Two-way Mixtures Research Articles

Auxinic Herbicide Mixtures for Controlling Multiple Herbicide-Resistant Kochia in Fallow

Kochia resistant to glyphosate (Roundup), chlorsulfuron (Glean), and dicamba (Banvel or Clarity) has become quite common in the U.S. Great Plains, whereas multiple resistance to additional herbicides, including fluroxypyr (Starane Ultra), atrazine (AAtrex), and metribuzin (Sencor) has also been reported recently. Effective management of these multiple herbicide-resistant (MHR) kochia populations warrants the need of alternative herbicide strategies. The main objective of this research was to investigate the efficacy of auxinic herbicides, including Duplosan (dichlorprop-p), Weedone (2,4-D), Clash (dicamba), and/or Pixxaro (premix of halauxifen and fluroxypyr) alone or in various combinations for controlling MHR kochia. Separate greenhouse and field experiments were conducted at the Kansas State University Agricultural Research Center (KSU-ARC) in Hays, KS. Greenhouse studies included an MHR kochia population (resistant to glyphosate, dicamba, fluroxypyr, chlorsulfuron, atrazine, and metribuzin) from Garden City, KS, and a susceptible (SUS) kochia population from Hays, KS. The postemergence (POST) applied herbicide programs, including Clash Weedone, Duplosan alone or in tank-mix combinations were tested. Field experiments were conducted in a fallow field at KSU-ARC with a natural infestation of kochia population with multiple resistance to glyphosate and dicamba. Herbicides, including Duplosan, Weedone, Clash, and Pixxaro were tested alone or in tank-mix combinations. Results from greenhouse study indicated that Clash, Duplosan, and Weedone applied alone provided inadequate control (5 to 42%) of MHR kochia at 21 days after treatment (DAT). In contrast, control of SUS population was 83 to 92% with Clash and Duplosan alone treatments. Tank-mixing Duplosan with Clash and/or Clash + Weedone significantly improved visible control (72 to 90%) of MHR kochia as compared to Duplosan, Clash, or Weedone alone treatments. Similarly, tank-mixing Clash to Duplosan or Pixxaro (two-way mixtures) and to Duplosan + Weedone, Pixxaro + Duplosan or Pixxaro + Weedone (three-way mixtures), provided an excellent control (91 to 97%) of MHR kochia compared to Clash, Pixxaro, Weedone, and Duplosan alone treatments in a field study. Altogether, these results suggest that tankmixing Clash with Duplosan and/or Pixxaro can potentially provide synergistic effect in controlling MHR kochia in fallow fields.

Genotype by environment interaction for area under the disease-progress curve (AUDPC) value in spring barley using additive main effects and multiplicative interaction model

The objective of this study was to assess genotype by environment interaction for area under disease progress curve values in spring barley grown in South-West Poland by the additive main effects and multiplicative interaction model. The study comprised of 25 spring barley genotypes (five cultivars: Basza, Blask, Antek, Skarb and Rubinek as well as all possible 10 two-way mixtures and 10 three-way mixtures combinations), evaluated at two locations in 4 years (eight environments) in a randomized complete block design, with four replicates. Area under disease progress curve (AUDPC) value of the tested genotypes ranged from 75.3 to 614.3, with an average of 175.3. In the AMMI analyses, 13.43% of the AUDPC value variation was explained by environment, 37.85% by differences between genotypes, and 18.20% by genotype by environment interaction. The mixture Basza/Skarb is recommended for further inclusion in the breeding program due to its low average AUDPC value (98.8) and is stable (AMMI stability value = 6.65).

Crop mixtures: does niche complementarity hold for belowground resources? An experimental test using rice genotypic pairs

Genotypic mixtures have been receiving a growing interest as genetic diversity could increase crop productivity. Resource-use complementarity is an expected key underlying mechanism, provided that varieties in the mixture differ in resource-related traits, notably root traits. We aimed at examining how trait differences and resource-use complementarity drive biomass production of genotypic mixtures. Four rice (Oryza sativa) genotypes including two Near-Isogenic Lines only differing in root depth were grown in monoculture and in two-way mixtures in pots under two levels of phosphorus supply. We analyzed the relative difference between mixture biomass and the best monoculture biomass in relation to between-genotype phenotypic distance on ten resource-related traits. Mixtures never outperformed the best monoculture. However, relative mixture productivity increased with increasing between-genotype distance in biovolume, specific leaf area and top soil root biomass. This was mainly driven by a “selection effect”: trait differences led to competitive ability differences and the dominant genotypes tended to gain more in mixture than the subdominant genotypes lost compared to monoculture. Rather than trying to minimize competition through resource-use complementarity, we argue that promoting interactions between genotypes that have different competitive abilities may be a more promising approach to design productive crop mixtures.

