Parasitic Fungi

Parasitic organisms that cause disease are called pathogens.

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. They can be spread airborne, soil-borne, water-borne, seed- or clone-borne, or vector-borne. Airborne inoculum can travel for great distances, while soil-borne inoculum is rarely spread any great distance. Many pathogens can be dispersed by more than one mechanism.


Continuous infection chains

The survival of most plant pathogens requires the repeated infection of host plants, which is known as the infection chain; it can be continuous or discontinuous. Continuous infection chains can include the same or alternative hosts. The parasite survives by repeatedly infecting plants of the same host species or infects the main crop species and another species, the alternative host. If the alternative host shows no disease symptoms, it is called a disease carrier. The parasite forms no resting structures, and is dependent on the existence of a susceptible host species.  

Some plant pathogens 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 insects, and are referred to as alternate hosts. Unlike the case of alternative hosts, the alternate host is a necessary step in the infection cycle. Without an alternative host, the infection chain breaks.

Discontinuous infection chains

Discontinuous infection chains usually involve an epiphytic, saprophytic or resting phase. During an epiphytic phase, the pathogen survives on 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.

Fungi and nematodes form resting structures that enable them to survive long periods without a suitable host, or when the environment is 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. 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.


Inoculum can be classified as primary or secondary inoculum. Primary inoculum consists of propagules of a pathogen that initiate the disease cycle in a new growing season while secondary inoculum distributes the pathogen within the main growing season of the crop.

Inoculum can be carried from plant to plant by air currents, through the soil, by water splash, or by a vector species, such as an insects or animals. Some bacteria and fungi are spread by sticking to the outside of insect vectors. Disease can also be spread in plants, 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.

Common fungal diseases include powdery mildew, rust, leaf spot, blight, root and crown rots, damping-off, smut, anthracnose, and vascular wilts.


These fungal diseases are very host-specific. Powdery mildew can be observed on lilacs, roses and several other plants. The symptom is a white or gray, powdery growth on leaves and stems. Powdery mildew usually would not kill a plant; it would weaken them instead. Adequate sunlight and air circulation can be provided to lower relative humidity and using fungicide sprays for high-profile plants to control powdery mildew.


Rusts are also host-specific. Masses of easily noticed, orange or dark red spore masses on leaf tissue are observed. Rusts commonly develops during cool weather. They are spread by wind and splashing water and require water to reproduce and for infection. Controls include watering early in the day and applying protective fungicidal sprays.


These diseases, which include water mould diseases, attack a wide variety of plants. They damage roots and crowns of plants. The damage to the root system results in poor growth, yellowing or stunting of the plant. Root rot fungal pathogens are found in almost all soils. However, they do not thrive in well-drained soils. They can live in soil for years in a latent state. Proper cultural practices, such as correct planting depth and improving drainage, are important controls.


Examples of Parasitic Fungi

Cordyceps ophioglossoides

Beauveria bassiana

Claviceps purpurea

~ by mb0804myco on July 29, 2008.

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