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PATHOGEN SURVIVAL AND DISPERSAL OF PLANT PARASITES

The survival of a parasite between cropping seasons and its effective dispersal to uninfected plants are crucial aspects of the plant disease cycle. If either of these is prevented, the disease will not occur. Most pathogens possess mechanisms to survive intercrop periods or periods of unfavourable environmental conditions. The spread of inoculum can be airborne, soil-borne, water-borne, seed- or clone-borne, or vector-borne. Airborne inoculum can travel for great distances, even across oceans, while soil-borne inoculum is rarely spread any great distance. Many pathogens are dispersed by more than one mechanism.

SURVIVAL

Continuous infection chains

The survival of most plant pathogens requires the repeated infection of host plants. This is known as the infection chain. The infection chain can be continuous, or discontinuous (incorporating a resting phase). Continuous infection chains can involve the same or alternative hosts. The parasite survives by continually infecting plants of the same host species, or, if the host species has a dormant or intercrop period, alternately infecting the main crop species and another, often related, species, the alternative host. If the alternative host does not display disease symptoms, it is called a disease carrier. The parasite does not form resting structures, and it is dependent on the presence of a susceptible host species. If the crop species grows throughout the year, for example, in tropical areas, the parasite can survive by continuously infecting new individuals of the same host species. Where the crop is not grown year-round, self-sown individuals of crop species that grow by the side of the road or as weeds in paddocks can act as hosts for parasites between cropping seasons.

There are some plant pathogens that cannot be transferred directly from one plant to another plant of the same species. They require another, completely unrelated, species to act as a vector. These vectors are usually, but not always, insects, and are referred to as alternate hosts. Unlike the case of alternative hosts, which are used opportunistically and where necessary, the alternate host is a necessary step in the infection cycle. If there is no alternate host available, even if there are susceptible plants available, the infection chain is broken.

Discontinuous infection chains

Discontinuous infection chains usually involve an epiphytic, saprophytic or resting phase. During an epiphytic phase, the pathogen survives on the surface of their host or other plants in a non-parasitic relationship as an epiphyte. Pathogens that go through a saprophytic phase survive during intercrop periods on diseased plant debris or other organic matter on or in the soil. Some of them can compete very effectively with the normal soil microflora. Others specifically inhabit the diseased plant debris. Fungi and nematodes are able to form resting structures that enable them to survive long periods without a suitable host, or when environmental conditions are unfavourable. The resting spores (oospores, teliospores or chlamydospores) of some fungi can survive for twenty years or more. Some of them are triggered to germinate only by secretions from the roots of suitable plants, reducing the risk of germinating without an available host. Other fungi produce sclerotia, which can also survive in the soil for periods ranging from months to years. Fungi can also produce sexual fruiting structures (such as cleistothecia, perithecia and pseudothecia) during the resting stage. Some fungi go through a resting stage after infection, called a latent infection. For example, loose smut fungus of wheat infects wheat embryos in the flowers, becomes dormant and is activated again when the seed germinates. When the plant matures, the fungus produces teliospores in the place of inflorescences. Nematode eggs can survive for long periods in egg cysts or gelatinous egg masses, which reduce the rate of egg desiccation.

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DISPERSAL OF INOCULUM

Inoculum can be classified as primary or secondary inoculum. Primary inoculum consists of propagules of a pathogen that start the disease cycle in a new growing season. Secondary inoculum distributes the pathogen within the main growing season of the crop. It is usually secondary inoculum that leads to the development of epidemics.

Inoculum can be carried from plant to plant by air currents, through the soil, by water splash, or via a vector species, such as an insect, other animal, fungus or plant. Insect vectors often have piercing and sucking mouthparts, penetrating the plant surface and providing an infection pathway for viruses, phytoplasmas and plant parasitic protozoa that they are carrying inside their bodies. Some bacteria and fungi are spread by sticking to the outside of insect vectors. Disease can also be dispersed in planting material, such as clones and seeds. Seed-borne inoculum can be mixed in with the seed during harvesting, attached to the surface of the seed, or present inside the seed, having already infected the seed or embryo. About one fifth of known plant viruses are dispersed via seed.

Link See Fungal Biology for more information on dispersal of fungal pathogens.

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DISTRIBUTION OF DISEASED PLANTS

The distribution of diseased plants within a population (see Figure 3 below) can provide information about the source of inoculum or the nature of the vector. Random distribution of diseased plants is not common. It can be a result of seed-borne infection, insect-borne inoculum or airborne inoculum being introduced from a long way away. Aggregations of diseased plants are more common, indicating a random distribution of inoculum, which has then spread from the originally infected plants. A disease dispersed by aphids, or a root-infecting fungus might be expected to produce a pattern of aggregation as the inoculum spreads from the initial diseased plants. Formulae and computer programs exist that can analyse the pattern of disease spread, and help to locate the source or the vector of the inoculum.

Regular distribution of diseased plants is highly unusual in the field. However, it could occur in vegetatively propagated crops if all planting material was infected, or if a previous crop that was evenly spaced left pathogens in the soil that were able to infect the next crop. Patch distribution of disease is characteristic of soil-borne diseases, such as root-rotting fungi or nematodes, or diseases carried by soil-inhabiting vectors. Soil-inhabiting organisms usually spread very slowly, as do the diseases carried by these vectors, hence the patchy distribution. A very common pattern of disease distribution is a gradient. Gradients usually indicate that the source of the inoculum is outside the crop, and the steepness of the gradient is proportional to the closeness of the source. Other factors can influence the slope of the gradient, such as how the disease is spread (eg. Crawling insects v flying insects). Vector movement has a much greater influence on the spread of disease than vector number. A few active vectors will spread the disease much more rapidly that many static vectors.

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