flora and fauna, both in the water and on the banks.

Arable crop production can benefit from the presence of parasitoids and predators of crop pests in shrub lands or natural meadows alongside the arable fields (Dyer and Landis, 1997).

Natural areas can also provide pollinators or protection against wind and pesticide drift. These ecological zones can function as an attraction for ecotourism as well, which may benefit the overall farm operation. Moreover, in some European countries, farmers obtain payments from the government for maintaining natural landscapes. One needs to be aware, however, that certain pests and diseases may also overwinter in these natural areas; for example, root aphids that are a lettuce pest over­ winter in poplar trees (see also section 2.2.4.1 and Chapter 4.4).

However, in general, organic farmers have experienced beneficial effects from natural areas on their farms.

Fig. 2.1.1. Organic fields with hedgerows. Arable land with flowering hawthorn and briar (left) and meadow with fruiting Sorbus sp. and blackberries (right) in the Netherlands. (Left, courtesy F. Smeding; right, courtesy A. H. C. van Bruggen)

18 • Chapter 2.1 2.1.2.3. I ntegration

of crop and animal production for diversification

The combination of plant and animal production, either in one farm operation or through collaboration with nearby farms, can be highly beneficial by promoting nutrient cycling and general soil health. Integration of crop and animal production diversifies the farming system, expanding the number of crops grown in rotation. Thus, forage leys are added to the rotation, which improves soil structure and fertility. An added benefit of a mixed farm (or collaborating farms) is the availability of animal manure to fertilize crops. Hygiene rules apply to the application of animal manures to food crops (see Chapter 4.9).

2.1.2.4. C rop

rotation and sequence to avoid replant problems caused by root diseases

Crop rotations are considered essential for healthy organic cropping systems for the following reasons: 1) to diversify the farming operations and stabilize income; 2) to obtain an even distribution of use of various nutrients, because crops vary in their nutrient requirements; 3) to maintain soil structure, since crops vary in rooting structure and composition; 4) to improve soil moisture holding capacity; 5) to avoid replant problems caused by root diseases (caused by pathogens or nematodes); and 6) to avoid buildup and selection of weeds, insect pests, and pathogens adapted to specific crops. These last two aspects are generally considered the most important benefits of crop rotations (National Research Council, 1989). Despite the advantages of crop rotation, there are also some disadvantages, such as the greater variety of equipment and skills needed and the higher proportion of less profitable crops. However, the benefits of crop rotation outweigh the disadvantages (see also Chapter 4.2).

The choice of the rotational crops is on the one hand determined by characteristics of the crops, like nutrient requirements and release (e.g., legumes providing nitrogen), rooting structure, and susceptibility to pathogens and nematodes or allelopathic effects and on the other hand by practical consid-

erations, such as timing of the operations and work load, availability of machinery, and profitability.

In stockless crop production (without animals), relatively more leguminous crops are needed in the rotation, in particular as cover crops, so that the nitrogen fixed is incorporated into the soil rather than removed with the harvest. Besides nitrogen-­ fixing leguminous crops, such as various clover, vetch, pea, or bean species, grain crops are frequently planted to catch the residual nitrogen. Examples are the winter cover crops ryegrass, rye, barley, oats, and spelt and summer cover crops such as sorghum and sudangrass. Nonleguminous broadleaf crops, e.g., buckwheat and Phacelia tanacetifolia, and various cruciferous crops, such as mustard, have also been used (Fig. 2.1.2). Many of these cover crops can have direct beneficial effects on crop health by suppressing pests, diseases, nematodes, and weeds (see Chapters 3.3 and 4.8).

Cover and catch crops should also be varied from year to year to avoid buildup of diseases and pests in these crops, and some have the capacity to suppress diseases (see Chapter 4.8 for details). This is especially important with legumes (see Chapter 5.4). Moreover, cover crop mixtures (Fig. 2.1.2) usually perform better than single species thanks to optimal niche utilization and reduced spread of diseases and pests (Hiddink et al., 2009).

