Conyza bonariensis glyphosate tolerance as affected by origin temperature and growth stage

Abstract

Glyphosate was patented as a broad-spectrum, non-selective, systemic herbicide in 1974. In 1996, glyphosate-resistant Lolium rigidum (rigid ryegrass) was reported in Australia. Since then 34 other weeds the world over have evolved resistance to this herbicide. The first case of glyphosate resistance in South Africa was in rigid ryegrass in vineyards. Resistance to glyphosate in Conyza bonariensis (L.) Cronquist (flax-leaf fleabane) was reported in 2003 in the Breede Valley, Western Cape. Glyphosate resistant Conyza canadensis (horseweed fleabane) reportedly becomes sensitive at low temperatures (below 12 oC). If the resistance mechanism is vacuolar sequestration, low temperatures will prevent glyphosate to be translocated into the vacuole. Tank mixtures with foliar manganese and other foliar-applied nutrient elements, in particular cations such as Mn2+, Zn2+, Fe2+, etc, are known to decrease glyphosate efficacy. The aim of the present study is to determine if growth stage has an influence on glyphosate tolerance in C. bonariensis, if highly glyphosate-tolerant C. bonariensis plants become sensitive at low temperatures, and to assess if there is an effect between high manganese levels and response to glyphosate, as well as if mancozeb (fungicide containing Mn and Zn) influences the glyphosate-manganese interaction. C. bonariensis seed was collected at 12 locations. At the four to six leaf stage six dosages of glyphosate was applied: 0, 0.25, 0.5, 1, 2 and 4 times the recommended dosage (2 L ha-1 Roundup Turbo). Fresh mass were measured at 21 days after treatment (DAT). Data was subjected to ANOVA. GR50 and Resistant / Sensitive values were calculated. From the screening experiment two highly tolerant, two tolerant and two susceptible populations were identified. The growth stage experiment was conducted in the exact same manner as the screening experiment, with growth stage as an added factor. Plants were treated with glyphosate at two different growth stages (10 – 12 and 16 – 20 leaves). A second screening experiment with four populations from the same area was performed. Plants from a highly tolerant population were grown in the greenhouse up to the four leaf stage and then moved to the temperature gradient table. Plants were exposed to temperature ranges of 8 - 13 oC, 15 - 20 oC and 22 - 27 oC, respectively. Plants were treated with glyphosate at 0, 0.5, 1, 2 and 4 times the recommended dosage and fresh mass measured at 21 DAT. The manganese experiment had three factors: manganese level, location and glyphosate dosage. Seed from a sensitive and highly tolerant population were planted in a seeding tray. Seedlings were transplanted to a hydroponic system containing three different nutrient solutions with different manganese levels. Plants were treated with glyphosate at the 4-6 leaf stage at 1 and 2 times the recommended dosage. The rest of the materials and methods are the same as for the temperature experiment. The final experiment was performed to examine if mancozeb has an influence on the efficacy of glyphosate when applied to C. bonariensis. Mancozeb was applied before and after glyphosate to susceptible C. bonariensis plants. For two of the populations in the screening experiment all of the replicates survived the recommended dosage, and hence, they were classified as highly tolerant. Two other populations were classified as tolerant and two as sensitive. There is a clear difference in the sensitivity of the various populations to glyphosate as well as populations from the same area. These six populations were further used in the growth stage experiment where the same results were obtained regarding the sensitivity of the populations at the 10 - 12 leaves growth stage. Plants at the 16 – 20 leave growth stage are much more tolerant to glyphosate. The second screening experiment showed that C. bonariensis plants from the same area differ in the tolerance towards glyphosate. Highly glyphosate-tolerant plants did not become susceptible at a low temperature. Susceptibillity to glyphosate increased at the higher temperatures. Therefore, vacuolar sequestration is probably not the mechanism of resistance responsible for the high tolerance to glyphosate. There was no replication of manganese treatments in the manganese experiment and therefore differences could not be tested. The unique methodology employed in this experiment is, however, of value. Mancozeb did not have an influence on glyphosate efficacy when applied either before or after glyphosate. If resistance to glyphosate develops in C. bonariensis and in other species on a wider scale than is currently the case in South Africa it will be a big problem for farmers in various cropping systems. Therefore, the label must be followed very strictly to ensure that plants are treated at the correct dosage and growth stage to ensure that populations are not incorrectly referred to as resistant.