Photo: Supplied
Three species of Conyza spp can be a problem in South Africa: Conyza bonariensis (flax-leaf fleabane); C. canadensis (horseweed fleabane); and C. sumatrensis (tall fleabane).
Flax-leaf fleabane can grow up to 1,2m tall and has side branches longer than the main stem. The leaf margins are sharply toothed, and both leaf surfaces are dull green and hairy. The flowers are white and occur in narrow, terminal plumes on the main stem and side branches.
Horseweed fleabane is shorter than the other Conyza species. Like them, it has white flowers, but these are smaller, and the stems branch only at the top of the plant. The leaves are finely toothed, yellowish green and less hairy.
Tall fleabane can grow more than 2m high and its side branches are shorter than the main stem. The flowers are white and the fruits straw-coloured.
All three species are semi-woody with sturdy taproots, and usually occur together.
Control
Conyza can easily be controlled with shallow cultivation and is therefore not usually a problem in a conventional tillage system. In conservation- or no-till systems, however, it can become a serious threat.
To control the weed, apply either a pre-emergence herbicide before the weed reaches the rosette stage, or a post-emergence broadleaf herbicide. A number of herbicides are registered for the control of Conyza in wheat and maize (see table).
| Herbicides registered for the control of fleabane on maize and wheat | |
| Active ingredient | Time of application |
| MAIZE | |
| 2,4-D/dicamba | Post-emergence of weed and crop, but before the crop is 30cm tall. Apply when the rosette is smaller than 15cm in diameter and growing actively. |
| atrazine/sulcotrione | Pre- or post-emergence. When applied post-emergence, add an adjuvant. |
| glyphosate | Post-emergence, only on glyphosate-tolerant maize. |
| WHEAT | |
| 2,4-d/dicamba | Apply between growth stages seven and 13 of the crop. |
| carfentrazone-ethyl | Apply between growth stages seven and 13 of the crop. |
| dicamba | Only for use in tank mixtures with Enhancer (10g to 12g) and Reaper (10g) and an adjuvant. |
| metsulfuron-methyl/thifensulfuron-methyl | Apply before four- or five-leaf stage of the weed. |
| glyphosate | Pre-emergence. |
| Pre-emergence. | |
| Note: Some Conyza spp show resistance to glyphosate, and control will therefore vary. | |
Herbicide effectiveness decreases with an increase in weed size, so try to control the weed before it sheds its seeds. Timing of application is crucial; unfortunately, farmers have often not finished harvesting before it’s the optimal time to spray.
Follow the specific instructions of each product used for spraying the various Conyza spp.
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