2.1.2.5. S oil

health management

Soil health has been considered more or less synonymous with soil quality, defined as “the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health” (Doran et al., 1996). However, soil health is related mainly to biological factors, while soil quality also includes soil chemical and physical factors (van Bruggen and Semenov, 2000). Soil functions include life-­support processes, i.e., plant anchorage and nutrient supply, water retention, and conductivity (Fig. 2.1.3), support of microbial diversity and soil food webs, and ecological regulatory functions such as nutrient

Fig. 2.1.2. Cover crops. Phacelia tanacetifolia (left) and a mixture of grass and white clover (right). (Cour­tesy M. R. Finckh)

General Principles of Organic Plant Production • 19 cycling, carbon sink and carbon dioxide source, remediation of pollutants, sequestration of heavy metals, and suppression of pathogens and pests. Soil and ecosystem health is characterized by biological diversity, connectedness, and stability. Thus, outbreaks of plant diseases can be considered indicators of instability and poor ecosystem health (van Bruggen and Semenov, 2000; van Bruggen and Termorshuizen, 2003; Ghorbani et al., 2008).

Soil health also includes soil fertility, i.e., the ability of the soil to provide sufficient plant nutrients, which can be improved by judicious management of organic matter. Of special importance is the incorporation of relatively recalcitrant (slow-­decomposing) organic matter into the soil that increases microbial and microfaunal activity and diversity and enhances organic matter turnover. This results in more oligotrophic conditions (low in easily available nutrients) and a more stable biological community with relatively more K-­strategists (slow-­

growing organisms with long generation times and relatively low numbers of offspring). Recalcitrant materials are, for example, manure or compost, including grass/clover multiyear leys in the rotation or fibrous winter cover crops that will be turned under (see Chapter 3.2 for details on the effects on the soil microbial community and soil health). Finally, soil tillage needs to be minimized to reduce disruption of the soil food web.

A healthy soil (see Chapter 3.2. for definition) has high soil fertility based on nutrients supplied from the slowly decomposing organic matter. However, nutrients removed in the harvested products need to be replenished. Nitrogen can be replenished by having sufficient nitrogen-­fixing crops in the rotation; phosphorus can be supplied by adding ground rock phosphate or manure, from which phosphorus is released slowly or more rapidly, respectively; and potassium can be supplied from patentkali, a natural mineral of potassium and magnesium sulfate.

Without this last fertilizer, potassium may become a yield-­ limiting factor in the long run (Mäder et al., 2002).

2.1.2.6. G eneral

Fig. 2.1.3. Lower left, healthy soil of a traditional dairy farm on which or-

ganic methods (e.g., surface application of manure) are used and upper right, farm on which conventional methods (fertilizer and injection of slurry) are used. The field on the right has water puddles caused by subsidence (oxidation of organic matter), while the field on the left is dark green because of better soil structure and innate fertility. (Cour­tesy A. H. C. van Bruggen)

tillage decisions

Conventional tillage operations (disking and harrowing) are commonly carried out to a depth of about 20–30 cm. The main reasons for turning the soil this deeply have been weed and disease control and preparation of a clean, fine seedbed. However, there are serious disadvantages to these tillage operations, in particular soil erosion and, in the long run, soil compaction and crust formation. In short, loss of soil structure and fertility is a serious, long-­term consequence. In recent decades, alternative tillage systems have been developed and implemented in some agricultural areas. No-­till farming systems have been widely adopted in the midwestern United States with corn and soybean rotations and in the wheat-­producing areas in the western United States and Canada. No-­till approaches are used more widely in the United States than in Europe. However, minimum tillage (to a depth of 3–15 cm) is becoming more common in organic and integrated farming systems (Fig. 2.1.4), especially in German-­speaking countries. Minimum tillage has resulted in preservation of the food web, as evidenced by more diverse and numerous microfauna in integrated or low-­input farming

Fig. 2.1.4. Left, special plow, “Stoppelhobel,” for shallow cutting and turning of stubble, with high steel weights on top of plow shares. Right, plants are cut at about 6-cm depth and turned and soil is crumbled. This operation is followed by disking after a few hours of drying. When the operation is performed after a few dry days, earthworms are not harmed. (Cour­tesy M. R. Finckh)

20 • Chapter 2.1

systems (Fan et al., 1993). On the other hand, soil temperatures remain slightly lower in no-­till systems in early spring, delaying crop emergence.

