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The vast majority of plant pathogens are fungi Link to Fungal Biology, however, plant diseases are also caused by insects, bacteria, nematodes, viruses and phytoplasmas. Disease-like symptoms can also be caused by abiotic factors, such as temperature, light, chemical agents and water or nutrient deficiencies.

Symptoms of disease include death and destruction of host tissue, wilting, abnormal growth and differentiation and discolouration of host tissue. Some parasites, called necrotrophs, secrete enzymes that kill host tissue, extract nutrients from the cells and then live in the dead tissue. The necrotic lesions caused by pathogens can be localised or extensive. Local necrotic lesions appear as discrete necrotic areas, while extensive, or spreading lesions spread until the whole organ or plant is killed. Wilting occurs when water loss is greater than water intake. It results from either: interference with water and nutrient absorption at the roots, interference with water conduction within the plant (i.e. infection of the vascular tissue), or loss of control of transpiration. Abnormal growth and differentiation results from deviation from the complex balance of interrelated reactions that take place in plants. Parasites can alter the hormonal balance in plants causing an abnormal increase in the size or number of cells, resulting in abnormal growth and differentiation, for example, the formation of galls. Discolouration of tissue is most commonly by chlorosis or mosaics of leaves, both of which can have a number of causes. Anything that interferes with the production of chlorophyll causes leaves to turn yellow, or chlorotic. Mosaicism is a symptom of many virus infections and is characterised by alternating light and dark green areas on the leaves.

Table 1. Some of the most common terms used to describe symptoms of plant diseases.
Blight A disease characterised by widespread death of plant tissue.
Canker A sunken necrotic lesion often of a main stem, branch or root.
Damping-off Collapse and rot of seedlings near soil level before emergence or soon after emergence caused by Pythium spp., Phytophthora spp., Fusarium spp., and Rhizoctonia spp.
Dieback Partial defoliation, twig and branch death and even complete death of plants.
Downy mildew White or grey 'bloom' on leaves and stems caused by production of sporangiophores and sporangia by members of the Peronosporales (downy mildew fungi).
Gall An abnormal growth or swelling produced as a result of pathogenic invasion.
Mosaic Patchy variation of normal green colour in leaves, usually light and dark green mosaic, symptomatic of many viral diseases.
Powdery mildew White powdery 'bloom' on the plant surface caused by the production of fungal mycelium, conidiophores and conidia by members of the Erysiphales (powdery mildew fungi).
Pustule A blister-like spore mass breaking through a plant epidermis.
Rot Disintegration of tissue, often caused by enzymes or toxins produced by pathogens.
Rust Rust-coloured pustules formed by members of the Uredinales (rust fungi).
Scab A discrete, superficial roughened lesion.
Smut A disease characterised by black spore masses on leaves, stems or inflorescences, caused by members of the Ustilaginales (smut fungi).
Vascular wilt A disease in which the pathogen is confined to the vascular system of the host and in which wilting is a characteristic symptom; plants lose their turgidity and become flaccid, leaves collapse.

Link to Washington State University Plant Disease Gallery.


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