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Potato Common Scab


Potato Common Scab

Usually appear in

Spring to Summer

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Caused By:-     The Bacteria Streptomyces scabies


Potato Common Scab is a bacterial disease of potato skin, which cause rough, scabby patches on the surface of the tubers.
Common scab is caused by the bacteria Streptomyces scabies which can be serious on potatoes, beetroot, carrots, radishes, swedes and turnips.
This bacteria exist in the soil, either as free-living S. scabies or as spores They invade the surfaces of potato tubers by means of a tiny thread that pushes directly through the tender skin of the young forming potato. It continues to grow until the crop is harvested. The infected areas on the potato respond by laying down a corky layer, which actually limit the spread. As each layer is invaded by the bacteria, new cork formation takes place until a mature scab spot is produced. Common Scab does not develop further after the potatoes are dug, but the organism remains alive all winter in storage.
Commmon Scab is worse when soil conditions are dry when tubers form as this encourages the skin to crack and allow the bacteria to enter. Older tuber has a thicker protective layer on its surface and is not readily affected by this bacteria although it become more aggressive on any given crop where it is grown year after year without crop rotation.
Severe attacks of scab can lead to massive distortion of tubers, reminiscent of wart disease caused by Synchytrium endobioticum, a serious but now very rare potato disease.
Light attacks of both diseases are only superficial and do little to affect eating quality as after cooking the scab pulls away with the skin, but they are commercially important because cosmetic damage lowers the value of the crop. Severe attacks can lead to cracking of the skin and rotting of the tubers.
Picture by Clemson University - USDA Cooperative Extension Slide Series,


In a garden where irrigation is available, scab can be reduced by keeping the soil reasonably wet for several weeks while the young tubers are beginning to form as this prevents the skin from cracking thereby allowing the ingress of the bacteria.
. Do not plant potatoes after brassicas have been grown as lime is usually added to the soil for brassicas and this increases the ph of the soil and make it more prone to scab. Adding organic matter when planting should reduce the soil pH to about 5.2 which is ideal for potatoes.
If possible avoid rotation planting of potatoes after plants which are susceptible to S. scabies are grown and avoid Red Clover as it is said to increase the problems with common scab.
Try to get all the potatoes out from the ground at harvest time.
Water from the base of the plant rather than spraying potatoes.
After harvest, check for signs severe scab and remove any suspect tubers as they may rot from secondary fungal invasion in store.
Use only good quality seed potatoes obtained from certified suppliers
Grow resistant varieties of potatoes as some show resistance to this bacteria.
Visit The British Potato Variety Database for more information.
Fungicides for the control of Common Scab is not recommended, but are available to some farmers.

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