Effect of bacterium-herbicide combinations on tropical soda apple

Erwinia carotovora subsp. carotovora, a plant pathogenic bacterium, was combined with a one-quarter rate of two herbicides, triclopyr (3,5,6-trichloro-2-pyridinyloxy acetic acid and dicamba (3,6-dichloro-o-anisic acid), to control tropical soda apple (TSA) (Solanum viarum). E. c. subsp. carotovora inoculated at the growing point of TSA plants was consistently recovered from the stem and roots from 5 to 35 days after inoculation indicating its persistence and spread in host tissue. Plants in greenhouse studies were inoculated with a suspension of E c. subsp. carotovora and Silwet L-77 (1.25%) alone or combined with reduced rates (25%) of triclopyr or dicamba at 0.28 kg ai/ha, or each herbicide alone. Control plants were inoculated with water and Silwet L-77. At 72 and 112 days after treatment (DAT), the percentage of leaf area injured (LAI) per plant was estimated using a scale of 0–100, where 0=no injury and 100%=entire plant necrotic. Injury to plants was necrosis and wilting of leaf, petiole, and stems. Injured plants regenerated from root buds, and plants that regenerated were rated at 0% LAI (no visible injury). Plants inoculated with E. c. subsp. carotovora and Silwet L-77 or water and Silwet L-77 treatments had 0% LAI. Plants treated with E. c. subsp. carotovora and triclopyr or dicamba had 82.5% and 42% LAI at 72 DAT and did not regenerate by 112 DAT. Plants treated with triclopyr had 82.0% LAI and dicamba-treated plants had 36.5% LAI at 72 DAT. Twenty percent and 32% of the plants treated with triclopyr and dicamba regenerated by 112 DAT, and had 62% and 4.5% LAI, respectively. Plants treated with dicamba and the bacterium had significantly higher LAI (82.5%) compared to those treated with dicamba (4.5%) at 112 DAT. In field trials, plants treated with the bacterium and Silwet L-77 combined with triclopyr or dicamba had 87.5% and 50% LAI at 72 DAT, respectively, and no plants regenerated. Plants treated with triclopyr had 96% LAI at 72 DAT, and 12% of these plants regenerated. At 112 DAT, the LAI on plants was 84%. Plants treated with dicamba had 47.5% LAI, and 15% of the plants regenerated. At 112 DAT, the LAI was 32.5%, which was significantly lower than the LAI on the dicamba-bacterium treated plants. The combination of E. c. subsp. carotovora and dicamba caused greater plant injury and than the herbicide alone and completely prevented the regeneration of plants.