Plant Disease Management

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.

Prevention of infection:

Many vaccines are primarily intended to prevent disease and do not necessarily protect against infection. Some vaccines protect against infection as well. Hepatitis A vaccine has been shown to be equally efficacious (over 90% protection) against symptomatic disease and asymptomatic infections. Complete prevention of persistent vaccine-type infection has been demonstrated for human papillomavirus (HPV) vaccine. Such protection is referred to as “sterilizing immunity”. Sterilizing immunity may wane in the long term, but protection against disease usually persists because immune memory minimizes the consequences of infection.

Reducing the levels of inoculums:

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.

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.

Crop Rotation:

Rotation of susceptible and resistant crops is one of the oldest practices used to control disease. It remains an important practice against many diseases, where a specific control, such as host resistance, is not available. Rotation is particularly effective in controlling soil- and stubble-borne diseases. The success of rotation in disease reduction depends upon many factors, which include the ability of a pathogen to survive in the absence of its host and the host range of the pathogen. Those that have a wide range of hosts will be controlled less successfully. For example, sclerotinia stem rot of canola (Sclerotinia sclerotiorum) has been recorded on about 400 different plant species worldwide. In Saskatchewan, it is known to occur on 13 field crops. Pathogens that live indefinitely in the soil are less likely to be curtailed by rotation than those that can survive only brief periods apart from their hosts.

Transmission of pathogens via seed, the presence of susceptible volunteer crops and weeds that harbor the pathogens, and the distribution of pathogens by wind and other means will reduce the benefit derived from a crop rotation. For instance, rotations are ineffective in controlling rusts in small grain cereals because the rust fungi do not overwinter in western Canada. Inoculum from the south is disseminated by wind in summer. In a similar fashion, diseases such as barley yellow dwarf and aster yellows depend largely on the northward movement of insect vectors, often over several hundred kilometres.

Crop rotation should be used in conjunction with other cultural practices to maximize its benefit. In establishing a rotation, select crops that are as diverse as possible. In general, a disease may infect closely related species, but will not injure those that are unrelated to the natural host (e.g. net blotch in barley).

Cereal Grains:

Crop rotation is recommended to control a number of leaf blights of small grain cereals. Leaf diseases such as net blotch (Pyrenophora teres), scald (Rhynchosporium secalis), and speckled leaf blotch (Septoria passerinii) of barley, and tan spot (Pyrenophora tritici-repentis) and septoria blotches (Septoria spp.) of wheat are generally not carried long distances by wind. The pathogens overwinter in crop debris and on the seed. A break of two - three years between susceptible crops will markedly reduce or control these diseases. This rotation break will allow previously infected crop residue to decay in the soil. Sanitation to eliminate volunteer host plants and the use of clean seed is important.

Soil-borne diseases such as take-all (Gaeumannomyces graminis) may be controlled when wheat follows non-susceptible crops, such as oat, pea, flax, sweetclover, sunflower, or fallow. Barley is less susceptible than wheat, but is still affected by the disease. Including barley as a break crop in the rotation should not be considered as it maintains the disease for future susceptible wheat crops. Unlike take-all, common root rot of cereals is caused by several fungi which are both seed and soil-borne. The control of this disease is more difficult to achieve through rotation, particularly in areas where mainly cereals are grown. Non-host crops for common root rot include canola, flax and legumes. However, lentil can be infected by a specific rot root organism (Fusarium avenaceum) which also infects cereal crops.

Ergot occurs on a wide range of cereal plants and grasses. It is most prevalent on rye and triticale but also attacks durum wheat, common wheat and barley, in decreasing order of susceptibility.A rotation allowing one year between successive crops of rye, triticale, wheat or barley will significantly reduce or control this disease. If native or forage grasses border a cereal crop, mowing the grasses at heading will reduce ergot in the adjacent cereal crop.

Oilseed Crops:

Canola acreage is now limited in organic agriculture; however, the two major diseases that affect this crop are blackleg (Leptosphaeria maculans) and sclerotinia stem rot.  For blackleg control, a rotation to non-susceptible crops for a period of three to four years is vital. Blackleg survives in canola stubble which generally takes a long time to decompose. Mustards (brown, oriental and yellow), cereals, and legume crops are resistant to this disease and can be safely used in a rotation.

At one time, sclerotinia was thought to be controlled moderately well by crop rotation. Research has indicated that little difference exists among crops in two, three or four year rotations. Rotations of four years or longer may offer some control. This length of rotation may not be practical in canola growing areas. Sclerotinia also attacks pulse crops, sunflower, mustard, sweetclover and flax. This further limits the crops that can be used in a rotation to manage the disease.

