Tackling the scourge of blackleg in South Africa

With blackleg being amongst the worst diseases of canola, South African reserachers are conducting trials to find the most resistant cultivars and most effective fungicides for local conditions. Dr Gert van Coller of the Western Cape Department of Agriculture offers advice on control of the disease. Glenneis Kriel reports.

- Tackling the scourge of blackleg in South Africa

While the extent of losses in South Africa is uncertain, canola farmers in Australia generally lose between 10% and 15% of their harvests annually to blackleg, with figures as high as 90% being recorded in some instances. This is according to Dr. Gert van Coller, plant pathology scientist at the Western Cape Department of Agriculture and a speaker at a Knowledge Hub event presented by Syngenta and John Deere in Napier in the Western Cape in October.

Global losses to the disease are estimated at over R16 billion annually.

FUNGAL SPECIES

Blackleg is caused by two fungal species, namely Leptosphaeria maculans and L. biglobosa. These can affect any part of the plant and result in leaf and pod lesions, and stem and crown canker. L. maculans, the more virulent of the two, is by far the most common in South Africa. It causes crown canker, which has a major impact on yield by inhibiting nutrient and water transportation in plants, as well as stem canker and upper canopy infection.

L. biglobosa causes more superficial canker on the upper parts of the stem and is therefore less damaging.

To complicate matters explains Van Coller, there are a number of races of L. maculans, and scientists are currently researching the occurrence of these in the Western Cape, where most of South Africa's canola is planted.

DISEASE CYCLE

The blackleg pathogen survives on infected canola stubble left on the land after harvest. This produces fruiting bodies called pseudothecia, which are visible as small black dots on the stubble. These in turn release sexual spores, called ascospores, after the first autumn rains, with 2mm to 3mm of rain being enough to initiate the disease.