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DISEASE MANAGEMENT: CULTURAL MANAGEMENT PRACTICES

Introduction

Cultural disease management practices are the measures undertaken by humans to prevent and control disease by manipulating plants. In the case of low-return crops, these might be the only forms of disease management that are economically viable. Cultural management can include reducing the amount of initial inoculum, reducing the rate of spread of an established disease, or planting a crop at a site that is not favourable to pathogens because of its altitude, temperature, or water availability.

REDUCING THE LEVELS OF INOCULUM

Practices that reduce the initial levels of inoculum include selecting appropriate planting materials, destruction of crop residues, elimination of living plants that carry pathogens, and crop rotation. The selection of appropriate planting material can involve planting resistant cultivars, planting a number of mixed cultivars, using certified seed and ensuring that disease is not spread on vegetative propagating material or on equipment. Many agricultural systems are characterised by dense populations with genetic homogeneity (monocultures). Once a disease becomes established in such a plant community, it can rapidly spread to epidemic proportions. Hence the value of planting mixed cultivars, which incorporate a range of resistances.

The destruction of crop residues, which can harbour many pathogens, by burying, burning or removal is an important practice performed between cropping seasons. How this is done depends on the type of crop, the type of pathogen and the size of the crop. Burying crop residues can destroy some pathogens, if ploughed in deeply enough, but some pathogens can survive, and even benefit from this process if it serves to spread them throughout the field. Burning crop debris is a common practice, especially for cereal crops, and it is a successful method of destroying many pathogens. Its success depends on the intensity of the fire. There are disadvantages to burning also, such as loss of nutrients, smoke pollutions, increased soil erosion and contributing to the greenhouse effect. Because burning can completely destroy a source of inoculum, it is commonly used in eradication campaigns against newly introduced pathogens.

The elimination of living plants that carry pathogens can include both remnant or diseased crop plants and also wild plants or weeds that can act as alternative hosts between seasons. Some rusts, for example, cannot complete their life cycle in the absence of an alternate host, where they undergo sexual recombination. Removing alternate hosts can delay an outbreak, but often inoculum finds its way to crop plants via wind or another vector.

Crop rotation refers to the successive planting of different crops in the same area, sometimes with a fallow, or resting, period in between crops. The most successful practice rotates crops over periods that are longer than the survival period of the pathogen, thus reducing or eliminating inoculum in the interval before a susceptible crop is planted there again. It tends not t be very effective against pathogens that survive for a long time in the soil.

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REDUCING THE RATE OF SPREAD OF DISEASE

The rate of disease spread can be reduced by controlling the spacing of plants, humidity, moisture levels, and amount of sunlight. These factors are more easily controlled in a greenhouse environment, but some can be manipulated in the field also. In general, wider spacing of plants reduces the speed with which disease moves between them, and reducing moisture levels can inhibit infection by pathogens. The amount of time that the plants and soil are wet can be reduced by watering in the morning, spacing plants further apart, pruning or training to reduce canopy cover, and orienting rows of plants with relation to the prevailing wind. The use of shade cloths can help to control fungi that need UV light in order to sporulate.

Tillage practices have indirect effects on the spread of plant pathogens, although, some forms of inoculum can be spread extensively during tillage. Tillage can bury pathogens in the topsoil in deeper where they are less likely to cause disease. Preparation of seed beds can greatly alter the soil's texture, aeration, temperature, moisture levels and density. Tillage can also influence nutrient release in the soil and can generally benefit the crop. Farming practices have moved away from regular tillage, reducing damage to roots and spread of pathogens caused by tilling machinery. However, minimum tillage can also encourage some pathogens, such as those that feed on crop residues left on the surface of the soil.

Sowing practices, such as changing time, depth and direction of sowing, and changing the density of the crop can protect plants from pathogens to which they are susceptible only at certain stages of their development. Changing the time of sowing can exploit weather conditions that are unfavourable to the pathogen, thus reducing crop losses. This might require the use of a specific cultivar that is adapted to the selected growing period, but might also be susceptible to different pathogens. The depth of sowing can have a bearing on the chance of infection, as the seedling's pre-emergence stage, which is usually more susceptible to attack, is longer when seeds are planted deeper. However, deeper planting can stimulate germination.

Crop density and disease incidence are usually correlated, mainly due to the ease with which inoculum is transferred between plants when they are close together and their leaves and roots are able to touch. Also affecting plant susceptibility in densely planted crops is the microclimate created by their crowding. Temperatures are more uniform, humidity is increased and leaves stay wet for longer, all of which favour the development of disease.Crop density can be manipulated by sowing, pruning, thinning, trellising, fertilisation, water management, staking and harvesting some plants or plant parts.

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INTERCROPPING

The practice of planting more than one crop in alternating rows, or intercropping, can reduce disease by increasing the distance between plants of the same species, and creating a physical barrier between plants of the same species. Intercropping is more labour intensive the more crops there are, but it is usually beneficial. How successful intercropping is depends partly on the combination of crop plants chosen, since some combinations can actually make disease worse by providing an alternate host or a stimulus that encourages germination of inoculum on the neighbouring species.

