Lisa M. Behnken

Comparison of Herbicide Tactics to Minimize Species Shifts and Selection Pressure in Glyphosate-Resistant Soybean

Abstract There are significant concerns over the long- and short-term implications of continuous glyphosate use and potential problems associated with weed species shifts and the development of glyphosate-resistant weed species. Field research was conducted to determine the effect of herbicide treatment and application timing on weed control in glyphosate-resistant soybean. Ten herbicide treatments were evaluated that represented a range of PPI, PRE, and POST-only application timings. All herbicide treatments included a reduced rate of glyphosate applied POST. PRE herbicides with residual properties followed by (fb) glyphosate POST provides more effective control of broadleaf weed species than POST-only treatments. There was no difference in soybean yield between PRE fb POST and POST-only treatments in 2008. Conversely, PRE fb POST herbicide treatments resulted in greater yield than POST-only treatments in 2009. Using PRE fb POST herbicide tactics improves weed control and reduces the risk for crop yield loss when dealing with both early- and late-emerging annual broadleaf weed species across variable cropping environments. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. CHEAL; common waterhemp, Amaranthus rudis Sauer AMATA; giant ragweed, Ambrosia trifida L., AMBTR.

Giant Ragweed (Ambrosia trifida) Seed Production and Retention in Soybean and Field Margins

As herbicide-resistant weed populations become increasingly problematic in crop production, alternative strategies of weed control are necessary. Giant ragweed, one of the most competitive agricultural weeds in row crops, has evolved resistance to multiple herbicide biochemical sites of action within the plant, necessitating the development of new and integrated methods of weed control. This study assessed the quantity and duration of seed retention of giant ragweed grown in soybean fields and adjacent field margins. Seed retention of giant ragweed was monitored weekly during the 2012 to 2014 harvest seasons using seed collection traps. Giant ragweed plants produced an average of 1,818 seeds per plant, with 66% being potentially viable. Giant ragweed on average began shattering hard (potentially viable) and soft (nonviable) seeds September 12 and continued through October at an average rate of 0.75 and 0.44% of total seeds per day during September and October, respectively. Giant ragweed seeds remained on the plants well into the Minnesota soybean harvest season, with an average of 80% of the total seeds being retained on October 11, when Minnesota soybean harvest was approximately 75% completed in the years of the study. These results suggest that there is a sufficient amount of time to remove escaped giant ragweed from production fields and field margins before the seeds shatter by managing weed seed dispersal before or at crop harvest. Controlling weed seed dispersal has potential to manage herbicide-resistant giant ragweed by limiting replenishment of the weed seed bank. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR; soybean, Glycine max (L.) Merr. Conforme las poblaciones de malezas resistentes a herbicidas se hacen incrementalmente más problemáticas en la producción de cultivos, estrategias alternativas de control de malezas se hacen cada vez más necesarias. Ambrosia trifida, una de las malezas agrícolas más competitivas en cultivos en hileras, ha evolucionado resistencia a múltiples sitios bioquímicos de acción de herbicidas dentro de la planta, lo que ha hecho necesario el desarrollo de métodos nuevos e integrados de control de malezas. Este estudio evaluó la cantidad y duración de la retención de semilla de A. trifida creciendo en campos de soja y márgenes de campos adyacentes. La retención de semilla de A. trifida fue monitoreada semanalmente durante las temporadas de cosecha desde 2012 a 2014 usando trampas de colección de semilla. Las plantas de A. trifida produjeron un promedio de 1,818 semillas por planta, con una viabilidad potencial de 66%. En promedio, A. trifida inició la dispersión de semilla dura (potencialmente viable) y suave (no-viable) el 12 de Septiembre y continuó durante Octubre, con una tasa promedio de 0.75 y 0.44% del total de semillas por día, durante Septiembre y Octubre, respectivamente. Las semillas de A. trifida permanecieron en las plantas hasta la temporada de cosecha de soja en Minnesota, con un promedio de 80% del total de las semillas estando retenidas al 11 de Octubre, cuando la cosecha de soja en Minnesota había sido completada al 75%, en los años de este estudio. Estos resultados sugieren que existe una cantidad de tiempo suficiente para remover A. trifida que haya escapado al control en campos de producción y en márgenes de campos antes de que la semilla sea liberada de la planta, mediante el manejo de la dispersión de semilla de malezas antes o durante la cosecha. El controlar la dispersión de semillas de malezas tiene el potencial de manejar A. trifida resistente a herbicidas al limitar el suministro de nuevas semillas al banco de semillas de malezas.