Ridge tillage has also been promoted to conserve soil. In this system, crops are grown on permanent beds (National Research Council, 1989). Erosion is significantly reduced, and problems such as low spring temperatures and soil compaction experienced in no-­till systems are not encountered. Only the tops of the ridges are tilled, often resulting in fewer weed problems than in conventionally tilled fields.

All these forms of conservation tillage leave crop residues on or near the surface, which may provide habitats for pests and plant pathogens (National Research Council, 1989) but also provide food for soil-­ inhabiting beneficial organisms, especially earthworms. When conservation tillage is supplemented by other natural disease-­control measures on organic farms, outbreaks of root and foot diseases may not be a problem (van Bruggen, 1995). Moreover, weed growth can be suppressed under plant debris. Narrow strips may be cultivated in no-­till fields to allow the soil to warm sooner in the spring and promote germination of the crop, while still suppressing weed growth under the remaining mulch (Liebman et al., 2001).

2.1.3. Short-­Term Management Decisions Short-­term strategic management decisions for crop production relate to 1) selection of seed or plant material, 2) spatial diversity, 3) weed-­control measures, 4) biological control of insects and diseases, 5) additional fertilization, and 6) refuse management.

2.1.3.1. S election

of seed or plant material

Farmers select their crops mainly on an economic basis, dependent on their expertise and preferences. Growers often specialize in certain crops. However, when designing alternative farming systems, one should consider the development of underexploited local crops, especially in tropical and subtropical regions.

Modern plant-­ breeding and seed-­ production techniques have narrowed the genetic base of crop plants. Plant breeding has resulted in more uniform crops with higher yields and quality under conditions of conventional agriculture. However, in the process, certain flavors and scents may have been lost. Organic farmers sometimes prefer old heirloom varieties because of their specific qualities, including at times better local adaptation. In addition, specialty markets may exist for certain crops.

An example of certain cereals is given in Chapter 5.2. However, just as with modern cultivars, often the levels of resistance to diseases and pests are not sufficient and there is a need for cultivars adapted to organic conditions (Lammerts van Bueren et al., 2002). Overall, plant cultivars used in organic agriculture are mostly the same as those in conventional agriculture, except in indigenous agroecosystems where local land races may still be used.

Organic agriculture is still dependent largely on conventional plant-­breeding programs. Nevertheless, breeding for organic agriculture is an important and rapidly growing research sector, because the requirements for cultivars adapted to organic farming systems are quite different from those adapted to conventional farming systems (Lammerts van Bueren et al., 2002). In particular, cultivars for organic production need

a better nutrient-­searching capacity, weed-­suppressing capacity, and general pest and disease resistance (see Chapter 4.5 for details). For example, the cereal ideotype for organic farming is quite different from a conventional one. Plants should be taller with a large distance between the flag leaf and the head and should have early high tillering, flat leaf angles, and high nutrient-­use ability, especially for organic nutrients (Fig. 2.1.5).

These properties make it harder for splash-­dispersed pathogens to move upward in the canopy and make the crops generally more competitive against weeds. There are a number of additional requirements, especially with respect to seedborne disease resistance, that are discussed in Chapter 5.2. Moreover, use of genetically modified organisms is not allowed in organic agriculture worldwide. Thus, with the increase in genetically modified plant cultivars, separate plant-­ breeding programs for organic agriculture are warranted. In fact, many national programs in Europe now include organic cultivar testing trials (Przystalski et al., 2008).

There is a special need for new cover and intercrop cultivars and species for organic farming that can be used to increase soil fertility and health by adding organic matter; conserving nutrients in the off-­season; protecting soil from erosion; and suppressing weeds, pests, and diseases (e.g., through competition and biofumigation effects).

Vegetable producers may use direct seeding or transplanting methods. Transplants are often used to save time in the field to catch the early market, to allow a winter catch crop to continue to grow in early spring before turning it under, to reduce labor costs, to avoid competition with weeds (for small-­seeded, slow-­ growing crops such as onions or celery), or to avoid damping-­ off problems in the field (for hybrid crops with expensive seeds).

