Avena Sterilis Research Articles

English

Seven species of oat (Avena) were evaluated for their relative drought tolerance under soil moisture stress. The plant height, leaf area production and biomass yield reduced under soil moisture stress. Among the species tested, minimum reduction in height was recorded in Avena vaviloviana, Avena abyssinica and Avena sterilis at vegetative and flowering stages. Significant decrease in leaf area production was recorded at vegetative stage, whereas at flowering stage, the decrease in leaf area production was marginal in A. sterilis followed by A. abyssinica predicting their more adaption to stress environment. The increase in specific leaf weight (SLW) of all the species of Avena showed increase in leaf thickness, exhibiting high water retention capacity under soil moisture stress condition which is a requisite trait for drought tolerance. Soil moisture stress imposed at vegetative and flowering stages reduced fresh biomass yield in all the species. Minimum reduction in dry biomass accumulation under stress environment at vegetative stage was recorded in A. sterilis followed by A. strigosa and A. sativa, exhibiting their tolerance to drought at early stages of growth. However, at flowering stage, minimum decrease in dry biomass production was recorded in A. sterilis (3.47%) followed by A. marocana (12.56%) indicating their relative drought tolerance at flowering stage of crop growth. A significant positive correlation between total leaf area and dry biomass (r2=0.738) under stress environment indicates that dry biomass accumulation was governed by total leaf area production. A. sterilis accumulated maximum fresh and dry biomass under soil moisture stress with minimum reduction over the non stress environment, indicating its drought tolerance potential as compared to other genotypes tested.   Key words: Avena, biomass, flowering stage, leaf area, soil moisture stress, vegetative stage.

Plasticity and variability in the patterns of phytolith formation in Asteraceae species along a large rainfall gradient in Israel

Silicification and phytolith formation in Poaceae species growing in arid and semi-arid regions are commonly thought to be positively correlated with silica and water availabilities and with transpiration. To expand our understanding of this phenomenon, we chose to study species of the Asteraceae, the largest dicotyledonous family. We measured phytolith concentrations in eight Asteraceae species (three non-spiny and five spiny) and one Poaceae species (Avena sterilis), as reference, along a large climatic gradient of 80–900mm mean annual rainfall in Israel.Phytolith concentration in A. sterilis was found to be positively correlated with water availability and with a minor effect of soil silica availability. The three non-spiny Asteraceae species have a partially similar pattern of changes in silicification according to water availability in more humid habitats, but with higher phytolith concentrations in the most arid site. The five spiny Asteraceae species have various patterns of phytolith formation, not consistently related to water and soil silica availabilities. These results demonstrate that patterns of phytolith formation in the Asteraceae differ among taxa and also from the situation in grasses.

Allelopathic Effects of Barley Extract (Hordeum vulgare) on Sucrose Synthase Activity, Lipid Peroxidation and Antioxidant Enzymatic Activities of Hordeum spontoneum and Avena ludoviciana

During two surveys, allelopathic effects of barley (Hordeum vulgare) extract on growth, photosynthesis and lipid peroxidation were assessed in Hordeum spontoneum and Avena ludoviciana seedlings in Islamic Azad University, Shoushtar Branch (Iran). The experiments were carried out under completely randomized design with 6 replications and 4 treatments (foliar application of 10, 20, and 30 % barley extract solutions and water as control). Remarkable decrease was observed in photosynthesis, sucrose synthase activity, antioxidant enzymatic activities of target weed species in line with the increase in barley extract percentage. Malondialdehyde reached its top concentration in Hordeum spontoneum and Avena ludoviciana plants (0.067 μmol/g FW) treated with 30 % barley extract solution; the lowest Shoot dry weight of Avena ludoviciana (0.58 g) and Hordeum spontoneum (0.53 g) were obtained with 30 % barley extract solution treatment. This study supported the assumption that barley extract inhibited the Hordeum spontoneum and Avena ludoviciana seedling growth through increasing lipid peroxidation.

