Frost Influences Control of Tropical Soda Apple with Aminopyralid

Brent A. Sellers, Assistant Professor of Agronomy, 3401 Experiment Station, Range Cattle Research and Education Center, University of Florida-IFAS, Ona 33865; Jason A. Ferrell, Assistant Professor of Agronomy, 301A Newell Hall, P.O. Box 110500, Department of Agronomy, University of Florida-IFAS, Gainesville 32611; and J. Jeff Mullahey, Professor, 5988 Highway 90, P.O. Box 3634, North Florida Research and Education Center, University of Florida-IFAS, Milton 32572

Tropical soda apple (Solanum viarum) is a perennial weed that has spread from Florida throughout the southeast USA since the late 1980s. Previous research showed that triclopyr, picloram, and glyphosate were the most effective herbicide options for S. viarum control (1,3). However, picloram is not registered for use in Florida and glyphosate is injurious to desirable forage grasses; triclopyr at 1.1 kg/ha was the standard herbicide recommendation for S. viarum control in Florida pastures. Triclopyr, however, did not provide adequate S. viarum control when plants were fruiting, making a mowing treatment necessary 60 days before triclopyr application. Additionally, triclopyr is rapidly degraded in the soil, allowing new seedlings to emerge within 60 days after application.

The registration of aminopyralid, a new pyridinecarboxylic acid, for use in pastures and rangeland allowed ranchers to apply an herbicide for S. viarum control at any developmental stage. In addition to the excellent postemergence control observed compared to triclopyr, aminopyralid has been shown to provide soil residual activity for at least 215 days after treatment (2). This high level of efficacy, coupled with its low use rate of 0.08 to 0.12 kg/ha, has made aminopyralid the standard herbicide for S. viarum control in pastures and rangeland in Florida.

Grower acceptance is often slow unless they have personally observed the results of a specific treatment. To increase grower awareness of the advantages of aminopyralid, several demonstration plots were established in north, central and south Florida in 2005 and 2006. These plots were established and treated with aminopyralid at different times of the year when a suitable patch of tropical soda apple was present. Aminopyralid was applied to different tropical soda apple patches approximately every two months in each region of the state. However, there were instances (June and August) when a suitable patch could not be located in each region. In general, demonstration plot size was 30 by 30 m with a 3 m buffer (untreated check). The untreated check was included to demonstrate the effects of aminopyralid on established S. viarum plants and residual seedling control. Aminopyralid was applied at 0.12 kg ai/ha with a spray boom calibrated to deliver 280 liter/ha.

Tropical soda apple control was > 95% one month after treatment in all demonstration plots regardless of when aminopyralid was applied. Demonstration plots treated in April, June, August, October, and December had high levels (>95%) of both seedling and established S. viarum control two months after application. However, previously established S. viarum plants started to regrow from the rootstock two months after the February application (Fig. 1), but residual seedling control is comparable to other application dates across the state. Plants that survived the February application timing did not grow as well as untreated plants, but flowering and fruiting did occur.

Fig. 1. Response of established tropical soda apple to aminopyralid applied in February. Note the regrowth from the perennial rootstock.

Environmental factors often influence herbicide efficacy. Failure to control S. viarum with aminopyralid in February was likely due to frost before or soon after treatment. Frost, while not uncommon in north and central Florida, does not occur every year in south Florida. Therefore, aminopyralid should not be applied when frost is likely to occur. In Florida, it is recommended that aminopyralid should not be applied from January through March.

Literature Cited

1. Call, N. M., Coble, H. D., and Pereze-Fernandez, T. 2000. Tropical soda apple (Solanum viarum) herbicide susceptibility and competitiveness in tall fescue (Festuca arundinacea). Weed Technol. 14:252-260.

2. Ferrell, J. A., Mullahey, J. J., Langeland, K. A., and Kline, W. N. 2006. Control of tropical soda apple (Solanum viarum) with aminopyralid. Weed Technol. 20:453-457.

3. Mullahey, J. J., Cornell, J. A., and Colvin, D. L. 1993. Tropical soda apple (Solanum viarum) control. Weed Technol. 13:172-175.