Transplants must be organically produced without exception.

An important issue is the health of plantlets grown in warm, moist greenhouses, conditions that may favor bacterial diseases. Arable, large-­seeded crops are generally seeded directly.

2.1.3.2. S patial

diversity

In conventional agriculture (and mostly still in organic agriculture), crops are grown as pure stands to minimize plant competition and maximize labor efficiency, especially with respect

Fig. 2.1.5. Wheat cultivar Butaro bred for organic production (in the back-

ground) compared with a conventionally bred modern wheat cultivar. Note differences in height, leaf angles, and distance between flag leaves and heads. (Cour­tesy M. R. Finckh)

General Principles of Organic Plant Production • 21 to cultivation and harvesting operations. There are, however, negative effects of a genetically and phenotypically uniform crop. In particular, pests and diseases can spread more easily in such a crop (Finckh and Wolfe, 2006). Diversity is one of the cornerstones of sustainable pest and disease management (see Chapter 4.4).

There are several reasons, in addition to pest and disease management, that a variety of crops and cultivars contribute to agricultural sustainability. First, an integrated system cannot rely solely on a limited number of crops to provide the region with all the plant nutrient needs. Second, integration of biodiversity allows for the conservation and further development of local indigenous genetic resources (Finckh, 2008).

In traditional agroecosystems, such as the remnants of the Mayan agroecosystem in Central America, food crops are generally mixed (e.g., maize, beans, and squash) to provide food security (Mannion, 1995). Frequently, even trees are integrated into such farming systems (agroforestry). Recently, research has been done on intercropping in temperate regions. In particular, crops that occupy different niches and do not compete at crucial time periods in the growing season, such as leeks and celery, can be more productive than individual crops (Fig. 2.1.6) (Baumann et al., 2000).

Organic perennial crops are frequently grown with a permanent cover crop to supply nitrogen, prevent leaching losses, and minimize soil erosion. Soil organic matter is increased and soil structure improved by a cover crop (Miller et al., 1989).

However, cover crops can sometimes harbor plant pathogens such as the bacterium Pseudomonas syringae that can cause

leaf and blossom blight of apple and pear and kill branches (Lindow, 1996). In Europe, organic fruit trees are sometimes grown as “high-­trunk” trees, with a permanent pasture underneath where cows may graze.

2.1.3.3. W eed-­control

Ecologically based weed management strategies begin with the premise that no single tactic will be successful in the face of genetically heterogeneous weed populations, range expansions by dispersing weed species, variable weather conditions, and changes in crop management practices. As stated by Liebman (2000), “Rather than relying on a single ‘large hammer,’ such as herbicide technology, to suppress weeds, ecologically based strategies seek to integrate many ‘little hammers’ that act in concert to stress and kill a wide range of weed species at many points in their life cycles.” Organic growers are generally aware of this principle and try to prevent the buildup of weed seed banks by various tactics. There are several strategies to try to prevent weed emergence and growth. 1) Avoiding large concentrations of readily available mineral nitrogen in the beginning of the season prevents germination of many small-­seeded weed species at that time. 2) Keeping the soil covered by crops, such as catch crops in winter, prevents germination of weed seeds because of competition for light and nutrients. 3) The use in the rotation of crops such as red clover or alfalfa that produce allelopathic chemicals reduces weed seed germination. 4) Incorporation of cover crops may attract certain soil-­dwelling insects (for example, fly grubs) that may eat young weed seedlings.

5) Certain tillage and residue management techniques can suppress weed emergence and growth (see discussion of ridge tillage above). Moreover, in irrigated agriculture, trickle irrigation limits weed germination compared with furrow irrigation or sprinkle irrigation, since the soil surface is for the most part kept dry. 6) Any measure that prevents weeds from flowering and setting seeds needs to be applied.

During the cropping season, organic farmers can use mechanical weed control, hand-­weeding, flaming, or biological control (e.g., Fig. 2.1.7). No organic herbicides are currently available. While some work is being directed especially toward mycoherbicides, it is unlikely that they will play a role in organic farming because weeds do not usually occur in contiguous pure stands in normal organic farming situations.