Bioherbicidal Effects of Essential Oils Isolated from Thymus fallax F., Mentha dumetorum Schult. and Origanum vulgare L.

The chemical composition of essential oil isolated by hydrodistillation from the ground part of Mentha dumetorum Schult. collected from Tokat province in 2010 and Thymus fallax Fisch. & Mey. and Origanum vulgare L. collected from Ordu province in 2009 The main components of Thymus fallax were thymol (41.48 %), o-cymene (26.75 %), ζ-terpinen (15.84 %), terpinoline (2.11 %) that of Origanum vulgare were thymol (50.41 %), carvacrol (12.96 %), 2-bornene (11.28 %), ζ-terpinen (8.80 %), o-cymene (6.68 %) and that of Mentha dumetorum were car vone (39.64 %), eucalyptol (14.34 %), dihydrocarvone (12.78 %), limonene (7.79 %). To deter mine herbicidal activities of the essential oils two layers of filter paper were placed petri dish (6 cm diameter) then seeds of Avena sterilis L., Datura strumarium L., Cucumis sativus L. and Lactuca sativa L. were homogeneously distributed on filter paper. Filter papers were thoroughly moistened using distilled water. Piece of filter paper was glued inner parts of each petri dish's lid. Four different concentrations (0, 3, 5 and 7 µL/petri dish) of the essential oil were applied to the filter paper pieces. Then lid of each petri dish was closed immediately and sealed with parafilm. Petri dishes were incubated at 12 h dark-12 h light periods with an average temperature of 24 oC for 7 days. All of the essential oils tested inhibited seeds germination, roots and shoots growths of A. sterilis, L. sativa, D. strumarium, C. sativus. Consequently, it was determined that essential oils of the plants had bioherbicid effects on seed germination and growths of some crops and weeds, but more detailed studies should be done under field conditions.

Factors Affecting Postdispersal Weed Seed Predation in Barely Fields

Seed predation can be exploited as a mean of natural weed control. Field experiments were conducted in 2007 in four barely fields in Mashhad, NE of Iran to determine the effects of seed covering tissues, seed distribution patterns and interactions between seed density and background seed density on postdispersal seed predation. Five weed species (Avena ludoviciana, Hordeum spontaneum, Sinapis arvensis, Rumex obtusifolius and Rapistrum rugosum), three seed densities (50, 100 and 150 seed dish-1), two background seed densities (with and without), three distribution patterns (random, even, and aggregate), and two level of seed covering tissues (with and without) were arrange in a factorial randomized complete block design. Seed covering tissues had significant negative effect on predation and the most its effect was observed for H. spontaneum. Seed predation was also affected by seed distribution patterns. The highest and lowest seed predations of all species were observed from aggregate and random treatments respectively. The interaction between density and background seed density affected seed predation. Results show that seed predation can play an important role in decreasing the weed seeds on the soil surface and thus, in soil seed bank. Therefore, seed predation could be considered as a control technique in integrated weed management.

Reduced herbicide rates for an effective weed control in competitive wheat cultivars

A study was carried out over 2 years (2008/2009 and 2009/2010) in Greece, in order to evaluate the weed control using reduced rates of the herbicide mesosulfuron-methyl+iodosulfuron-methyl-sodium of four major weeds (Avena sterilis, Phalaris minor, Papaver rhoeas and Sinapis arvensis), grain yield and yield components of five wheat cultivars (Bob, Cosmodur, Meridiano, Quadrato and Simeto). The results indicated a significant differentiation between the several wheat cultivars regarding their weed competitive ability, in terms of dry biomass and seed production. In the case of cultivars such as Simeto the efficacy of 50% reduction of the herbicide recommended rate on wild oat remained high, since it resulted to a reduction of biomass and seed production up to 81 and 98% compared with the untreated plots. In most cultivars studied, the reduction of the herbicide rates by 25 to 50% resulted to a no significant reduction of grain yield compared to the recommended rate. The study revealed that some wheat cultivars with enhancing weed competitiveness can improve the efficacy of reduced herbicide rates. Consequently, sustainable cropping systems could be further developed through the integration of such agronomic practices, while in parallel, care on the threat of herbicide resistance development should be also taken.

