2019 Abstracts (New Orleans, LA)
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|In other work we have established that in a herbicide-susceptible population of ryegrass (Lolium rigidum), minor genes for resistance to the Accase herbicide diclofop-methyl can be selected and enriched by a few cycles of recurrent selection at a sub-lethal diclofop-methyl rate resulting in a significant level of resistance. It is not known, at what frequency those minor resistance genes are and what their specific contribution towards resistance is (e.g. additive or multiplicative effect). Simulation modeling, with a set of parameters established, may help in evaluating the number of genes involved in the selected resistance trait, the initial gene frequencies and their relative contribution to the observed resistance level. However, to have an accurate estimation of genetic parameters involved, it is important to know what the real (lowest) herbicide sensitivity baseline. Here, we studied whether we could shift the population in the opposite direction by selecting for greater susceptibility to herbicides. L. rigidum plants were cloned (divided into two identical portions) at the six-leaf stage. One series of cloned plants was treated with a very low diclofop-methyl dose (94 g ha-1). The untreated clone of an individual observed to be highly herbicide-susceptible was selected and bulk-crossed to obtain the selected seed progeny. Two consecutive cycles of selection towards greater herbicide susceptibility were conducted. Dose-response studies were conducted comparing the selected lines with the original population VLR1. A significant increase in herbicide susceptibility was observed (selected : unselected LD50 ratio = 0.5). The cross resistance profiling of the twice-selected line, compared to the original line VLR1, revealed not only that the selected population was more susceptible to diclofop-methyl but that there was a concomitant increased susceptibility to two other ACCase herbicides (haloxyfop and sethoxydim) and to the different mode of action ALS herbicide chlorsulfuron. The results indicated that minor genes for resistance were present in a herbicide-susceptible ryegrass population. These minor gene traits may be removed from a susceptible population of ryegrass further increasing the level of susceptibility over the unselected line. Importantly, increased susceptibility was also observed to other ACCase herbicides and a different mode of action suggesting the possibility of herbicide metabolism mediated by these unspecified minor gene traits. This was expected and it is consistent with the cross-resistance pattern exhibited by the highly resistant population selected at low doses of diclofop-methyl. The information obtained in this study may greatly contribute to understand the dynamics and the genetic base involved in resistance evolution subsequent to low dose herbicide selection.
|Propane flaming could be included as an additional tool for weed control program in organic wheat productions. However, the crop tolerance to broadcast flaming must be determined first. Therefore, field experiment was conducted at the Haskell Agricultural Laboratory, Concord, NE in 2008 to determine winter wheat response to six propane rates applied at three growth stages of the crop, including: shoot elongation (SE), first node (FN), and boot stage (BS), which corresponded to the 4.0, 6.0, and 10.1 stages according to the Feeks scale of wheat growth and development. The propane rates included: 0, 12, 31, 49, 68, and 87 kg/ha (0, 2.5, 6.5, 10.5, 14.4, and 18.4 gal/acre). Flaming treatments were applied utilizing an ATV mounted flamer moving at a constant speed of 6.5 km/h (4 m/h). The response of wheat to propane flaming was evaluated in terms of visual injury at 7, 14, and 28 DAT, effects on yield components (spikes/m2, seeds/spike, and 100-seed weight) and grain yield. Crop responses were described by log-logistic model. Overall response of wheat to propane flaming varied among growth stages and propane rates. In general, all three growth stages were sensitive to flaming and presented similar visual damage at 28 DAT. In particular, SE had the least yield loss and the least affected yield components compared to BS, which was the most susceptible stage to flaming resulting in highest yield loss, and largest loss of all yield components. Yield reduction is main concern with broadcasts flaming. Preliminary curve analysis suggested that arbitrarily acceptable yield reduction of about 5% (eg. threshold level) was achieved with the propane rate of about 5 kg/ha, regardless of the flaming stage. Based on our previous studies, propane rate of 60 kg/ha was needed for control most weed species, however such propane rate caused unacceptable yield losses of 25%, 32%, and 43% in this study for SE, FN, and BS, respectively. Therefore, due to unacceptable yield losses, we do not recommend the use of broadcast flaming in wheat at the above tested growth stages. email@example.com