Because pathogens are unlikely to cause extensive damage in diversified stands, however (see Chapter 4.4), pathogen-­based herbicides (and biological weed control) are of interest only in specific situations, such as when invasive weeds take over large areas. In addition, not many biological weed-­control agents have been approved for use because of the potential risks that the pathogens have wider host ranges than anticipated. On the other hand, biological weed control by ducks or geese has been practiced in traditional agroecosystems (for example, in rice in Southeast Asia; see Chapter 5.3) as well as by organic farmers in temperate zones.

2.1.3.4. P est-­and

Fig. 2.1.6. Mixed crop of leeks and celery in Switzerland. (Cour­ t esy

D. Baumann)

measures

disease-­control measures

Two basic strategies of biological control in the narrow sense can be distinguished: 1) enhancing biological control by supporting endemic natural enemies (competitors, antagonists, predators, or parasites) by means of habitat management; and 2) inundative biological control by release of predators, parasites, or competitors (Letourneau and van Bruggen, 2006). This includes seed treatments and drenching of transplants.

22 • Chapter 2.1

Fig. 2.1.7. Left, special mechanical weeding apparatus with spiked teeth, a winged cutting device to weed between rows, and rubber fingers to weed around single plants without pulling them out. Right, tractor-pulled support for hand weeding. Workers lie down to weed while being driven across the field. (Cour­tesy M. R. Finckh)

Enhancing natural control is preferred in organic farming as a system-­immanent measure (Alabouvette et al., 2006). Natural control can be enhanced by increasing the diversity in the food webs in the agroecosystem. Food web diversity can be enhanced by increasing vegetation diversity, for example, by planting beneficial insectary plants and establishing natural habitats. Soil biodiversity and root disease suppression can be enhanced by increasing recalcitrant soil organic matter.

“Inundative biological control” implies the application of specific biocontrol agents, which is allowed in organic agriculture provided that no petroleum-­based synergists or carriers are used in the formulations. Although a great deal of research has been conducted on specific biological control agents against plant pathogens, insect and nematode pests, and weeds, relatively few species have been registered for field use (see Chapters 4.7 and 4.8 for more details). On the other hand, biological control of insect pests by release of natural enemies (antagonists, predators, and parasitoids) has been quite successful, especially under controlled conditions such as in greenhouses.

For example, the greenhouse whitefly, Trialeurodes vaporariorum, has been controlled successfully by a parasitic wasp, Encarsia formosa, that lays eggs in the larvae of the whitefly. Similarly, the herbivorous spider mite Tetranychus urticae has been controlled in commercial greenhouses by the predatory mite Phytoseiulus persimilis. In the field, products based on various Bacillus thurigiensis subspecies and neem extracts are widely used. Despite these success stories, biological control agents are rarely applied in organic fields (Langer, 1995). More details on biological control of plant diseases are presented in Chapters 4.7 and 4.8 and the examples of specific crops studies in Part 5.

2.1.3.5. A dditional

fertilization

The nutritional needs of organic crops are met mainly by long-­term strategies such as inclusion of legumes in the rotation and application of animal manure or compost before a particular crop is planted. Sometimes, animal slurries (mixtures of manure and urine) are used immediately before a crop is seeded to enhance nitrogen availability at the beginning of the season or even as a top dressing at crucial developmental times, but this may result in serious weed problems during early crop growth or after crop harvest.

Vegetable crops sometimes receive additional fertilization during the cropping season; organic fertilizer materials that are used for this purpose can be animal or plant based, e.g., fish emulsion, blood meal, or seaweed extracts. Many organic farmers are reluctant to use these materials in the field, however, because they may create potential weed problems and may promote fast, lush crop growth, resulting in crops with less intensive taste. Also, high nitrogen supply may increase susceptibility to various diseases and pests (Huber and Thompson, 2007).

The situation is different in the greenhouse, where additional fertilization is needed to keep the crops growing and producing for approximately 9 months and economics preclude the extensive use of green manure and cover crops (see Chapter 2.4).

