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Scab or freckle is a fungal disease of stone fruit caused by Venturia carpophila (synonym Cladosporium carpophilum). The disease affects most stone fruit but is most important on mid and late season peaches and nectarines, and on apricot and almond. It may cause loss or downgrading of fruit quality and is more severe in southern Victoria.
Symptoms
Scab can affect fruit, leaves and shoots. It first appears on fruits as small dark spots about six to eight weeks after petal fall. On mature fruits, the fungus forms small, circular, sooty-brown spots or freckles which become scabby. These can merge to form large, irregular dark brown lesions. When infection is severe, the fruit can crack, shrivel and fall prematurely. On apricot fruit the disease should not be confused with Shot-hole, which causes raised scabs on the fruit surface; by contrast Scab lesions are pale green and remain flush with the fruit surface. On peaches, lesions are flat, circular black spots up to 3 mm in diameter. When nectarines are affected, the skin loses its pigment and becomes pale green to cream in colour. The centre of each spot is dark with the development of spores.
Leaf infections appear as sooty or olive blotches on the underside of leaves, and as dark lesions running along the mid-rib and petiole. Severe leaf infection can cause defoliation, but in some cases, little or no leaf infection can be found even when the fruit is badly affected.
On the shoots, small brown lesions with slightly raised margins may appear. The margins of these lesions become olivaceous where the pathogen is sporulating.
Life-cycle
The pathogen overwinters on twig lesions, and spores are blown or splashed onto developing fruits, leaves and shoots. Infections are most severe during wet weather in spring and summer. There is a long incubation period of around 45 days before symptoms are seen. The fungus can also overwinter in infected leaves which fall to the ground in autumn.
Control
The disease can be controlled by sprays of suitable fungicides at shuck fall (the time when dried floral remains fall from the developing fruitlet) and again seven to eight weeks later. Pruning out infected shoots should also assist by reducing the potential carry-over of the disease.
For effective pest and disease control, correct diagnosis is essential. A commercial diagnostic service is available at the DPI PIRVic, Knoxfield Centre. For further information, contact the Diagnostic Service. ph: (03) 9210-9222 or fax (03) 9800 3521.
The previous version of this note was published in December 1999.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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