Identifying Optimum Herbicide Mixtures to Manage and Avoid Fenoxaprop-p-Ethyl Resistant Phalaris minor in Wheat

ABSTRACT Use of herbicide mixtures has been advocated as most effective strategy for avoidance and management of herbicide resistant weeds. Effect of twelve selected treatments of four herbicides (clodinafop-propargil, metribuzin, pinoxaden and sulfosulfuron) two-way mixtures at different doses was investigated against fenoxaprop-p-ethyl resistant and susceptible populations of P. minor grown along the wheat plants. In repeated experiment, herbicides mixtures were applied at 3 to 4 leaf stage of P. minor under greenhouse conditions. All the herbicide mixtures were effective to control resistant as well as susceptible P. minor. Mixtures having 75% lethal dose of each mixture component provided best control against P. minor. Mixtures with 50% lethal dose of each herbicide also provided more than 80% control of P. minor. Surviving P. minor plants after exposure to herbicide mixtures showed reduced growth and seed production potential. No mixture combination produced phytotoxic effects on wheat plant up to 75% of lethal dose of each mixture component. Mixtures including clodinafop-propargil + metribuzin, pinoxaden + sulfosulfuron and pinoxaden + metribuzin at 100% dose of each mixture component produced minor phytotoxic effects on wheat plants and caused no reduction in terms of ultimate growth and grain yield. However, mixture of sulfosulfuron + clodinafop-propargil at 100% dose of each component was phytotoxic to wheat and caused significant reduction in term of growth and grain yield. So, farmers can use these mixtures even at 75% of recommended dose of mixture component to control susceptible and resistant P. minor in wheat.

Influence of genotypic mixtures on field pea yield and competitive ability.

Darras, S., McKenzie, R. H., Olson, M. A. and Willenborg, C. J. 2015. Influence of genotypic mixtures on field pea yield and competitive ability. Can. J. Plant Sci. 95: 315-324. Field pea breeding programs have been very successful at improving plant and disease resistance; however, limited success has been achieved in improving the competitive ability of field pea. A study was conducted to determine whether growing field pea in two-way genotypic mixtures could improve the crop's yield and competitive ability. A second objective was to determine if genetic relatedness had any effect on the mixing ability of genotypes. Genotypes were chosen on the basis of pedigree and included two sister lines (CDC1987-3 and CDC1897-14), their common parent (Eclipse), and a distantly related genotype (Midas). The four genotypes were grown as pure stands and as all possible two-way mixtures in field experiments conducted at Lethbridge and St. Albert, Alberta, from 2010 to 2011. The results revealed that CDC1897-3×Eclipse s...

Influence of genotypic mixtures on field pea yield and competitive ability

Darras, S., McKenzie, R. H., Olson, M. A. and Willenborg, C. J. 2015. Influence of genotypic mixtures on field pea yield and competitive ability. Can. J. Plant Sci. 95: 315–324. Field pea breeding programs have been very successful at improving plant and disease resistance; however, limited success has been achieved in improving the competitive ability of field pea. A study was conducted to determine whether growing field pea in two-way genotypic mixtures could improve the crop's yield and competitive ability. A second objective was to determine if genetic relatedness had any effect on the mixing ability of genotypes. Genotypes were chosen on the basis of pedigree and included two sister lines (CDC1987-3 and CDC1897-14), their common parent (Eclipse), and a distantly related genotype (Midas). The four genotypes were grown as pure stands and as all possible two-way mixtures in field experiments conducted at Lethbridge and St. Albert, Alberta, from 2010 to 2011. The results revealed that CDC1897-3×Eclipse suppressed the model weed (barley); it reduced seed production by 47% (442 kg ha−1) and 61% (391 kg ha−1) compared with the same components within pure stands at Lethbridge 2010 and Lethbridge 2011, respectively. The same mixture also reduced model weed (barley) biomass production by 61% (831 kg ha−1) at St. Albert in 2010, and by 41% (1372 kg ha−1) at Lethbridge in 2010. Although mixtures demonstrated the potential to improve field pea competitive ability, results were not consistent across site-years. However, some mixtures did improve yield and competitive ability over the most poorly competitive genotypes in pure stand.

