Investigation into the mortality of potted Chromolaena odorata (L.) R. M. King & H. Robinson (Asteraceae)
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The neotropical semi-woody perennial shrub Chromolaena odorata (L.) King and Robinson (Asteraceae: Eupatorieae) has become a serious invasive weed in the sub-tropical regions of southern Africa, resulting in the initiation of an insect biocontrol research programme at Cedara in the KwaZulu-Natal province. The programme has experienced difficulties in growing and maintaining healthy potted C. odorata plants for research, for almost ten years. In an attempt to identify possible causes of mortality, plants were grown on elevated grids in two trials; (winter-spring 2003) and (autumn-winter 2004) in Durban (subtropical region, within the natural range of C. odorata) and Cedara (temperate region, outside its natural range). Plants that established at all sites, in Trials 1 and 2, did not display any visual symptoms associated with mortality; however, results from Trial 1 indicated that plants grown in a tunnel at Cedara were physiologically stressed evidenced by lower transpiration rates despite higher vapour pressure deficits, compared with plants grown in a tunnel in Durban. Environmental stress was further indicated by the chlorophyll fluorescence ratio and a change in allocation of total non-structural carbohydrate (TNC) within Cedara-grown plants compared with Durbangrown plants. An attempt to relate mortality symptoms of unhealthy C. odorata plants with TNC content of the roots was unsuccessful but unhealthy plants had lower levels of N, P and K than healthy plants. Although mortality seem to be exacerbated by pathogenic fungi in saturated soil, infection was reduced by treating the soil with Previcur N and Benlate drench. Morphological adaptations of plants exposed to waterlogged soils include hypertrophied lenticels and new lateral roots; these were observed on the stem base of unhealthy C. odorata plants placed on the ground, that had reached a critical stage of die-back or had died back and re-sprouted. Examination of the root anatomy of C. odorata plants indicated an absence of cortical aerenchyma (interconnected system of air spaces), which may explain plant mortality in waterlogged pots wherein levels of oxygen in the soil may be reduced. C. odorata stock plants at Cedara have been placed on elevated grids since August 2004 and two years later plant mortality has been significantly reduced. Presently, the only feasible option is to grow potted C. odorata plants on elevated grids at Cedara because this allows for adequate drainage, aeration of soil and air-nipping of roots, and spacing plants on the grids also improves penetration of light.