2.1.3.6. R efuse

management

In conventional as well as in traditional agriculture, crop debris or stubble is sometimes burned (in some regions even on a large scale) to 1) facilitate seeding the next crop, especially if the stubble and debris break down slowly; and 2) control diseases that survive in the debris and stubble.

This practice is, however, not desirable because of the burden placed on the environment and society as a result of the small particles in smoke and the release of greenhouse gases.

The loss of carbon (in the form of CO2) makes this practice unacceptable to organic farming, in which all available organic matter must be recycled to maintain and enhance soil fertility.

Better alternatives are working the (preferably shredded) debris into the soil to provide substrate to the soil food web in the form of recalcitrant matter as described above (see also Chapter 3.2. for details) or to leave it on the surface for slower decomposition (mainly by fauna and fungi) to prevent erosion.

Because nutrients are removed from farming systems in the form of agricultural products, these nutrients need to be replaced in the form of organic matter. This organic matter can originate from crop refuse, for example, from the cotton industry, wineries, or sugar refineries. This material should be composted before it is returned to the land in order to kill pathogens and weed seeds and to return partially humified, more stable material that has various benefits (such as root disease suppression, slow release of nutrients, and improved soil structure).

Many organic farmers produce their own compost (Fig. 2.1.8).

General Principles of Organic Plant Production • 23

Fig. 2.1.8. Elongated compost heaps covered by fleece next to a green-

house operation in the Netherlands. (Cour­tesy W. Blok)

However, compost piles need to be large enough to reach high temperatures (to kill pathogens and weed seeds). Also, it might be better that compost piles not be placed directly next to the vegetable production site. Animal manure can also be included in compost piles.

2.1.4. References Alabouvette, C., Olivain, C., and Steinberg, C. 2006. Biological control of plant diseases: The European situation. Eur. J. Plant Pathol.

114:329-­341.

Baumann, D. T., Kropff, M. J., and Bastiaans, L. 2000. Intercropping leeks to suppress weeds. Weed Res. 40:359-­374.

Codex Alimentarius Commission. 1999. Guidelines for the Production, Processing, Labelling and Marketing of Organically Produced Foods. GL 32-­1999. http://www.codexalimentarius.net/web/standard _list.jsp Doran, J. W., Sarrantonio, M., and Liebig, M. A. 1996. Soil health and sustainability. Adv. Agron. 56:2-­54.

Dyer, L. E., and Landis, D. A. 1997. Influence of noncrop habitats on the distribution of Eriborus terebrans (Hymenoptera: Ichneumonidae) in cornfields. Environ. Entomol. 26:924-­932.

European Commission. 2007. Council Regulation (EC) No 834/2007 of 28 June 2007 on organic production and labeling of organic products and repealing Regulation (EEC) No 2092/91. Off. J. Eur. Union.

L 189/1-­L 189/23. http://eur-­lex.europa.eu/LexUriServ/LexUriServ. do?uri=OJ:L:2007:189:0001:0023:EN:PDF Fan, Y., Liebman, M., Groden, E., and Alfred, A. R. 1993. Abundance of carabid beetles and other ground dwelling arthropods in conventional versus low-­input bean cropping systems. Agric., Ecosyst.

Environ. 43:127-­139.

Finckh, M. R. 2008. Integration of breeding and technology into diversification strategies for disease control in modern agriculture.

Eur. J. Plant Pathol. 120:399-­409.

Finckh, M. R., and Wolfe, M. S. 2006. Diversification strategies. Pages 269-­308 in: The Epidemiology of Plant Disease. B. M. Cooke, D. G. Jones, and B. Kaye, eds. Springer, Dordrecht, Netherlands.

Ghorbani, R., Wilcockson, S., Koocheki, A., and Leifert, C. 2008. Soil management for sustainable crop disease control: A review. Environ. Chem. Lett. 6:149-­162.

Hiddink, G. A., Termorshuizen, A. J., and van Bruggen, A. H. C. 2009. Mixed cropping and suppression of soilborne diseases, a re-

view. Pages 119-­146 in: Sustainable Agriculture Reviews 4: Genetic Engineering, Biofertilisation, Soil Quality and Organic Farming.

