A canker is a localised spot of dead tissue.
It often starts as a small but definite dead area, then spreads, perhaps girdling a stem.
This can then virtually "ringbark" a stem, killing growth further up. Cankers are caused by both bacteria and fungi.
They commonly develop from a damaged or wounded area of tissue.
If there is dieback from the tips of a plant, the cause may be a blight, or a canker which is "choking" the plant somewhere below the tip.
Citrus canker (ie. a bacteria: Xanthomonas campestris pv. citri), is one of the most serious citrus diseases, worldwide.
It affects most important commercial citrus varieties causing necrotic (ie. dead) spots or patches over the fruit, leaves and stems.
Fruit quality is impaired, and developing fruit drop from the tree before they mature.
The lesions (ie. dead patches) appear firstly as small, slightly raised, water soaked, dark green, circular spots. They look the same whether on fruit, leaves or stems. As they develop, they become grey to white, and break open developing a soft depressed centre. The edge of these lesions is sharply defined (ie. it has distinct boundaries). On fruit the lesions are usually between 1 and 6cm in diameter. On leaves they can be larger.
The only effective control of citrus canker is to destroy infected plant parts (in early stages of infection) or entire infected plants if the infection is severe.
This disease originally comes from Asia, but has now spread widely. Citrus canker was eliminated in the USA in 1949 by a massive program which involved burning millions of affected trees. Despite such drastic measures, it reappeared in the USA in 1984.
Australia, New Zealand and South Africa all have citrus canker. Countries which are free of citrus canker, maintain strict quarantine/prohibition controls to prevent it spreading to their countries.
Rose Canker (Coniothyrium fuckelii)
Healthy plants are rarely infected. Some damage to the plants is essential for the fungus to enter the rose plant - usually poor pruning technique or when branches rub against itself. The fungus grows down the stem causing necrosis of the infected material. Ridges frequently occur between infected and non-infected plant material. In badly infected cases, spore bearing, black spots may appear to spread the disease.
Control: Improve pruning techniques and hygiene. Prune off diseased material about 100 mm below infected sections. Make a clean, angled cut above a bud using sterilised secateurs. There is no successful fungicidal treatment.
Want to Learn More?
Check out publications in the ACS Bookshop: www.acsbookshop.com
For more information on Courses on Pests & Diseases:
In Australia: www.acs.edu.au/Courses/General-Horticulture-courses.aspx
In the UK: www.acsedu.co.uk/Courses/General-Horticulture-Courses.aspx