Cowpea Cultivar Mixtures for Stable and Optimal Leaf and Seed Yields in a Maize Intercropping System

Cowpea [Vigna unguiculata (L.) Walp.] is an important crop in many African countries, though its use as a leafy vegetable has not received adequate research attention. Leaf and grain yields are low and unstable, especially in highly variable climates in marginal areas. A study was conducted to evaluate the impact of cowpea cultivar mixtures on leaf and seed yield and stability in a cowpea–maize (Zea mays L.) intercropping system. Four cowpea cultivars and a local landrace check were used in monoculture or in different mixture levels. When leaves were harvested, seed yield was reduced on average by 57% and 59%, on-station and on-farm, respectively, with large variation among treatments. The local landrace check had the highest leaf yields on-farm where it produced a mean of 25 g/plant/2-weekly harvesting interval. It also conveyed positive mixture effects; however, yield stability across successive harvests was lowest, indicating its capacity to react to positive environmental changes. Some mixtures of more than two cultivars maintained more leaf yield stability over time across successive leaf harvests. The highest positive relative mixture effects on leaf and seed yields of up to 100% and 193%, respectively, were obtained in two-way mixtures, indicating that these may offer the best combinations for improved yields and to screen for favorable and unfavorable cultivar components.

Common reed (Phragmites australis) control is influenced by the timing of herbicide application

Herbicides are typically used as the primary method of weed control. Since common reed (Phragmites australis subsp. australis) infestations in terms of density are relatively large in the State of Nebraska, USA, determining the most appropriate timing of herbicide application is critical for developing a weed management programme. Therefore, several field studies were conducted in 2007 and 2008 at three locations along the Platte River, Nebraska, with the aim of assessing the effect of herbicide selection and timing of application on common reed control. Three herbicides (glyphosate, imazapyr, and imazamox) were applied either alone at two doses or as two-way mixtures on three growth stages of common reed, including vegetative, flowering, and seed filling stages. Both doses of imazapyr (280 and 560 g active ingredient [a] ha−1) provided the highest level of control (≥ 92%) across all three timings, while imazamox provided the lowest level of control. For example, imazamox applied alone at 280 and 560 g a ha−1 provided poor control (≤ 60%) across all three timings at the highest rating date. Imazapyr and glyphosate provided the highest levels of control (90%) by the end of the first growing season and into the next growing season (390 to 450 days after treatment, DAT), regardless of the herbicide application time. Imazamox and glyphosate provided the lowest level of control (< 30%) at the first application time at 450 DAT, but slightly improved control with later timings (74% and 85% control at the flowering and seed filling stages, respectively). Stem density decreased in all herbicide applications and timings except for imazamox at both doses during the vegetative growth stage, which was not significantly different than the untreated control.

Biocontrol with Trichoderma species for the management of postharvest crown rot of banana.

Lasiodiplodia theobromae and Colletotrichum musae cause the postharvest crown rot disease complex of banana. In vitro experiments evaluated the effect of twelve isolates of Trichoderma spp. from the soil of organic banana orchards (‘native isolates’) and eight isolates of Trichoderma spp. from culture collections (‘introduced isolates’) on the two pathogens. The native and introduced Trichoderma spp. had varied antagonistic effects against the two pathogens. Eight Trichoderma spp. isolates effective in the in vitro assays were evaluated singly on fruits both at room temperature and in cold storage. Single antagonists did not satisfactorily control crown rot on the fruits as compared with the fungicide carbendazim. However, two isolates of T. viride , one of T. harzianum and one of T. koningii performed well when applied singly, and these were selected for evaluation in isolate mixtures. There was very little antagonism between these isolates. Of 11 two-way, three-way and four-way mixtures of these isolates, the four-way and a three-way mixtures reduced crown rot incidence, both at room temperature and in cold storage, giving better control than carbendazim. The study identified consortia of compatible Trichoderma antagonists with superior biocontrol potential for the management of the postharvest crown rot complex of banana.

