Potato Blackleg - pectobacterium atrosepticum is a bacterial disease that affects potato tubers and is virtually impossible to detect.
It is often more prevalent after wet growing seasons, and where tubers have been stored wet.
This is one reason for not saving potato tubers to resow the following year, as the disease can overwinter unnoticed in the tuber.
Similarly the disease can sometimes be found in tubers that were missed at harvest time or in haulms that have inadvertantly been dug in.
Although quite rare, infected tubers can be missed by commercial seed merchants, then sold on to the unsuspecting grower.
The first sign might be the appearance of pale green to yellow limp stunted foliage, followed by black greasy lesions at the base of the stem, this in turn can lead to foliage collapse.
The disease then spreads to the tubers which rot if badly affected.
Black leg is best controlled by buying disease free seed potatoes and practising crop rotation.
Growing varieties that are less susceptible to the disease can also help!
Prior to planting tubers inspect tubers and destroy any that show signs of rot.
This rotting may not be due to the blackleg bacterium but it is safer to destroy them rather than take the risk of contaminating your soil.