E. Lichtfouse, ed. Springer, Berlin.

Huber, D. M., and Thompson, I. A. 2007. Nitrogen and plant disease.

Pages 31-­44 in: Mineral Nutrition and Plant Disease. L. E. Datnoff, W. H. Elmer, and D M. Huber, eds. American Phytopathological Society, St. Paul, MN. Lammerts van Bueren, E. T., Struik, P. C., and Jacobsen, E. 2002.

Ecological aspects in organic farming and its consequences for an organic crop ideotype. Neth. J. Agric. Sci. 50:1-­26.

Langer, V. 1995. Pests and diseases in organically grown vegetables in Denmark: A survey of problems and use of control methods. Biol.

Agric. Hortic. 12:151-­171.

Letourneau, D., and van Bruggen, A. H. C. 2006. Crop protection. Pages 93-­121 in: Organic Agriculture: A Global Perspective.

P. Kristiansen, A. Taji, and J. Reganold, eds. Commonwealth Scientific and Industrial Research Organisation (CSIRO), Collingwood, Victoria, Australia.

Liebman, M. 2000. Opportunities to integrate soil, crop, and weed management in low-­external-­input farming systems. Pages 26-­31 in: Professional Societies and Ecologically Based Pest Management: Proceedings of a Workshop. National Academy of Sciences, National Academies Press, Washington, DC. Liebman, M., Mohler, C. L., and Staver, C. P. 2001. Ecological Management of Agricultural Weeds. Cambridge University Press, Cambridge, U.K. Lindow, S. E. 1996. Role of immigration and other processes in determining epiphytic bacterial populations: Implications for disease management. Pages 155-­168 in: Aerial Plant Surface Microbiology.

C. E. Morris, P. C. Nicot, and C. Nguyen-­The, eds. Plenum Press, New York.

Lotter, D., Seidel, R., and Liebhardt, W. 2003. The performance of organic and conventional cropping systems in an extreme climate year. Am. J. Altern. Agric. 18:146-­154.

Mäder, P., Fliessbach, A., Dubois, D., Gunst, L., Fried, P., and Niggli, U. 2002. Soil fertility and biodiversity in organic farming. Science 296:1694-­1697.

Mannion, A. M. 1995. Agriculture and Environmental Change: Temporal and Spatial Dimensions. John Wiley & Sons, Chichester, U.K. Miller, P. R., Graves, W. L., and Williams, W. A. 1989. Cover crops in California agriculture. Univ. Calif. Div. Agric. Nat. Resour. Leafl.

21471.

National Research Council. 1989. Alternative Agriculture. National Academy of Sciences, Washington, DC. Ohlander, L., Lagerberg, C., and Gertsson, U. 1999. Visions for ecologically sound agricultural systems. J. Sustainable Agric. 14:73-­79.

Przystalski, M., Osman, A., Thiemt, E., Rolland, B., Ericson, L., Ostergaard, H., Levy, L., Wolfe, M., Büchse, A., Piepho, H. P., and Krajewski, P. 2008. Comparing the performance of cereal varieties in organic and non-­organic cropping systems in different European countries. Euphytica 163:417-­433.

U.S. Department of Agriculture, Agricultural Marketing Service.

2000. National Organic Program; Final Rule. 7 CFR Part 205. Federal Register 65 (246). http://www.ams.usda.gov/AMSv1.0/getfile? dDocName=STELPRDC5087165 van Bruggen, A. H. C. 1995. Plant disease severity in high-­input compared to reduced-­input and organic farming systems. Plant Dis. 79:976-­984. van Bruggen, A. H. C., and Semenov, A. M. 2000. In search of biological indicators for soil health and disease suppression. Appl. Soil Ecol. 15:13-­24. van Bruggen, A. H. C., and Termorshuizen, A. J. 2003. Integrated approaches to root disease management in organic farming systems.

Australas. Plant Pathol. 32:141-­156.

Vermeulen, G. D., and Mosquera, J. 2009. Soil, crop and emission responses to seasonal-­controlled traffic in organic vegetable farming on loam soil. Soil Tillage Res. 102:126-­134.