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MULCHING AND SOIL AMENDMENTS

Mulches are used to conserve moisture and organic matter and reduce erosion in the soil. They are usually some kind of organic matter, such as straw, sawdust, manure or aquatic plants. Some manufactured products, such as plastics, asphalt paper and paper are also used. If crop residues are used for mulch they can provide a food source and attractive environment for pathogens and increase the incidence of a disease. Mulch or crop residues can also influence disease incidence by altering the immediate environment. Water retention, nutrient enrichment of the soil, decrease in soil temperature, weed inhibition and seedling protection are all effects of mulching that can influence disease development. Spread of soil-borne diseases that rely on splash-dispersal can be reduced, but other diseases, for which the altered environment is favourable, can be enhanced. The addition of organic amendments to the soil can also reduce disease influence by increasing the activity of competing or predatory micro-organisms in the soil, although it can also increase the incidence or severity of diseases caused by pathogens that thrive on the amendment.

Flooding

Flooding can be used as a form of disease management, as in the example of growing rice in paddy fields. Its primary purpose is to reduce weeds, but it can also reduce the number of fungal propagules, insects and nematodes in the soil. While flooding can aid the destruction of crop debris carrying inoculum, it can also carry propagules of some pathogens growing in the flood waters. Flooding has variable success in disease management, depending on the pathogens present.

Irrigation

Most forms of irrigation play a detrimental, rather than beneficial role in plant disease management. For example, irrigation during dry seasons prevents the desiccation of propagules during drought, thereby increasing the level of inoculum. The irrigation water can also carry propagules and spread disease, unless it is treated before use. Similarly, overhead watering can prolong leaf wetness, increasing the likelihood of germination and infection by fungal spores. It can also facilitate splash-dispersal of inoculum. However, irrigation does not always encourage disease, and in some cases it can be used to actively reduce the level of inoculum and retard disease development. Alternate drying and wetting of soil encourages the activity of micro-organisms that destroy sclerotia, and brief overhead watering can wash spores away without causing long periods of leaf wetness. Trickle or drip irrigation is generally the least likely to encourage disease, since water is delivered directly to root zones and at a rate insufficient to disperse pathogens.

Roguing

The removal and destruction of diseased plants, or rogues, can prevent further spread of disease, but is labour intensive. For that reason, it is practicable only in small plantings, or where labour is cheap or if the crop is very valuable. Effective roguing requires the removal of diseased plants as soon as possible after symptoms are observed. In some cases, the diseased plants as well as those immediately surrounding them, are destroyed in an attempt to halt spread of the disease.

Fertilisers and crop nutrition

A well-balanced supply of soil nutrients will result in healthy, vigorous plants, which should have a greater chance of withstanding attack by pathogens that unhealthy plants would. However, many pathogens also thrive under ideal growth conditions, particularly biotrophic pathogens, such as viruses. The major nutrients that influence plant and pathogen success are nitrogen, phosphorous, potassium and calcium.

The application of nitrogenous fertilisers delays crop maturity by prolonging vegetative development. This can increase the risk of infection, since plants are more susceptible to disease at that stage of development.   High nitrogen levels could also influence the production of metabolites by the host, with various outcomes for disease development. In some cases, infection is enhanced, in other cases it is inhibited. The addition of nitrogen to the soil can also alter the activity of micro-organisms and change the micro-environment in the crop due to increased canopy cover and crowding of foliage. The addition of phosphorous fertilisers affects different crops and diseases in different ways, sometimes encouraging and sometimes inhibiting disease. The mechanisms through which phosphorous influences plant disease are not well understood. Potassium generally inhibits disease development, counteracts some of the disadvantages of nitrogen fertilisers, and promotes the healing of wounds in plants, all of which reduce disease. However, potassium's effects can be variable, directly stimulating or inhibiting the penetration, multiplication, survival and establishment of a pathogen. Calcium is a necessary nutrient for the composition of plant cell walls. An adequate supply of calcium produces cell walls more resistant to penetration by facultative pathogens. High levels of calcium can also raise the pH of the soil, disadvantaging any pathogens that favour acidic soils. However, soils high in calcium can also promote the development of some diseases.

Essentially, the use of fertilisers has mixed effects on plant disease. Which fertiliser can be used without promoting disease will depend on the starting soil conditions and, importantly, the pathogens that are present.

Strip farming

Strip farming is a similar practice to intercropping, where areas of one crop are separated from each other with strips of another crop, the rationale being that the two different crops are unlikely to share the same pathogens, thus reducing the rate of spread. The main disadvantage of strip farming is the fact that it is relatively labour intensive.

Trap and decoy crops

Trap crops of susceptible plants are grown on land known to contain pathogens. They become infected and are then destroyed before the pathogens' life cycles are complete, thus reducing the amount of inoculum in the area. Decoy crops stimulate the hatching or germination of pathogen eggs or propagules, but the pathogens are unable to establish and infection of the decoy host and die, again reducing the amount of available inoculum. The ideal decoy crop is one that has economic value and can be used in a routine crop rotation.

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