Organic and conventional management of mixtures of wheat and spring cereals

Cereal mixtures may provide both organic and conventional producers with a more sustainable approach in reducing weed pressure, crop rotation flexibility, improved yield stability, buffering against pests and diseases, minimizing soil variability and increasing animal feed value. We examined the response of small grain mixtures containing wheat, oats, barley and triticale to varying degrees of natural competition and environmental stress at three locations in central Alberta, Canada. One modern and one heritage hard-red spring wheat cultivar, along with one cultivar each of oats, barley and triticale and eighteen two-way mixtures, were planted on organic and conventional land at seven location-years between 2003 and 2005. Average yields were 30% to 70% lower on organically managed sites. Monocrop barley yielded 43% and 16% higher than the site average at two organic locations. Our results suggest two main conclusions: (1) on conventionally managed land, wheat-barley mixtures exhibited potential for yield maintenance and weed suppression, and; (2) on organically managed land, competition with weeds had a large negative effect on yield (>30%). The 25:75 mixtures of wheat and oats, and all mixtures of Park (a heritage) wheat and Manny barley exhibited yield potential similar to or (up to 1.0 t ha−1) greater than monocrop yield. Manny barley mixtures exhibited weed suppressive capabilities.

A Comparison of Flumioxazin and Rimsulfuron Tank Mixtures for Weed Control in Potato

In 2004 and 2005, field research was conducted in Idaho to compare tank mixtures of flumioxazin at the 53 g ai/ha potato use rate with comparable tank mixtures of rimsulfuron at 26 g ai/ha for broad-spectrum weed control. Flumioxazin in two-way tank mixtures with metribuzin, EPTC, pendimethalin, S-metolachlor, or ethalfluralin provided greater than 90% hairy nightshade control, which was comparable with control by similar rimsulfuron two-way tank mixtures. Flumioxazin plus metribuzin or rimsulfuron were the only two-way mixtures with flumioxazin consistently providing 90% or greater redroot pigweed, common lambsquarters, and green foxtail control. Control of these weeds by any of the rimsulfuron two-way mixtures was almost always greater than 90%. Three-way tank mixtures containing flumioxazin or rimsulfuron controlled hairy nightshade, redroot pigweed, and common lambsquarters similarly, and control ranged from 89 to 100%. When metribuzin was not included with flumioxazin in three-way mixtures, control was 80 to 97% and not always comparable with the 89 to 100% control by similar rimsulfuron mixtures. Green foxtail control by flumioxazin or rimsulfuron three-way mixtures usually was similar and greater than 90%.

Impact of density and disease on frequency-dependent selection and genetic polymorphism: experiments with stripe rust and wheat

Frequency-dependent disease impacts may contribute to the maintenance of genetic diversity and sexual reproduction in plant populations. In earlier work with experimental wheat (Triticum aestivum) populations at a single density, we found that stripe rust (caused by Puccinia striiformis) created frequency-dependent selection on its host but competitive interactions between host genotypes reduced the potential for disease to maintain genetic polymorphisms in this highly self-pollinated species; the weaker competitor actually exhibited positive disease-mediated frequency-dependent selection. Based on these results we predicted that at low density, where the overall level of competition is lower, disease would have a stronger impact relative to competition and thus be more likely to maintain genetic polymorphisms; at low densities the greatest effect of disease for negative frequency-dependent selection should be seen in the weak competitor. Here we report on results with wheat stripe rust in which we altered both the frequency and density of host genotypes in factorial combinations of two-way mixtures where each host genotype was attacked by its own specialized race of rust. Within each density disease levels increased with genotype frequencies, creating frequency-dependent disease attack at all densities. Similarly, disease created negative frequency-dependent selection on its host at all densities, as a genotype’s fitness was often greater at low than high frequency when disease was present. Disease levels increased with plant density in 1997 but decreased in 1998. While increasing plant density reduced absolute fitness, presumably as a result of increased competition, a genetic polymorphism was not more likely to be maintained at low than high density as we had predicted. Within each density, the impact of disease was insufficient to reverse the slope of the relationship between absolute fitness and planted frequency from positive to negative for the less competitive host genotype, thus preventing the maintenance of a genetic polymorphism.

