Blackleg –    Dickeya Dianthicola (formerly Erwinia chrysanthemi)

Symptoms   Haulm symptoms vary according to the timing of infection and on environmental conditions.  Early infection may kill plants before emergence causing uneven spacing along rows.  Later infected plants become yellowish and stunted; some or all stems show a brown/black area for a few centimetres above the point of attachment to the seed tuber.   Seed tubers invariably rot.  In wet conditions lesions quickly develop into a soft rot, so that the bottom of the stems turn into an inky, wet mess. Tuber infection can occur via the stolons or via the lenticels and wounds; a greyish-black soft rot quickly develops.

Sources and Factors Favouring Development

Predominantly seed-borne, spread may also be through insects, aerosols and irrigation water; these secondary ways of transmission may be significant in the early stages of seed multiplication when there is little contamination of stocks.  Most commercial stocks of seed carry erwinia bacteria, but it is only in certain conditions that these latent infections become active. Cool (10-15°C) wet soils at planting, followed by high temperatures (above 20°C) after plant emergence favour disease development and spread, but Blackleg can be severe in dry seasons.

Control          Having seed tested to know the levels of erwinia can be used as an indicator of how much blackleg infection is likely in the growing crop. Planting seed shallow and into well-drained soils at 10-13°C planting may help reduce the incidence of Blackleg infection. Avoid cut seed of susceptible varieties.  The use of limited generation seed may help minimise the risk of infection.