Tillage and weed control effects on productivity of a dry seeded rice–wheat system on a Vertisol in Central India

A 3-year (2006–2007 to 2008–2009) field study was conducted to evaluate the effect of four tillage systems {zero tillage (ZT) in rice (Oryza sativa L.) and wheat (Triticum aestivum L.), conventional tillage (CT) in rice and wheat, and two rotational tillage sequences that alternated between CT and ZT} with three weed management levels (weedy check, recommended herbicide and herbicide+1 hand weeding) in rice–wheat cropping system on a clay-loam soil. Continuous ZT increased the population density of awnless barnyard grass [Echinochloa colona (L.) Link] and rice flat sedge (Cyperus iria L.) in rice but reduced the population of wild oats [Avena ludoviciana (L.) Dur.] and common lambsquarters (Chenopodium album L.) in subsequent wheat. Pendimethalin (1.0kga.i.ha−1) fb 2,4-D (0.50kga.i.ha−1) significantly reduced the population density of E. colona, C. iria and sessile joyweed [Alternanthera sessilis (L.) D.C.], but did not control pink node flower (Caesulia axillaris Roxb.) in rice. In wheat, clodinafop propargyl (0.06kga.i.ha−1) fb 2,4-D (0.50kga.i.ha−1) significantly reduced the population of A. ludoviciana, but had no effect on toothed burclover (Medicago hispida Gaertn.). Continuous ZT resulted in significantly higher yield of rice (2.94Mgha−1), wheat (4.45Mgha−1) and rice–wheat system (7.39Mgha−1) compared to continuous CT (2.35, 3.86 and 6.21Mgha−1, respectively). For the total soil depth sampled (0–20cm), weed seed population was significantly greater under continuous ZT (165 weeds 500g−1 soil) and CT (101 weeds 500g−1 soil), compared to rotational tillage (71–85 weeds 500g−1 soil). Rotational tillage systems significantly reduced the seed density of C. iria, A. ludoviciana and M. hispida compared to continuous ZT or CT. Total weed seed density was 66% lower in herbicide treated than in untreated plots. Continuous ZT with effective weed management using recommended herbicide+1 hand weeding was more remunerative and energy efficient. These results suggest that conventional till-based rice–wheat system could be replaced with zero-till-based crop establishment method with effective weed control to save labor and energy in Vertisols of Central India.

Controlling annual weeds in cereals by deploying crop rotation at the landscape scale: Avena sterilis as an example

Weed control through crop rotation has mainly been studied in a nonspatial context. However, weed seeds are often spread beyond the crop field by a variety of vectors. For weed control to be successful, weed management should thus be evaluated at the landscape level. In this paper we assess how seed dispersal affects the interactions between crop rotation and landscape heterogeneity schemes with regard to weed control. A spatially explicit landscape model was developed to study both short- and long-term weed population dynamics under different management scenarios. We allowed for both two- and three-crop species rotations and three levels of between-field weed seed dispersal. All rotation scenarios and seed dispersal fractions were analyzed for both completely homogeneous landscapes and heterogeneous landscapes in which more than one crop was present. The potential of implementing new weed control methods was also analyzed. The model results suggest that, like crop rotation at the field level, crop rotation implemented at the landscape level has great potential to control weeds, whereby both the number of crop species and the cropping sequence within the crop rotation have significant effects on both the short- and long-term weed population densities. In the absence of seed dispersal, weed populations became extinct when the fraction of each crop in the landscape was randomized. In general, weed seed densities increased in landscapes with increasing similarity in crop proportions, but in these landscapes the level of seed dispersal affected which three-crop species rotation sequence was most efficient at controlling the weed densities. We show that ignoring seed dispersal between fields might lead to the selection of suboptimal tactics and that homogeneous crop field patches that follow a specific crop rotation sequence might be the most sustainable method of weed control. Effective weed control through crop rotation thus requires coordination between farmers with regard to cropping sequences, crop allocation across the landscape, and/ or the fraction of each crop across the landscape.

