Chapter 12

Plant Disease Management for Organic Crops

Cultural Practices in Controlling Crop Diseases

Cultural practices are invaluable tool that a grower should consider in designing an integrated disease control system. Cultural control aims to prevent contact with the pathogen, to create environmental conditions unfavorable to the pathogen, or to reduce the amount of pathogen inoculum available to the infected crop plants. Some cultural control practices include host eradication, crop rotation, sanitation, tillage, improving crop growth conditions, and selection of resistant cultivars to name a few. The remainder of this section discusses ways of improving plant growth and creating environments in the field that discourage spread of pathogens from one host to another.

Site Selection

A grower can incur significant losses if susceptible crops planted in fields known for having a history of soil pathogens. Plant-pathogenic fungi such as Armillaria, Fusarium (the wilt-causing species), Plasmodiophora, Sclerotium, and Verticillium are true soil inhabitants and will persist in soil for many years, even in the absence of a plant host. Soilborne fungi such as Phytophthora, Pythium, and Rhizoctonia often are much more widespread, so site selection might be less of an option in avoiding these pathogens. If possible, avoid planting crops in low-lying, wet areas given the higher incidence of soilborne diseases.

Selection of Resistance Cultivars

The use of resistant cultivars is one of the most important and economical components of an integrated disease management program. Resistant cultivars offer one of the most successful approaches to the control of pathogens of many crops, especially those diseases that cannot be controlled by other means. The term resistance usually describes the plant host's ability to suppress or retard the activity and progress of a pathogenic agent, which results in the absence or reduction of symptoms.

Planting Pathogen-free and High-quality Seed

Planting pathogen-free and high-quality seed is a critical first step in managing diseases. This is particularly important for vegetatively propagated material such as tubers, bulbs, and slips. True seed can also contain pathogens, however, and all can serve as the source of entry of pathogens into new areas.

Timing of Planting Dates

Alternating the time of planting to avoid high levels of pathogen inoculum or conditions conducive for development of a particular disease can lead to reduced severity of some crop diseases. For example, early-planted fields of soybeans may have a greater incidence of seedling blights caused by Fusarium solani and Pythium if planted early in cool and wet soils. The incidence of these two diseases can be reduced by delaying planting until the soil warms up.

Seeding Practices

Deeper seeding may promote germination but it also lengthens the (usually) susceptible pre-emergence seedling phase. Smuts and seedling diseases caused by Fusarium spp. and Rhizoctonia spp. are more serious if seeds are planted too deeply. Similarly, potato seed pieces are more readily attacked by Rhizoctonia if planted too deeply.

Crop Density

Crop density can exert considerable influence over disease incidence due to the ease with which the pathogen inoculum can be transferred closely between closely spaced plants. In closely planted crops, temperatures are more uniform, humidity is higher, and foliage is wetter for longer periods of the day, all of which provides favorable conditions for pathogen infection and subsequent development. Diseases such as downy mildew and Sclerotinia stem rot (white mold) are greatly, affected by high humidity.

Crop Rotation

Some pathogens that causes diseases survive in the soil from year to year in one form or the other, usually as sclerotia, spores, or hyphae. Continuously cropping the same crop builds up the population levels of any soilborne pathogen of that crop that may be present. The populations can potentially build up so large that it becomes difficult to grow that crop without yield losses. However, by rotating crops soilborne pathogens will eventually decline without a suitable host. The most successful rotations employ intervals between susceptible crops, which are longer than the known survival period of pathogens. Crop rotations are typically for a definite time-period (usually 2 or 3 years in most cases). A longer time-period of four to six years may be required for control of some diseases of brassicas, potatoes, and cotton. The crop rotation may also include a fallow period in which land is “rested” from production.

Tillage Practices

Deeply burying infested crop debris and pathogen survival structures by moldboard plowing reduces disease incidence. For this to work, the residue must be buried deeply enough that it is not pulled back up during seedbed preparation and cultivation. Burying diseased material is especially useful against pathogens that produce sclerotia and those that infect only aboveground plant tissue. However, deep, full inversion plowing decreases soil health by burying beneficial organisms that live in the top few inches of the soil profile.

Eradication of Hosts to Reduce Level of Inoculum

Crop plants remaining from previous seasons, weeds or wild plants may act as hosts providing a source of inoculum in the new growing season. For example, grasses such as Hordeum leportnum are hosts of the wheat take-all fungus Gaeumannomyces graminis. Many dicotyledonous weeds are infested with root-knot nematodes (Meloidogyne spp.) and other nematodes.

Intercropping

The incidence of disease is often less in mixed plantings than in monocultures because the distance between similar plants is greater than in more intensive growing systems so it is less likely that propagules or vectors of pathogens will successfully move from one host to another. The intervening plants pose physical barriers to the dissemination of aerial pathogens or their vectors. However, the incidence of disease may be higher in intercropped plantings.

Physical Removal and Destruction of Crop Residues

Crop residues provide suitable substrates for many pathogens. Physically removing and destroying (e.g., burning, burying, etc.) crop residues are important cultural control practice performed during intercrop periods. The effect of destroying crop residues on particular pathogens depends on the type of crop (annual, perennial, or harvested product), the extent of the cropping area and the survival mechanisms and host ranges of the target pathogens.

Sanitation in Preventing the Build-up of Pathogens

Wash soil off of farm equipment, including brushing off soil particles from shoes. These practices are especially important to prevent movement of soilborne pathogens such Sclerotinia sclerotiorum (causal agent of White mold), Phytophthora capsiciVerticillium dalhiae, and different species of Fusarium. A power washer is an important piece of equipment in the battle against these diseases.

Cover Crops

Disease suppressive cover crop rotations may provide an additional tool for managing soilborne diseases. The effect, however, is highly variable, differing between locations, and between years. At least three mechanisms are at work.

Mulching

Mulching, the application of a covering layer of material to the soil surface, is a commonly used cultural practice, especially in horticulture. Natural materials used for mulching include cereal straw and stalks, crop debris, sawdust, leaves, grass, compost, and manure. When crop residues are used as mulch they provide many pathogens with a food source as well as an environment in which to live and reproduce and can, therefore, increase the incidence of a disease.

Irrigation Management

Irrigation management is clearly an important factor when it comes to disease control. Regardless of the irrigation method a grower chooses (furrow, sprinkler, or drip), timing and duration of irrigations should satisfy crop water requirements without allowing for excess water.

Plant Nutrition

A properly nourished plant is able to withstand or tolerate the attack of pathogens much better than a plant that has nutrient deficiencies or has been excessively fertilized. For example, fertilizing with phosphates can delay the onset, and lessen the severity of take-all in barley (Gaeumannomyces graminus) and reduce the incidence of potato scab (Streptomyces scabies).

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