Other diseases such as white rust and staghead can be a problem on polish canola. Since both of these are stubble-borne, crop rotation is an effective method of control.

Pulse Crops: 

Ascochyta blight can infect numerous pulse crops such as field pea, lentil and fababean. Each crop has a specific ascochyta blight fungus. Thus, one does not have to worry about ascochyta from one pulse crop infecting another in a rotation.

With lentil, follow at least a three year rotation (i.e. two year break) involving a cereal, oilseed or other pulse crop. This should be longer (up to four years) if the pulse crop residue is resistant to breakdown (e.g. fababean). Stubble residue is the main source of inoculum for this disease. In field pea, mycosphaerella blight (Mycosphaerella pinodes) can also be introduced as airborne inoculum.

Powdery mildew (Erysiphe polygoni) is specific to field pea. Crop rotation will help, depending on climatic conditions. Dew formation, and lack of rainfall, favours the development of the disease. Inoculum is spread by wind and, once established, powdery mildew increases very rapidly. Field isolation may assist in reducing infection which occurs by wind movement.

Root rot diseases can be a problem on pulse crops. Since the pathogens involved do not usually infect cereals, they can be reduced in severity by including a cereal crop or flax in a rotation. Unfortunately, control is complicated by the wide range of alternate non-crop host plants which these pathogens can infect.

Flax :

Flax is a crop with a limited range of disease problems at present. Serious diseases such as rust and wilt are controlled by disease resistant cultivars. Pasmo (Septoria linicola), seed decay and seedling blights that are specific to flax can be controlled by using rotations with several years away from flax. Careful harvesting and handling to prevent seed coat cracking can reduce seed decay and seedling blight.

Forage Legumes :

Forage legumes such as alfalfa, sweet clover and red clover have numerous disease problems that affect both leaf and root tissue. Almost all major diseases of forage crops are amenable to control through crop rotation. The wilts (verticillium and bacterial), crown and root rots and foliar diseases all survive on dead plant material. When a forage crop is plowed down and another crop is planted, there is a period after which the disease inoculum is reduced and dies out. This time period will vary, depending on the time needed for the forage residue to decompose.

 

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.

Resistant Cultivars

The use of cultivars resistant to the prevalent diseases in the growing region is the most efficient and cost effective means of disease control. There is little additional operating expense and no hazard to the producer or the environment. Equally important is the fact that crop residue of resistant cultivars constitutes less of a disease inoculum problem for future crops.

The use of resistant cultivars in conjunction with other crop management factors, such as field sanitation and crop rotation, can prevent many serious diseases from occurring.

The use of resistant varieties is particularly beneficial where crop rotation is of limited use. This is the case when the disease organism has any of the following properties:

·         The pathogen has a wide range of hosts (common root rots, seedling blights)

·         The pathogen has a long lived resting stage or persists in the soil for a long period of time (fusarium wilt in flax)

·         The pathogen has airborne spores (cereal grain rusts), or is carried by insects (yellow dwarf, aster yellows), that are spread over long distances

·         The pathogen has a very high rate of spread (powdery mildew)

·         The pathogen is seed-borne (cereal smuts, ascochyta blight of lentil, pasmo)

·         The use of the term "resistant" cultivar may be misleading. In the case of ascochyta blight in field pea, the difference in resistance ranges only from very poor to fair. With blackleg in canola, the level of "resistance" ranges from very poor to very good. Growing these "better" varieties does not mean that producers can forget about using rotations or any other management technique that will reduce the level of infection. Such variation in level of resistance within a crop, between varieties, occurs for a number of diseases.

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.

Integration of Disease Management Practices:

Disease organisms and their parasites, host crops, associated vegetation, soil and meterological conditions are all elements of a linked interdependent system.

Since crop plants are subject to more than one disease, methods for the control of each disease must be woven together. In addition, most diseases are transmitted in more than one fashion. This will require integration of several control and management strategies at more than one level. Disease control strategies, in turn, need to be combined with methods for weed, insect and other production concerns. For example, clean cultivation, achieved by burying infected crop residue in the soil, is a highly effective way of controlling many diseases. However, it may be an unacceptable production practice because of its effect on soil erosion and water conservation. Attempts to increase yields by applying higher rates of nutrients, such as nitrogen, can lead to increased leaf disease problems. Manure incorporated into the soil can reduce the level of numerous pathogens. An over-application, however, may create nutrient or chemical imbalances resulting in micronutrient deficiencies.

Integration of disease control and crop production practices must be done carefully and suit the producer, location and type of cropping system. Disease management systems must be devised to utilize control technology that works harmoniously within the cropping system while maintaining the pathogen population below economically damaging levels.