Scab is a general name for a disease that causes minor russeting of the plant skin tissue to more severe rough corky areas which may be pitted or raised.
Spots may be singular or join together to form larger lesions.
APPLE SCAB (Venturia inaequalis)
The same disease causes black spots on apple leaves and scabs on the fruit.
Infected fruit can develop deep cracks and grow into irregular shapes.
It is likely to occur if leaves remain wet at mild temperatures for 12 hrs or more (ie 13-16 degrees C).
It can be controlled by a regular spray program of copper-based sprays in early spring, followed by mancozeb, or zineb.
POTATO SCAB (Streptomyces scabies)
This affects root vegetables, such as potatoes, turnip & beetroot.
It starts as small spots on surface of the tuber or swollen root, spreading to become large scabs.
Control: Keep soil acidic, rotate crops and don't allow soil to become too dry while root crops are forming.
RHIZOCTONIA SCAB (Rhizoctonia solani)
This attacks a number of plants, most notably potatoes.
The disease is prevalent in cool moist conditions.
New shoots may rot before they appear above the ground. Mature leaves may be curl at the edges and be discoloured with reds or yellows.
The leaves of the growing tip of the plant may be crowded together on the stalk.
Tubers may be malformed, cracked or have depressions.
This fungus has various strains that attack different plants.
Control: Plant disease-free tubers and practice crop rotation.
CITRUS SCAB (Sphaceloma fawcettii)
This affects many citrus varieties, but rarely sweet orange.
It produces scabs on leaves, fruit & stems.
Control: Using a copper-based spray with white oil added, spray once in spring. Towards the end of summer, spray again with zineb.
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