The effect of diversity and spatial arrangement on biomass of agricultural cultivars and native plant species

Abstract Various spatial arrangements can be found within natural grassland plant populations and communities. In contrast, spatial arrangement diversity is typically not observed in agroecosystems. Little is known about the influence of spatial arrangement on the productivity and success of agricultural or native plants. Such information is of interest to farmers who want to increase yield and restorationists working with native ecosystems. Agricultural and native plants were planted in two-way mixtures that included combinations of cultivars, species, genera, or functional groups. Each combination was arranged as a random mixture within rows, alternating rows of the different genotypes or species, and as alternating pairs of rows. Aboveground biomass was determined for each mixture component and compared to monoculture controls. Though plot composition had the most consistent influence on aboveground biomass, spatial arrangement appeared to have some influence among agricultural cultivars. Whether native or agricultural, biomass was greater for mixtures containing both a grass and a legume.

Assessing Four-Way Mixtures of Winter Wheat Cultivars from the Performances of their Two-Way and Individual Components

Seven four-way bread wheat mixtures were compared to their components (individual cultivars and two-way mixtures) for septoria tritici blotch severity, grain protein content, and grain yield. Four-way mixtures and two-way mixtures performed better than the average of individual pure line components. Disease severity and protein content were most influenced by mixtures, with mixture efficiencies being greater in the four-way than in the two-way mixtures. Performances of four-way mixtures in terms of diseased leaf area reduction, grain yield, 1000-grain weight, and grain protein content were better predicted by performances of two-way mixtures than by those of pure lines. Our results suggest that two-way mixtures should be screened to remove unfavourable cultivar pairs and those binary mixtures that show complementarity should be selected in order to construct four-way mixtures.

Assessment of interactions between components of fungicide mixtures against Monilinia fructicola

Mixtures of fungicides with different modes of action can exhibit synergism, i.e. an inhibition of pathogen growth above that expected from independent action of the mixture components. Two-way mixtures of commercial formulations of propiconazole with either benomyl, captan, chlorothalonil, cyprodinil or vinclozolin were evaluated in vitro for potential synergism in inhibiting Monilinia fructicola, the causal agent of blossom blight and brown rot of stone fruits. Propiconazole was emphasized because of its widespread use and the recent detection of isolates of M. fructicola with reduced sensitivity to this fungicide. Experiments included each active ingredient at low, medium and high concentrations in all possible pairwise combinations. Inhibition of radial growth of two isolates of M. fructicola was not significantly different ( P>0.01) from that predicted by a simple model of independent action for any of the fungicide–concentration combinations, indicating absence of synergism between active ingredients. Results were similar when mixtures of propiconazole with either benomyl, chlorothalonil or cyprodinil were evaluated on peach fruit treated with fungicide. While fungicide mixtures are useful in delaying the development of fungicide resistance, they are unlikely to be used in practice unless synergistic interactions allow for applications at reduced concentrations. The absence of synergism suggests little incentive exists for favoring propiconazole-based fungicide mixtures over a rotating schedule of fungicides for control of and resistance management in M. fructicola.

Effects of competition on resistance gene polymorphism in a plant/pathogen system.

Puccinia striiformis, a fungal pathogen, has been shown to cause direct, negative frequency-dependent selection on its host, wheat (Triticum aestivum). This disease-induced frequency-dependent selection was not sufficient to maintain polymorphism for resistance genes in the host populations. The present study examines whether interactions between disease and competition could impact upon the maintenance of genetic polymorphism in a highly self-pollinated species such as wheat, where strong associations between traits are likely. Four different two-way mixtures of wheat genotypes, susceptible to different races of P. striiformis, were planted at different frequencies in both the presence and absence of disease. In order to examine the influence of competition and disease on the maintenance of genetic polymorphism, relationships between host absolute fitness and host frequency were studied for each genotype in the mixtures of plants both in the presence and in the absence of disease. In the absence of disease, the absolute fitness of the stronger competitor was often negatively frequency-dependent, or else it did not vary with host frequency; the absolute fitness of the weaker competitor was often positively frequency-dependent. Disease typically rendered the slopes between absolute fitness and genotype frequency more negative for the stronger competitor. However, the influence of disease was not strong enough to reverse the sign of the slope between absolute fitness and genotype frequency for the genotype that was the weaker competitor in the absence of disease. Thus, disease was unable to reverse the relative ranking of the two genotypes caused by competition and create the negative frequency dependence on both genotypes in a mixture that is required for the maintenance of genetic polymorphism.