Seedbank Depletion and Emergence Patterns of Giant Ragweed (Ambrosia trifida) in Minnesota Cropping Systems

In the midwestern United States, biotypes of giant ragweed resistant to multiple herbicide biochemical sites of action have been identified. Weeds with resistance to multiple herbicides reduce the utility of existing herbicides and necessitate the development of alternative weed control strategies. In two experiments in southeastern Minnesota, we determined the effect of six 3 yr crop-rotation systems containing corn, soybean, wheat, and alfalfa on giant ragweed seedbank depletion and emergence patterns. The six crop-rotation systems included continuous corn, soybean—corn—corn, corn—soybean—corn, soybean—wheat—corn, soybean—alfalfa—corn, and alfalfa—alfalfa—corn. The crop-rotation system had no effect on the amount of seedbank depletion when a zero-weed threshold was maintained, with an average of 96% of the giant ragweed seedbank being depleted within 2 yr. Seedbank depletion occurred primarily through seedling emergence in all crop-rotation systems. However, seedling emergence tended to account for more of the seedbank depletion in rotations containing only corn or soybean compared with rotations with wheat or alfalfa. Giant ragweed emerged early across all treatments, with on average 90% emergence occurring by June 4. Duration of emergence was slightly longer in established alfalfa compared with other cropping systems. These results indicate that corn and soybean rotations are more conducive to giant ragweed emergence than rotations including wheat and alfalfa, and that adopting a zero-weed threshold is a viable approach to depleting the weed seedbank in all crop-rotation systems. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR, alfalfa, Medicago sativa L., corn, Zea mays L., soybean, Glycine max (L.) Merr., wheat, Triticum aestivum L.

First report of Stemphylium globuliferum causing Stemphylium leaf spot on alfalfa (Medicago sativa) in the United States

