Kurt J. Hembree

Growth stage influences level of resistance in glyphosate-resistant horseweed

While glyphosate-resistant horseweed has not previously been reported in California, we suspected that it might exist, especially in noncrop areas. We collected horseweed seeds from two locations in the San Joaquin Valley and treated greenhouse-grown plants at different stages with different amounts of glyphosate. This study showed that a glyphosate-resistant biotype of horseweed exists in the noncrop areas of Dinuba, in Tulare County, and that the level of resistance may be influenced by the plants growth stage at the time of glyphosate application.

Evolution and spread of glyphosate resistance in Conyza canadensis in California.

Recent increases in glyphosate use in perennial crops of California, USA, are hypothesized to have led to an increase in selection and evolution of resistance to the herbicide in Conyza canadensis populations. To gain insight into the evolutionary origins and spread of resistance and to inform glyphosate resistance management strategies, we investigated the geographical distribution of glyphosate resistance in C. canadensis across and surrounding the Central Valley, its spatial relationship to groundwater protection areas (GWPA), and the genetic diversity and population structure and history using microsatellite markers. Frequencies of resistant individuals in 42 sampled populations were positively correlated with the size of GWPA within counties. Analyses of population genetic structure also supported spread of resistance in these areas. Bayesian clustering and approximate Bayesian computation (ABC) analyses revealed multiple independent origins of resistance within the Central Valley. Based on parameter estimation in the ABC analyses, resistant genotypes underwent expansion after glyphosate use began in agriculture, but many years before it was detected. Thus, diversity in weed control practices prior to herbicide regulation in GWPA probably kept resistance frequencies low. Regionally coordinated efforts to reduce seed dispersal and selection pressure are needed to manage glyphosate resistance in C. canadensis.

Glyphosate Resistance Is More Variable Than Paraquat Resistance in a Multiple-Resistant Hairy Fleabane (Conyza bonariensis) Population

Abstract Reduced control of some glyphosate-resistant hairy fleabane populations with paraquat has raised concerns about evolved multiple resistance to both glyphosate and paraquat. The objective of this study was to confirm the presence of multiple-resistant (glyphosate and paraquat) hairy fleabane populations in California. A series of dose-response experiments was conducted to evaluate the effect of glyphosate and paraquat in a known susceptible (S) and putative multiple-resistant (R) population of hairy fleabane. The greenhouse experiments were conducted during summer, fall, and winter under controlled temperature and natural light conditions. Multiple-resistant hairy fleabane was identified; however, the level of resistance to glyphosate varied substantially among seasons. During the summer, the glyphosate rate required to reduce growth by 50% (GR50) for the R population was 0.94 kg ae ha−1, 5.2-fold more than for the S population. In the fall and winter experiments, however, the R population response to glyphosate was similar to the S population with a GR50 of 0.22 kg ae ha−1 or less. Multiple-resistant plants were controlled in the fall and winter at rates that did not control the same population during summer. GR50 of paraquat varied among seasons (0.94, 0.24, and 0.07 kg ai ha−1 during summer, fall, and winter, respectively); however, plant mortality was more consistent. This is the first reported case of glyphosate–paraquat resistance in hairy fleabane and the multiple-resistant population could pose a significant challenge to annual no-till and perennial cropping systems in California. Further research on the mechanisms of resistance and the physiological factors underlying the seasonally variable response to glyphosate is needed. Nomenclature: Glyphosate; paraquat; hairy fleabane, Conyza bonariensis (L.) Cronq.

Postemergence Control of Glyphosate/Paraquat-Resistant Hairy Fleabane (Conyza bonariensis) in Tree Nut Orchards in the Central Valley of California

Abstract Hairy fleabane is an important weed in orchards and vineyards of California. Populations of glyphosate-resistant (GR) and glyphosate-paraquat-resistant (GPR) hairy fleabane have been documented in California but very little information is available on the efficacy of other POST herbicides on these populations. Greenhouse and field experiments were conducted to evaluate the efficacy of several POST herbicides registered in almond orchards on GPR, GR, and glyphosate/paraquat-susceptible (GPS) hairy fleabane plants. Plants were treated at the 8- to 12-leaf stage in greenhouse experiments, and at the bolting to flowering stage in field experiments. A sequential application of glyphosate (1,100 g ae ha−1) followed by paraquat (500 g ai ha−1) 14 d later did not control the GPR plants in any of the studies, but was effective in controlling the GR and GPS plants. Glufosinate at 1,050 g ai ha−1 or saflufenacil at 48.8 g ai ha−1 resulted in 90% or greater control of all populations in all studies, whereas glyphosate (1,100 g ae ha−1) + 2,4-D (1,090 g ae ha−1) resulted in inconsistent control (58 to 100%). The GPR population was not resistant to other common POST herbicide modes of action used in California tree nut orchards, and glufosinate and saflufenacil can provide growers effective management options for hairy fleabane in these crops. Nomenclature: 2,4-D; carfentrazone; glufosinate; glyphosate; paraquat; saflufenacil; hairy fleabane, Conyza bonariensis (L.) Conquist; almond, Prunus dulcis (Mill.) D.A. Web. Resumen Conyza bonariensis es una maleza importante en plantaciones de frutales y viñedos de California. Poblaciones de C. bonariensis resistentes a glyphosate (GR) y resistentes a glyphosate/paraquat (GPR) han sido documentadas en California, pero hay poca información acerca de la eficacia de otros herbicidas POST en estas poblaciones. Experimentos de invernadero y de campo fueron realizados para evaluar la eficacia de varios herbicidas POST registrados para uso en plantaciones de almendra sobre plantas de C. bonariensis GPR, GR, y susceptibles a glyphosate/paraquat (GPS). Las plantas fueron tratadas en el estadio de 8 a 12 hojas, en los experimentos de invernadero, y en el estadio de producción del tallo floral a la floración, en los experimentos de campo. La aplicación secuencial de glyphosate (1,100 g ae ha−1) seguida de paraquat (500 g ai ha−1) 14 d más tarde, no controló a las plantas GPR en ninguno de los estudios, pero fue efectiva para el control de las plantas GR y GPS. Aplicaciones de glufosinate a 1,050 g ai ha−1 o saflufenacil a 48.8 g ai ha−1 resultaron en 90% de control o más para todas las poblaciones en todos los estudios, mientras que glyphosate (1,100 g ae ha−1) + 2,4-D (1,090 g ae ha−1) resultó en control inconsistente (58 a 100%). La población GPR no fue resistente a herbicidas de otros modos de acción comunes usados en plantaciones de árboles de nueces en California. Glufosinate y saflufenacil pueden brindar a los productores opciones efectivas para el manejo de C. bonariensis en estos cultivos.

