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INFECTION PROCESS: DISEASE DEVELOPMENT

The amount of disease that develops in a plant community is dependent on properties of the host, the pathogen and the environment. The environment can affect both the susceptibility of the host (e.g. by creating stress in the plant) and the activity of the pathogen (e.g. providing moisture for spore germination). The pathogen and the host can affect each other's performance. The plant can also change its environment, by creating a microclimate around it.

Table 1. Factors that affect disease development.
Pathogen Host Environment
  • Presence of pathogen
  • Pathogenicity
  • Adaptability
  • Dispersal efficiency
  • Survival efficiency
  • Reproductive fitness
  • Susceptibility
  • Growth stage & form
  • Population density & structure
  • General health
  • Temperature
  • Rainfall / Dew
  • Leaf wetness period
  • Soil properties
  • Wind
  • Fire history
  • Air pollution
  • Herbicide damage

THE PATHOGEN

The presence or absence of a pathogen is the main factor that determines whether disease occurs. Introduction of a pathogen to an area from which it has previously been absent can cause major outbreaks of disease in plant communities. The amount of disease that develops is often determined by the pathogenicity of the main pathogen. The term pathogenicity relates to both the virulence (infection ability) and the aggressiveness (the vigour of the infection) of the pathogen. Pathogenicity is dependent on the pathogen's reproductive, dispersal and survival fitness.

The adaptability of the pathogen is also important in determining its ability to infect resistant hosts or to survive changed environmental conditions. Adaptability is determined by the pathogen's genetic flexibility and reproductive efficiency. The spread of a disease and the formation of epidemics is reliant on the pathogen's ability to disperse rapidly over long distances. The spores of cereal rusts, for instance, can be blown over vast distances in a few days, while soil-borne pathogens have little scope for extensive spread. For a pathogen to cause disease in successive seasons, it must be able to survive the intervening time. Some pathogens form spores or sclerotia that can survive in the soil for years, while others colonise alternative plant species until the season of their primary host comes around again. Beyond the mere presence of a pathogen, the number of infective propagules available to infect plants is a crucial factor in determining the amount of disease that develops. Generally, as the number of propagules increases, the level of disease increases, levelling off when the amount of disease reaches very high levels and there are few uninfected plants available. The survival of propagules, and therefore the number of propagules available to cause disease, is heavily influenced by environmental factors.

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THE HOST

The development of disease in a plant community relies on the presence of individual hosts that are susceptible to that particular pathogen. If the majority of the population is susceptible to the pathotypes of a pathogen in the vicinity, an epidemic can occur.The best way of controlling disease is by planting species or cultivars that are not susceptible to pathogens of that area. The occurrence of disease can also be influenced by the host plant's growth stage and form. Some diseases are common in seedlings, while others are typical of mature plants. The growth stage of the population can also affect the microclimate around the plants; for example, the humidity and sunlight levels under the canopy. The population structure and density will also affect the development of disease in a plant community. The density of the main host species and the proportion of other plants that are not hosts within the community will determine the rate and extent of epidemic development. Crop plants tend to be densely planted, with no other species in amongst them, making them more susceptible to rapid spread of disease. Extensive, dense plantations can host spectacular epidemics, particularly if a new pathogen is introduced to the area. In addition, the general health of the host plant before infection is important in determining the success of a disease. Necrotrophs do well on poorly growing plants, while biotrophs thrive on a healthy host plant.

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THE ENVIRONMENT

The presence of a pathogen against a particular plant will generally not cause serious disease unless the environmental conditions are favourable. This includes the aerial environment and the soil (edaphic) environment. Human attempts at controlling disease usually involve manipulating the environment in some way. For example, breeding wheat cultivars to tolerate dry conditions allows Australian farmers to plant the crop in areas that are not favourable for pathogens such as powdery mildew and leaf rust. Properties of the aerial environment that influence disease development include moisture levels, temperature and pollution.

Moisture is particularly important to pathogenic bacteria and fungi. Rain splash plays an important role in the dispersal of some fungi and nearly all bacteria, and a period of leaf wetness is necessary for the germination of most airborne spores. By using water for dispersal, propagules are dispersed at a time when they are likely to be able to germinate as well. Because the process of germination and infection takes time, the duration of leaf wetness also influences the success of the infection. The duration necessary for infection varies with temperature. Usually, a longer period of leaf wetness is needed to establish an infection in cooler temperatures, as germination and infection are generally accelerated in warmer conditions.

Temperature also affects the incubation, or latent, period (the time between infection and the appearance of disease symptoms), the generation time (the time between infection and sporulation), and the infectious period (the time during which the pathogen keeps producing propagules). The disease cycle speeds up at higher temperatures, resulting in faster development of epidemics. The period of leaf wetness, combined with temperature information can be used to predict outbreaks of some diseases (infection periods) and be used to time preventative treatments, such as spraying. A recently recognised aspect of the aerial environment that can influence disease in plants is air pollution. A high concentration of pollutants can affect disease development and, in extreme cases, damage the plants directly by causing acid rain.

The edaphic (soil) environment affects soil-borne diseases, largely by determining the amount of moisture available to pathogens for germination, survival and motility.   Germination and infection success also rely on the temperature of the soil. The fertility and organic matter content of the soil can affect the development of disease. Plant defences are weakened by nutrient deficiency, although some pathogens, such as rusts and powdery mildews, thrive on well-nourished plants. Other diseases thrive in soils that are specifically low in organic matter.

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INTERACTION BETWEEN FACTORS

The pathogen, the host and the environment interact, usually in ways that are difficult to quantify and predict. Control measures can include sowing of a crop species early, to avoid exposing seedlings to a disease during the time of year that provides the best environmental conditions for the pathogen.

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