Clubroot is caused by the soil-borne fungus Plasmodiophora brassicae. The disease occurs worldwide and only affects plants in the cruciferae family including canola, juncea canola (mustard), cabbage, cauliflower, brussel sprouts and broccoli.
In Australian vegetable brassicas, clubroot is widespread and causes significant yield losses. The Australian oilseed industry has been somewhat protected from clubroot as the major production areas for vegetable and oilseed brassicas are usually separated from one another. In addition, most Australian pathotypes of clubroot are only able to cause disease in the warmer months and require irrigation water for dispersal. However in recent years clubroot has been found in canola crops throughout the growing regions of Western Australia.
Swollen, galled roots are the most typical symptom of infected plants. This ranges from tiny nodules, to large, club-shaped outgrowths. The galls are at first firm and white but become soft and greyish brown as they mature and decay. Affected roots have an impaired ability to transport water and nutrients, (Figure 11.23).
Figure 11.23 Canola roots with severe club root infection. Photo: S. Marcroft
Resting spores of the fungus can survive in soil for many years, even in the absence of a susceptible host. Infection can occur at any stage of growth and is restricted to the roots. In the presence of susceptible roots, the spores germinate and release tiny motile spores that swim in free water to the surface of the rootlets, penetrate and form a fungal colony (plasmodium) inside the root cells. The fungus causes cells to enlarge and divide rapidly resulting in the characteristic galls. Late in the season resting spores develop in the infected roots and are released into the soil as the galls decay. Fields become infested mainly by the movement of soil on cultivation equipment and by seedling transplants.
In the Australian vegetable brassica industry several methods of control have been developed that may be useful for oilseed brassicas as follows:
- infested fields are kept free of susceptible crops and weeds for at least 5 years, to allow sufficient natural decay of the long-lived spores
- do not move cultivating equipment from infested to non-infested areas before thoroughly cleaning the equipment
- clubroot thrives in acid soils (pH <7.0), liming to increase soil pH (7.0-7.5) has been successful for vegetable brassicas but would be cost prohibitive in most canola areas.
More detailed information can be obtained from the DEDJTR AgNotes Series www.vic.gov.au/graindiseases
Victorian Winter Crop Summary www.vic.gov.au/victorianwintercropsummary
Blackleg Management Guide http://www.grdc.com.au /
Canola Blackleg Resistance Ratings
National Variety Trials (NVT Online) www.nvtonline.com.au
Australian Oilseeds Federation www.australianoilseeds.com
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