Blackleg, Aerial Stem Rot
Also known as Pectobacterium carotovorum, the bacteria that causes Blackleg (Soft Rot) is often found in potato tubers (this is why planting potatoes from the market is commonly frowned upon), but this doesn’t necessarily mean your plants will get this disease. Blackleg usually only becomes a problem in certain situations (warm, wet soil and damaged tubers) and may show itself at different stages of plant development.
In some situations the seed potato may rot in the ground before it sprouts, so no plant appears. The characteristic Blackleg disease is most common in wet weather and often occurs when plants are growing well (or even flowering). It shows itself as black slimy decay around the base of the stem (hence Blackleg). Leaves turn yellow between the veins, and then brown and roll up at the edges and die. The tuber rots from the stem end and becomes a slimy, smelly mass. In these situations plants are chlorotic and stunted and few tubers are produced. Tuber soft rot occurs in storage, when given the right combination of temperature (above 50 degrees F) and humidity. It may start in a single damaged tuber, but will move quickly through the closely packed tubers.
Though infection most often results from infected seed potatoes, the bacteria may also enter the plant through wounds in the tuber (such as from Scab or insect damage). The resulting disease will be the same though.
Aerial Stem Rot is caused by the same organism attacking the plant from the top down, through wounds or natural openings in the leaves. After infecting the leaves it travels down the stem to the developing tubers. Like most bacterial infections this is usually transported in water, so keep the foliage dry by using drip irrigation. If you must use sprinklers then it’s best to water in the morning or early evening, so leaves have a chance to dry out before nightfall.
The most important way to control this disease is by planting certified disease-free seed potatoes. If this disease has been a problem in the past, it is better to use small, whole tubers rather than cutting big ones into small pieces (which damages them before they even go in the ground). You can also plant them at a shallow depth to encourage quick emergence (hill them up later to increase soil depth). You should always rotate potato crops annually and try to remove all tubers from the soil, as any that remain in the ground can harbor the disease (the bacteria can’t survive outside of living plants for very long). Also give the plants plenty of room (for good air circulation) and make sure the soil is well-drained.
To minimize rot in storage it is important to ensure that all tubers are undamaged. If you are storing them in a root cellar, you should put them in crates or boxes where they can be conveniently inspected occasionally.
Image: Howard F. Schwartz, Colorado State University, Bugwood.org