Plant Diseases

Plant diseases have accompanied humans throughout their evolution, affecting the quality and quantity of food available to them. Wet years often resulted in the destruction of most or all of the crops grown by early human farmers, and famines were common. Some plant diseases had a strong influence on the course of history of certain countries. Most famous in recent times is Ireland's late blight epidemic of potatoes in 1845, which contributed to a famine resulting in the death of an estimated one million people and the emigration of another 1.5 million, primarily to North America.

Plant diseases and the associated science of plant pathology that studies causes of plant diseases, gained attention in Saskatchewan at the beginning of the 20th century, when severe epidemics of the stem rust fungus caused serious losses in wheat. As a result of the rust epidemic of 1916, with an estimated loss of 100 million bushels of wheat, the Dominion Laboratory of Plant Pathology was established on the campus of the University of Saskatchewan in 1919. This laboratory eventually became part of the Canada Department of Agriculture Research Station that evolved into the Agriculture and Agri-Food Canada Research Centre Saskatoon, where there is still a strong emphasis on the study of plant diseases. In the Department of Biology, where the laboratory was initially housed, courses on fungi and plant diseases were first offered in 1920 and a strong tradition in the study of plant diseases was established. Since then, several other private, federal and provincial organizations have been active in conducting research on diseases, developing disease management strategies or supporting farmers in Saskatchewan by offering diagnostic and advisory services.

Plant diseases are defined as plant disorders due to microoganisms or adverse environmental conditions. These eventually become visible in the form of symptoms such as spots, blights, rots, wilts, and galls. Depending on the disease, symptoms can be found on all plant parts such as roots, stems, flowers, and leaves. Microorganisms that cause diseases on plants are called pathogens and include viruses, phytoplasms, bacteria, fungi, and nematodes. Adverse environmental conditions that can result in plant disease include temperature, moisture, mineral nutrients, and pollutants at levels above or below the tolerance level of plants. Most plant diseases are, however, associated with microorganisms; and in Saskatchewan most of these are caused by fungi.

While in the early days research on plant diseases was focused on diseases of wheat such as stem rust, bunt and various root rots, further diversification of the cropping system brought new crops into Saskatchewan, and with them new diseases. Currently more than sixty arable and horticultural crops are grown in the province, and there are several hundred diseases that are causing more or less severe damage on those plant species. Damage can range anywhere from less than 1% to total crop failure. Because most fungi require moisture to be able to infect a plant, more damage is encountered in years with wet summers. A lot of research has concentrated on the breeding of resistant crop varieties as the most economical way of disease control, and on the development of strategies to reduce the impact of diseases by other means. Emphasis is placed on the development of so-called integrated disease management strategies that incorporate sound cropping practices, good crop rotations, healthy seed, the use of resistant cultivars, the use of fungicides only when required, and the use of biological control methods where available. Plant diseases also affect ornamental crops including trees, shrubs, and flowers in house gardens and parks. A well-known disease of a city tree species is Dutch Elm disease, which has migrated with beetles into Saskatchewan and is endangering elm trees in the province. Plant diseases are also common on forest trees and can cause serious economic damage in timber production. Rots and staining of wood due to fungi are the most common diseases found in forestry. Many fungi attacking trees belong to the commonly known mushrooms and bracket fungi, some of which are edible. Forest pathology is a special area in the large field of plant pathology, dealing specifically with those micro-organisms that infect forest trees. From 1947 until 1965 Saskatchewan had a forest pathology laboratory of the Forest Department, before research on forest pathology problems across the three prairie provinces was consolidated in Edmonton.

Sabine Banniza

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Further Reading

Agrios, G.N. 1997. Plant Pathology. San Diego: Academic Press; Bailey, K.L., B.D. Gossen, R.K. Gugel, and R.A.A. Morrall. 2003. Diseases of Field Crops in Canada. Ottawa: Canadian Phytopathological Society; Conners, I.L. 1972. Plant Pathology in Canada. Ottawa: Canadian Phytopathological Society.