ACCase-Inhibiting Herbicide-ResistantAvenaspp. Populations from the Western Australian Grain Belt

Avenaspp. are world weeds with many cases of evolved herbicide resistance. In Australia,Avenaspp. (wild oat and sterile oat) are a major problem, especially in grain crops. Acetyl-CoA carboxylase (ACCase)–inhibiting herbicides have been used extensively since the late 1970s forAvenaspp. control. However, continued reliance on these herbicides has resulted in the evolution of resistantAvenaspp. populations. Resistance across many ACCase-inhibiting herbicides was characterized in fourAvenaspp. populations from the Western Australian grain belt. Dose–response experiments were conducted to determine the level of resistance to the aryloxyphenoxypropionates and cyclohexanediones and to the phenylpyrazoline herbicide pinoxaden. On the basis of resistance index values, all four resistant populations exhibited high-level diclofop resistance but varied in the level of resistance to other ACCase-inhibiting herbicides tested. It is evident thatAvenaspp. populations from the Western Australian grain belt have evolved resistance to a number of ACCase-inhibiting herbicides.

Crop establishment, tillage and weed management techniques on weed dynamics and productivity of rice (Oryza sativa)-chickpea (Cicer arietinum) cropping system

A field study was conducted to study the effect of methods of rice establishment, tillage and weed management techniques in rice–chickpea cropping system. Treatments included four crop establishment techniques (transplanting, puddling and broadcasting sprouted rice seeds, i.e wet-seeding and dry seeding under conventional and zero tillage systems) in rice and two tillage (zero and conventional) and two weed control methods (weedy check and pendimethalin 1.0 kg/ha as pre-emergence + one hand weeding at 30 days after sowing) in succeeding chickpea. Rice field was infested with jungle rice [Echinochloa colona (L.) Link], Caesulia (Caesulia axillaris Roxb.), sessile joyweed [Alternanthera sessilis (L.) DC.], and rice flat sedge (Cyperus iria L.) and chickpea with wild oats (Avena ludoviciana Dur.) and toothed burclover (Medicago hispida Gaertn.). Results revealed that methods of rice establishment did influence the weed dynamics in rice– chickpea system. Mean yield of zero-till direct-seeded rice (3 262 kg/ha) was as good as that of puddle broadcast rice (3 343 kg/ha) and better than the transplanted rice (3 038 kg/ha). Effective weed control in preceding chickpea benefitted the succeeding rice crop. Methods of rice establishment and tillage did not influence the chickpea yield. Infestation of weeds caused 78.45% reduction in yield of chickpea. Maximum seed yield of chickpea (2 813 kg/ha) was noticed in pendimethalin followed by one HW under conventional tillage. Zero till direct-seeded rice followed by zero-till chickpea system was the best combination for maximizing system productivity, profitability and energy efficiency

Leafy and semi-leafless field pea competition with winter wild oat as affected by weed density

A 2-year field study was conducted in northern Greece using one leafy, long vine length (‘Olympus’) and one semi-leafless, short vine length (‘Hardy’) field pea ( Pisum sativum L.) cultivars to determine their ability to compete with winter wild oat (or animated oat) [ Avena sterilis spp. ludoviciana (Durieu) Gill & Magne]. Winter wild oat stems and dry biomass were increased with increasing weed density from 35 to 210 plants/m 2. Stem number and dry biomass of both field pea cultivars were decreased by 19–55% due to the competition of winter wild oat. However, the cv. ‘Olympus’ provided greater ability to compete and ability to withstand competition of winter wild oat than the cv. ‘Hardy’. In particular, stem number and dry biomass of winter wild oat grown in competition with the cv. ‘Olympus’ were 16–46% lower than those in competition with the cv. ‘Hardy’. In weed-free treatments, seed yield of cv. ‘Hardy’ was by 37% greater than that of cv. ‘Olympus’. However, in weedy treatments, pod number or seed yield losses of cv. ‘Olympus’ were up to 20% or 26%, respectively, lower than those of cv. ‘Hardy’. The results indicated that winter wild oat was a strong competitor against field pea. However, leafy, long vine length field pea had greater ability to suppress winter wild oat and should be used in sustainable production fields infected by this weed.

