Herbicides reduce the severity and sporulation of Phakopsora pachyrhizi in soybean with triple herbicide resistance.

BACKGROUND: Transgenic event DAS44406-6 (E3) makes soybeans that are herbicide [glyphosate (Gly), 2,4-dichlorophenoxyacetic acid (2,4-D) and glufosinate] and caterpillar resistant. The E3 soybean was commercially released for the 2021/2022 harvest in Brazil. We conducted this study to test whether Gly and 2,4-D applied alone and in a commercial mixture affect Asian soybean rust (ASR). Assays were conducted in detached leaves and in vivo, in a controlled environment using the herbicides Gly, 2,4-D and Gly + 2,4-D, and pathogen inoculation. Disease severity and spore production were evaluated. RESULTS: Only the herbicides Gly and Gly + 2,4-D inhibited ASR in detached leaves and in vivo. When applied preventively and curatively in vivo, these herbicides reduced the disease severity and spore production of the fungus. In vivo, inhibition of disease severity reached 87% for Gly + 2,4-D and 42% for Gly. A synergistic effect was observed with the commercial Gly + 2,4-D mixture. Application of 2,4-D alone in the in vivo assays did not reduce or increase disease severity. Gly and Gly + 2,4-D act residually in inhibiting the disease. Growing E3 soybeans may combine weed and caterpillar management benefits with ASR inhibition. CONCLUSION: Application of Gly and Gly + 2,4-D herbicides in resistant E3 soybean shows inhibitory activity for ASR. © 2023 Society of Chemical Industry.

Challenging glyphosate resistance EPSPS P106S and TIPS mutations with soybean competition and glyphosate: implications for management.

BACKGROUND: Eleusine indica (L.) Gaertn. (goosegrass) is a major weed in global cropping systems. It has evolved resistance to glyphosate due to single Pro-106-Ser (P106S) or double Thr-102-Ile + Pro-106-Ser (TIPS) EPSPS target site mutations. Here, experiments were conducted to evaluate the single effect of soybean competition and its combined effect with a glyphosate field dose (1080 g ae ha-1 ) on the growth and fitness of plants carrying these glyphosate resistance endowing target site mutations. RESULTS: TIPS E. indica plants are highly glyphosate-resistant but the double mutation endows a substantial fitness cost. The TIPS fitness penalty increased under the effect of soybean competition resulting in a cost of 95%, 95% and 96% in terms of, respectively, vegetative growth, seed mass and seed number investment. Glyphosate treatment of these glyphosate-resistant TIPS plants showed an increase in growth relative to those without glyphosate. Conversely, for the P106S moderate glyphosate resistance mutation, glyphosate treatment alone reduced survival rate, vegetative growth, aboveground biomass (34%), seed mass (48%) and number (52%) of P106S plants relative to the glyphosate nontreated plants. However, under the combined effects of both soybean competition and the field-recommended glyphosate dose, vegetative growth, aboveground biomass, seed mass and number of P106S and TIPS plants were substantially limited (by ≤99%). CONCLUSION: The ecological environment imposed by intense competition from a soybean crop sets a significant constraint for the landscape-level increase of both the E. indica single and double glyphosate resistance mutations in the agroecosystem and highlights the key role of crop competition in limiting the population growth of weeds, whether they are herbicide-resistant or susceptible. © 2022 Society of Chemical Industry.