Distribution of Conyza sp. in Orchards of California and Response to Glyphosate and Paraquat

Abstract Resistance to glyphosate in hairy fleabane and horseweed is a problem in orchards and vineyards in California. Population genetic analyses suggest that glyphosate resistance evolved multiple times in both species, but it is unknown if resistance to other herbicides is also present. Two approaches of research were undertaken to further evaluate herbicide resistance in Conyza sp. in the perennial crop systems of California. In the initial study, the distribution of Conyza sp. in the Central Valley, using a semistructured field survey, was coupled with evaluation of the presence and level of glyphosate resistance in plants grown from field-collected seed. In a subsequent study, single-seed descendants representing distinct genetic groups were self-pollinated in the greenhouse and these accessions were evaluated for response to glyphosate and paraquat. Conyza sp. were commonly found throughout the Central Valley and glyphosate-resistant individuals were confirmed in all field collections of both species. The level of glyphosate resistance among field collections varied from 5- to 21-fold compared with 50% glyphosate resistance (GR50) of the susceptible, with exception of one region with a GR50 similar to the susceptible. When self-pollinated accessions from different genetic groups were screened, the level of glyphosate resistance, on the basis of GR50 values, ranged from 1.7- to 42.5-fold in hairy fleabane, and 5.9- to 40.3-fold in horseweed. Three accessions of hairy fleabane from different genetic groups were also resistant to paraquat (40.1- to 352.5-fold). One glyphosate-resistant horseweed accession was resistant to paraquat (322.8-fold), which is the first confirmed case in California. All paraquat-resistant accessions of Conyza sp. identified so far have also been resistant to glyphosate, probably because glyphosate resistance is already widespread in the state. Because glyphosate and paraquat resistances are found across a wide geographical range and in accessions from distinct genetic groups, multiple resistant Conyza sp. likely developed independently several times in California. Nomenclature: Glyphosate; paraquat; hairy fleabane, Conyza bonariensis L. (Cronq.); horseweed, Conyza canadensis L. (Cronq.).

Weed Control in Orchards and Vineyards

Weed management in tree and vine cropping systems is a major factor in the establishment, production, and harvest of these important fruit and nut crops. Factors that influence weed control methods in orchards and vineyards include weed species and density, crop age, planting arrangement, and irrigation systems, as well as the growers time, equipment, available labor, and economic resources. Weed management techniques commonly include mechanical methods such as mowing or cultivation; chemical methods including preemergence or postemergence herbicide applications; and cultural methods such as cover cropping. Most managers will use several methods in an integrated approach to address weed management challenges and may alter their techniques in response to changing needs during the life of the orchard or vineyard.

Onion Growth, Yield, and Production Costs as Affected by Irrigation System

Onion (Allium cepa) production in California’s San Joaquin Valley (SJV) typically involves intensive tillage and sprinklers for crop establishment followed by drip irrigation (DR). Studies were conducted at Five Points, California, in 2011 and 2013 to compare minimum tillage (MT) practices under overhead irrigation (OH) and DR relative to crop growth, yield, and costs. The experimental design was a randomized complete block with four replications of each irrigation treatment. Crop growth in both systems was similar; however, there was a year X irrigation system interaction for crop yields. In 2011, yields were not affected by irrigation, but in 2013, irrigation type affected yield with the OH system having 15.4 t/ha higher production than the DR system (78.6 t/ha vs. 63.4 t/ha). Onion size was not affected by irrigation system. Cost estimates indicated that onion production could be

Growth and development of fall- and spring-planted populations of Conyza canadensis and C. bonariensis

564 per hectare lower with the OH system compared with the DR system, if yields were maintained. Because of cost savings from OH, in both years the OH system was more profitable than DR. The study showed that onions could be successfully grown with MT using the OH system in the SJV and similar amounts of water.

Transition to conservation tillage evaluated in San Joaquin Valley cotton and tomato rotations

ABSTRACT The growth and phenological development of spring- and fall-planted Conyza canadensis and C. bonariensis were studied in Fresno, CA, USA. A glyphosate-resistant (GR) and a glyphosate-susceptible (GS) population of each species were used. Time taken by each plant to reach the rosette, bolting, appearance of first bud, appearance of first open flower, and initial seed set were converted to growing degree days (GDDs). The fall- and spring-planted C. canadensis required similar GDDs to complete their life cycles but the GR type required fewer GDDs than the GS type. In C. bonariensis, the GDDs required to complete its life cycle differed between the fall- and spring-plantings but not between the GR and GS types. The total aboveground biomass of both fall- and spring-planted Conyzas at initial seed set was similar but the spring-planted GS C. canadensis produced more biomass than the GR type, whereas the fall-planted GS C. bonariensis produced more biomass than the GR type. Plant development based on GDD models can have global implications and help in planning for timely applications of post-emergence herbicides.