Hosts
Black leg: potato.
Blight: potato and tomato.
Symptoms
Black leg infections start as a slimy black lesion at the base of the stem, leading to collapse of the foliage, then spreads to the tubers which rot if badly affected.
Blight infections appear as slightly sunken brown patches on the surface with a reddish-brown discolouration in the flesh below. This rapidly becomes a foul-smelling slimy mess if secondary organisms invade the tuber.
Biology
Black leg is caused by a bacterium (Erwinia carotovora pv. erythroseptica) which spreads from contaminated seed to infect young stems. The bacterium then spreads through the stolons to infect the daughter tubers, which develop a soft rot from the stolon end. Closely related 'soft rot' bacteria are widely present in soil and infect tubers through wounds. Both of these problems are more severe in wet soils. Lightly contaminated tubers may appear healthy at lifting, but can then go on to develop soft-rotting during storage (this will then spread to adjacent tubers).
The blight fungus (Phytophthora infestans) spreads in wind-blown rain and infects the foliage, from where spores may be washed down during rain to infect the tubers. Another Phytophthora species (P. erythroseptica) is soil-borne and sometimes causes a disease called pink rot, but is not so serious.
Control
To reduce the risk of black leg use good quality, certified seed tubers. If one or two plants still develop the disease ensure that these are dug up and destroyed as soon as possible. If you use your own seed, do not save tubers from an infected crop.
If soft-rotted tubers are present when a crop is lifted, these should be disposed of. Take great care when lifting the rest of the crop to avoid damaging the tubers, and do not lift when the soil is wet. Use the tubers as soon as possible rather than storing them. If they must be stored, ensure they are dry and pick them over to remove any further diseased or damaged tubers before storing. Do not store in damp conditions or severe loss could occur. The disease is unlikely to spread to any extent if tubers are stored in cool, dry conditions.
The cultivars 'Cara', ‘Kondor’, ‘Orla, ‘Markies’, ‘Valor’ and ‘Remarka’, show some resistance to blight, as do the recently introduced 'Sarpo' cultivars (e.g. 'Tominia', 'Axona' and 'Mira'), which appear to have a high level of durable resistance.
Protective applications of a copper fungicide (e.g. Bordeaux Mixture or Murphy Traditional Copper Fungicide) or mancozeb (Dithane) can also be used.
Prompt removal of infected foliage can save the crop, if infections appear close to harvest. As with black leg and soft rots, do not attempt long-term storage of tubers from a blighted crop.
A number of other fungi can infect potato tubers to cause diseases such as dry rot, gangrene and watery wound rot. If you are uncertain as to the cause of the problem expert diagnosis may be required.
More information on potato blight