Preliminary evaluation of soil-residual herbicides for the control of silver-leaf bitter apple (Solanum elaeagnifolium Cav.)

Abstract

Solanum elaeagnifolium Cav., commonly known as silver-leaf bitter apple, silverleaf nightshade or “satansbos”, has become one of the most important emerging weeds in South Africa, with the potential to spread throughout some of the major crop growing areas of the country. Consequently, it has been declared a weed of national importance. Previous research revealed the difficulty in killing the extensive root system when using foliar applied herbicides. The small leaf area for herbicide absorption relative to the large root system and apparent inability to translocate sufficient herbicide beyond the root crown are believed to be some of the main reasons for the poor levels of control achieved. Since herbicides capable of being absorbed by the roots may be more effective, a study was undertaken to evaluate a range of soil-residual herbicides in the hope of improving control. While imazapyr 250 g ⁻¹ SL applied at 8 1 ha⁻¹initially caused the most significant reduction in weed population levels, it was ultimately tebuthiuron 500 g ⁻¹ SC and a bromacil/tebuthiuron mixture 250/250 g ⁻¹ SC applied at 32! ha⁻¹ that sustained the best long-term control. However, the high cost and long soil residual nature of these products would seriously limit their application value in cropping areas and sensitive habitats. Nonetheless, they may be useful for controlling isolated dense patches on fallow land and along roadsides, away from desirable vegetation, steep slopes or watercourses. The registration holders of these products are encouraged to conduct further evaluations using lower rates, as well as combining these products with foliar applied herbicides, to reduce costs and potential environmental impacts if used in sensitive habitats.