|Response to selection with sub-lethal glyphosate doses in Lolium vs. Avena populations The rate of resistance evolution in weed populations subjected to recurrent selection at sub-lethal doses relies on additive genetic variation for resistance, the frequency of such minor gene traits endowing herbicide resistance and the capacity of surviving plants to respond to selection (i.e. incremental accumulation towards polygenic resistance). In general, self-pollinated plant species, compared to cross-pollinated species, have higher inter-population genetic diversity, higher level of homozygosity, but lower intra-population diversity (fewer genotypes). Rigid ryegrass (Lolium rigidum Gaud.) is an obligate cross-pollinated species while wild oat (Avena fatua L.) is mainly self-pollinated. Both are damaging grass weed species in many agricultural systems. Rigid ryegrass is highly genetically variable and we have shown that ryegrass is particularly responsive to recurrent selection at sub-lethal herbicide rates. In three generations of selection a susceptible biotype of rigid ryegrass (VLR1) evolved high level of resistance to diclofop-methyl or moderate level of glyphosate resistance. Likely, existing minor resistance gene traits for each specific herbicide could be additively enriched through cross-pollination among surviving plants. Conversely, in a self-pollinated species this may not occur or be a much slower evolutionary process. Here, we subjected a herbicide-susceptible population of Avena to recurrent selection at sub-lethal glyphosate rates for two consecutive generations to understand the contribution of the reproductive system towards the evolution of herbicide resistance. Selection was also conducted on five different populations collected in different agronomic regions from the Western Australian wheatbelt and the selected seed progeny analyzed. After two cycles of selection no significant difference was observed in survival or plants biomass in the glyphosate-selected versus the unselected population. The five different populations displayed significantly different response to sub-lethal rates of glyphosate in dose-response studies. However, the first glyphosate-selected progeny from each population showed no response to selection. No consistent increase in survival and/or plant biomass was evident in the selected progenies compared to the unselected parents. The results obtained in Avena are consistent with that observed in self-pollinated mouse ear cress (Arabidopsis thaliana L.) in which, seven cycles of recurrent glyphosate selection caused no shift in glyphosate resistance. Therefore, we conclude that cross-pollination is a significant factor in herbicide resistance evolution from recurrent selection at sub-lethal herbicide doses. Cross-pollination enables the progressive accumulation of minor genes while self-pollination limits response to sub-lethal dose selection. Full herbicide rates should be used by farmers. Herbicide rate cutting should be avoided especially where major cross-pollinated weed species are present in the cropping system (e.g. rigid ryegrass in Australia).
|Weed flaming is a thermal weed control technique which has been proven to be very effective in several crops. Propane is the typical fuel source and its combustion in presence of air can produce flame and gas temperatures approaching 2000°C. These high temperatures are behind the principle of weed flaming, which is to boil the water in the plant cells, resulting in cell wall leakage and fatal damage to the targeted weed. From an engineering viewpoint, heat must be carefully controlled to maximize its effectiveness and to guarantee only weeds are being killed during flaming treatment. We have developed an improved heat control method that yielded in a higher quality treatment. Through the use of computational fluid dynamics and experimental measurements, a hood/torch device has been designed and compared to an open flame torch currently on the market. Both temperature measurements and field testing have been conducted in the comparison. Temperature measurements showed that the hood/torch device concentrated the heat of combustion, producing a larger high temperature core than the open torch. This also resulted in a longer exposure time to high temperature gases. Results from field tests showed the hood/torch device has the potential to reduce the propane consumption rate by 30-50% and to provide much greater control of flames in windy field conditions. (firstname.lastname@example.org).