Using mixing ability analysis from two-way cultivar mixtures to predict the performance of cultivars in complex mixtures

Cultivar mixtures are an alternative to monoculture for crop production. Methods for predicting the performance of cultivars in mixtures would facilitate the identification of the best cultivars for mixture formation. A prediction method [Theor. Appl. Genet. 81 (1991) 265] based on the combining ability analysis proposed by Gardner and Eberhart [Biometrics 22 (1966) 439] model II was evaluated for mixtures. Mean values for percent diseased leaf area (DLA) and yield under disease (YUD) of five club wheat cultivars and all possible two-way mixtures (across three Oregon locations) were used to estimate the relative contribution of each cultivar to the mixture mean ( I i ) and the predicted mean of all possible mixtures ( Y m) according to the MFC method. Estimated I i and Y m were compared to actual mixture means. I i allowed identification of the best cultivars for mixture formation for both DLA and YUD. Actual and predicted rank correlation coefficients for DLA and YUD of complex mixtures (more than two components) were highly significant ( P<0.01) for the mean of the three environments (0.87 and 0.78, respectively) and for the mean of the two most relevant environments (0.83 and 0.93, respectively). Similar results were obtained when I i and Y m were estimated from the means of the cultivars in pure stand (instead of being estimated from the mixing ability analysis). This was due to the relatively small competitive effects ( h i ) of the cultivars compared to their additive effects ( v i ). We extended our analysis to data from a yield study of two-way mixtures of eight soybean cultivars [Crop Sci. 29 (1989) 885; Agron. J. 81 (1989) 559], where additive and competitive effects had similar magnitude. Actual and predicted mixture means could not be compared because only two-way mixtures were included in the study. In this case, the mean yield of the cultivars in pure stand was not a good predictor of the performance of cultivars in mixtures. Ra 604, the third highest yielding cultivar, had a positive additive effect ( v i=53.8 kg/ ha ), but a highly negative competitive effect ( h i=−49.7 kg/ ha ) that resulted in a negative contribution ( I i ) to yield when mixed with the other soybean cultivars. Additive ( v i ) and competitive effects ( h i ) must be considered to obtain superior mixtures, and the advantage of I i is that it takes into account both effects ( I i= 1 2 v i+(k−1/k)h i , k is the number of cultivars). The MFC method may be a useful tool to select desirable cultivars to obtain complex mixtures.

Preemergence Weed Control in Potato (Solanum tuberosum) with Ethalfluralin1

Field studies were conducted to assess weed control and potato (Solanum tuberosum) tolerance to ethalfluralin. Ethalfluralin applied preemergence (PRE) alone at 1.05 kg ai/ha generally did not control weeds adequately. However, ethalfluralin at 1.05 kg/ha combined with either metribuzin at 0.28 kg ai/ha or rimsulfuron at 0.018 kg ai/ha controlled common lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus), and green foxtail (Setaria viridis) ≥ 98%, which was similar to control observed with several currently registered herbicide mixtures. Volunteer oat (Avena sativa) control with either ethalfluralin at 1.05 kg/ha plus EPTC at 3.4 kg ai/ha or ethalfluralin plus metribuzin was equal to registered two-way mixtures. Ethalfluralin plus metribuzin did not adequately control hairy nightshade (Solanum sarrachoides), but ethalfluralin mixtures with either rimsulfuron or EPTC controlled hairy nightshade equal to or better than the registered two-way mixtures evaluated. A sequential application of ethalfluralin PRE followed by rimsulfuron or rimsulfuron plus metribuzin postemergence (POST) did not improve hairy nightshade control compared to ethalfluralin plus rimsulfuron applied PRE. Potato tolerance to herbicide treatments applied PRE or POST to potato was evaluated in weed-free studies. Ethalfluralin alone or with metribuzin was compared to mixtures of metribuzin with either pendimethalin or EPTC. Initial visual injury with ethalfluralin PRE was ≤ 4% both years. In 1996, initial injury with ethalfluralin POST was ≤ 4% and U.S. No. 1 and total tuber yields were not affected by herbicide treatment or application timing. However in 1997, initial injury from POST ethalfluralin at 1.05 or 2.1 kg/ha was 2 or 8% and increased to 9 or 17%, respectively, at potato row closure. Averaged over all herbicide treatments, POST applications reduced U.S. No. 1 and total tuber yield 7% relative to PRE applications in 1997.Nomenclature: EPTC, S-ethyl dipropyl carbamothioate; ethalfluralin, N-ethyl-N-(2-methyl-2-propenyl)-2,6-dinitro-4-(trifluoromethyl)benzenamine; metribuzin, 4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one; pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine; rimsulfuron, N-[[(4,6-dimethoxy-2-pyrimidinyl)amino] carbonyl]-3-(ethylsulfonyl)-2-pyridinesulfonamide; common lambsquarters, Chenopodium album L. #3 CHEAL; green foxtail, Setaria viridis (L.) Beauv. # SETVI; hairy nightshade, Solanum sarrachoides Sendter # SOLSA; redroot pigweed, Amaranthus retroflexus L. # AMARE; volunteer oat, Avena sativa L. # AVESA; potato, Solanum tuberosum L. ‘Russet Burbank.’Additional index words: Injury, tolerance, AMARE, AVESA, CHEAL, SETVI, SOLSA.Abbreviations: PNW, Pacific Northwest; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Disease, frequency-dependent selection, and genetic polymorphisms: experiments with stripe rust and wheat.