Stemphylium leaf spot occurs in most areas where alfalfa (Medicago sativa) is grown. In the United States, Stemphylium botryosum is reported to be the predominant pathogen (1), although S. vesicarium and S. herbarum are also observed. S. alfalfae was isolated on alfalfa in Australia (4) and S. globuliferum was reported in Egypt and Korea. In April and May 2012, alfalfa plants with leaf spot symptoms were observed in Rosemount and Waseca, MN, and in Arlington, Tomah, and Waupaca, WI. Initial symptoms consisted of white to tan spots with a brown border, 2 to 3 mm in diameter, circular to oval, enlarging to 5 to 8 mm in diameter. Large lesions often coalesced. Small, narrow, brown lesions occurred on petioles. Lower killed leaves remained attached to the primary stem. Spots were larger than those caused by the cool temperature biotype of S. botryosum. Conidia formed on lesions after 48 h in a moist chamber. Conidia were removed with a fine glass rod, germinated on 1% water agar, and single hyphae transferred to V8 agar (V8A). After 2 weeks under room light, plates were placed under UV light to stimulate spore production. Conidia on host material were borne singly on straight, unbranched, smooth conidiophores, medium brown at the apex. Conidia were medium to dark brown with small papillae, subspherical with 3 to 4 transverse and 3 to 4 complete or near complete longitudinal septa, with a distinct constriction at the median transverse septum. Conidia were 27.5 to 32.5 μm long × 20 to 22.5 μm wide with a length/width (L/W) ratio of 1.2 to 1.5. Conidia on V8A were smaller, 25 to 30 μm long × 12.5 to 19 μm wide with a L/W of 1.6 to 1.8. Ascostromata 300 μm in diameter formed on leaves held at 4°C for 2 months as well as on culture plates after 1 month. Ascospores from leaves were golden brown to reddish, 40 to 42.5 × 20 μm, slightly broader in the upper half of the spore, with 7 to 8 transverse septa and one complete longitudinal septum with several incomplete septa. Ascospores from culture were smaller, 27.5 to 30 × 12.5 to 15 μm wide. These morphological features are consistent with the description for S. globuliferum (3). DNA was extracted from pure cultures of SAr301 and SWp202, isolated from plants grown in Arlington and Waupaca, respectively, and used to amplify ITS1-5.8S-ITS2 rDNA using primers ITS1 and ITS4, GPD with primers GPD1 and GPD2, EF-1α with EF446f and EF1473R, and the intergenic spacer between vmaA and vpsA with primers ATPF2 and GTP604R (2). In sequence comparisons made by BLASTn searches of GenBank, the ITS (KF479193), GPD (KF479194), and EF-1α (KF479195) sequences from S. globuliferum were different from the gene sequences of S. botryosum but identical to those from S. vesicarium, S. herbarum, and S. alfalfae. The vmaA-vpsA spacer sequence (KF479196) of S. globuliferum had 3 nucleotide differences from S. vesicarium and S. herbarum and 4 nucleotide differences from S. alfalfae, demonstrating that this sequence is useful for species discrimination. Conidia from strains SAr301 and SWp 202 were suspended at 104/ml in sterile water with 0.01% Tween 20 and used to inoculate 12 alfalfa plants using a handheld sprayer. Plants were kept at 100% RH for 48 h, then grown at 20°C with a 16-h photoperiod. After 2 weeks, lesions similar to those seen in the field were observed on leaves of all plants. Symptomatic leaves placed in moist chambers produced conidia with the size and morphology of S. globuliferum within 48 h. This is the first report to our knowledge of S. globuliferum causing disease on alfalfa in the United States. Cultures were deposited in the University of Minnesota Mycological Culture Collection. References: (1) W. A. Cowling et al. Phytopathology 71:679, 1981. (2) P. Inderbitzin et al. Mycologia 101:320, 2009. (3) E. G. Simmons. Mycologia 61:1, 1969. (4) E. G. Simmons. Sydowia 38:284, 1985.

Giant Ragweed (Ambrosia trifida) Emergence Model Performance Evaluated in Diverse Cropping Systems

Accurate weed emergence models are valuable tools for scheduling planting, cultivation, and herbicide applications. Multiple models predicting giant ragweed emergence have been developed, but none have been validated in diverse crop rotation and tillage systems, which have the potential to influence weed emergence patterns. This study evaluated the performance of published giant ragweed emergence models across various crop rotations and spring tillage dates in southern Minnesota. Across experiments, the most robust model was a mixed-effects Weibull (flexible sigmoidal function) model predicting emergence in relation to hydrothermal time accumulation with a base temperature of 4.4 C, a base soil matric potential of -2.5 MPa, and two random effects determined by overwinter growing degree days (GDD) (10 C) and precipitation accumulated during seedling recruitment. The deviations in emergence between individual plots and the fixed-effects model were distinguished by the positive association between the lower horizontal asymptote (Drop) and maximum daily soil temperature during seedling recruitment. This finding indicates that crops and management practices that increase soil temperature will have a shorter lag phase at the start of giant ragweed emergence compared with practices promoting cool soil temperatures. Thus, crops with earlyseason crop canopies such as perennial crops and crops planted in early spring and in narrow rows will likely have a slower progression of giant ragweed emergence. This research provides a valuable assessment of published giant ragweed emergence models and illustrates that accurate emergence models can be used to time field operations and improve giant ragweed control across diverse cropping systems. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR.