Resistant plants of Sumatran fleabane with unusual rapid necrosis (RN) symptoms after application of 2,4-D were characterized in previous studies. Field observations indicated variability in the occurrence of the RN caused by 2,4-D, but the causes of the variation are unknown. This study aimed to investigate the effect of environmental conditions, plant growth stage, and simultaneous and sequential herbicide mixtures with other auxin mimics on the occurrence of RN caused by 2,4-D. Application at temperature of 12 C delayed the symptoms and decreased the intensity of the RN but still resulted in plant survival to 2,4-D. The absence of light after herbicide application caused a slight delay in the symptoms, but the production of hydrogen peroxide and the size of the necrosed area were not affected by the light treatments before and after 2,4-D application. Changes in plant photosynthesis through inhibiting photosystem II do not prevent the occurrence of RN symptoms. The auxinic herbicides dicamba, triclopyr, and halauxifen-methyl do not cause RN symptoms and are effective at controlling the resistant biotype when applied without 2,4-D, but the effectiveness of these herbicides was reduced when sprayed on the resistant biotype either together, 4 h, or 24 h after 2,4-D. The RN phenotype does not occur for dicamba and triclopyr, even in advanced plant growth stages and high doses on the resistant biotype. The herbicides dicamba and triclopyr effectively controlled resistant plants, especially when sprayed at the initial growth stages. The results of this study identify environmental effects, plant development effects, and herbicide interactions that interfere with the occurrence of RN symptoms caused by 2,4-D in Sumatran fleabane. These data provide insights about the mechanisms behind the RN symptoms caused by 2,4-D and are important for identifying the causes of variability of the herbicide symptomology and performance under experimental and field conditions.

Nomenclature: 2,4-D; dicamba; triclopyr; halauxifen-methyl; Sumatran fleabane, Conyza sumatrensis (Retz.) E. Walker