Pathogens have the potential to maintain genetic polymorphisms by creating frequency-dependent selection on their host. This can occur when a rare host genotype is less likely to be attacked by a pathogen (frequency-dependent disease attack) and has higher fitness at low frequency (negative frequency-dependent selection). In this study, we used wheat genotypes that were susceptible to different races of the pathogen Puccinia striiformis to test whether disease created frequency-selection on its host and whether such selection could maintain polymorphisms for resistance genes in the wheat populations. Four different two-way mixtures of wheat genotypes were planted at different frequencies in both the presence and absence of disease. Disease created frequency-dependent selection on its host in some populations. Unknown factors other than disease also created frequency-dependent selection in this system because, in some instances, rare genotype advantage was observed in the absence of disease. Although the pathogen created frequency-dependent selection on its host, this selection was not sufficient to maintain genetic polymorphism in the host populations. In all cases where frequency-dependent selection occurred only in the diseased plots, one of the two genotypes was predicted to dominate in the population and the same genotype was predicted to dominate in both the presence and absence of disease. Only in cases where frequency-dependent selection was not caused by disease was there evidence that genetic polymorphisms would be maintained in the population. The frequency-dependent selection described in this study is a consequence of epidemiological effects of disease and differs from the time-lagged frequency-dependent selection resulting from coevolution between hosts and parasites. The impact of this direct frequency-dependent selection on the maintenance of genetic polymorphisms in the host population is discussed.

DISEASE, FREQUENCY-DEPENDENT SELECTION, AND GENETIC POLYMORPHISMS: EXPERIMENTS WITH STRIPE RUST AND WHEAT

Pathogens have the potential to maintain genetic polymorphisms by creating frequency-dependent selection on their host. This can occur when a rare host genotype is less likely to be attacked by a pathogen (frequency-dependent disease attack) and has higher fitness at low frequency (negative frequency-dependent selection). In this study, we used wheat genotypes that were susceptible to different races of the pathogen Puccinia striiformis to test whether disease created frequency-selection on its host and whether such selection could maintain polymorphisms for resistance genes in the wheat populations. Four different two-way mixtures of wheat genotypes were planted at different frequencies in both the presence and absence of disease. Disease created frequency-dependent selection on its host in some populations. Unknown factors other than disease also created frequency-dependent selection in this system because, in some instances, rare genotype advantage was observed in the absence of disease. Although the pathogen created frequency-dependent selection on its host, this selection was not sufficient to maintain genetic polymorphism in the host populations. In all cases where frequency-dependent selection occurred only in the diseased plots, one of the two genotypes was predicted to dominate in the population and the same genotype was predicted to dominate in both the presence and absence of disease. Only in cases where frequency-dependent selection was not caused by disease was there evidence that genetic polymorphisms would be maintained in the population. The frequency-dependent selection described in this study is a consequence of epidemiological effects of disease and differs from the time-lagged frequency-dependent selection resulting from coevolution between hosts and parasites. The impact of this direct frequency-dependent selection on the maintenance of genetic polymorphisms in the host population is discussed.Corresponding Editor: C. Lively