Sectioning remote imagery for characterization of Avena sterilis infestations. Part A: Weed abundance

Software was developed to spatially assess key crop characteristics from remotely sensed imagery. Sectioning and Assessment of Remote Images (SARI®), written in IDL® works as an add-on to ENVI®, has been developed to implement precision agriculture strategies. SARI® splits field plot images into grids of rectangular “micro-images” or “micro-plots”. The micro-plot length and width were defined as multiples of the image spatial resolution. SARI® calculates different indicators for each micro-plot, including the integrated pixel digital values. Studies on weed patches were done with SARI® using ground-truth data and remote images of two wheat plots infested with Avena sterilis at LaFloridaII and Navajas (Southern Spain). Patches of A. sterilis represented 47.5 and 19.2% of the field areas at the two locations, respectively; the infested areas were a combination of a few large and several small patches. At LaFloridaII, 2.1% of all patches were >500 m2 and 55.0% of all patches were smaller than 10 m2. Based on ground-truth weed abundance data, SARI® output includes geo-referenced and visual herbicide prescription maps, which could be used with variable-rate application equipment.

Sectioning remote imagery for characterization of Avena sterilis infestations. Part B: Efficiency and economics of control

Avena sterilis weed pressure categories can be discriminated in wheat through remote images taken at late stages of wheat senescence. Site-specific image processing was achieved with SARI®, an add-on software program for ENVI® developed to implement precision agriculture. Using the SARI software and crop-weed competition and economic models, the precision yield losses for each micro-plot can be estimated and herbicide prescription maps obtained. Simulation studies on control indicators and herbicide use efficiency were undertaken using real-time ground data and remote images of two wheat plots infested with Avena sterilis at LaFloridaII and Navajas (Southern Spain). The simulation indicated that precision application of herbicides would produce higher overall herbicide savings (OHS) compared to broadcast applications but would vary depending on the level of weed infestation, the decision making criteria (DMC) of applying herbicide above a weed infestation level and the size of the spray grid considered. For example, for areas with low levels of infestation (around 15%), the OHS was 20, 44, 81 and 90% for a DMC of 0, 10, 20 and 30%, respectively. SARI® also estimates the overall herbicide application efficiency (OHAE), a key agro-environmental index to estimate the efficiency of herbicide applications in weedy areas and the lack of herbicide applications in weed-free areas. Ideally, the OHAE is equal to 1 if weed control is complete and herbicide applications in weed-free areas are not necessary. The OHAE index is influenced by the size of the micro-plot and decision-making herbicide application criteria (DMC). The OHAE values increased as the size of the micro-plot decreased, regardless of the intensity of the weed infestation. For example, micro-plots of 20 × 6 m, 5 × 3 m and 1.2 × 1.5 m had OHAE values of 0.27, 0.57 and 0.76, for areas of low infestation, when averaged over the DMC. Generally, the OHAE values increased as the size of the micro-plot decreased regardless of the intensity of weed infestation. Based on actual weed abundance data, competition models and production costs, SARI® estimated wheat yield losses and economic net return for each micro-plot and herbicide application strategy. In both locations, weed infestation varied spatially from virtually weed-free micro-plots to 15 and 24% winter wheat yield loss in Navajas and LaFloridaII, respectively. Preliminary calculations indicate that net returns were slightly higher for areas with site-specific adjusted-rate applications than for the overall standard label rate application strategy. Both of these strategies provided considerably higher net returns compared to non-treated areas.

Sterile oat (Avena sterilis L.) cross-resistance profile to ACCase-inhibiting herbicides in Greece

Sterile oat (Avena sterilis) is the most widely distributed, abundant and harmful weed in the winter cereals grown in Greece. Its effective control has relied for the past three decades upon the ACCase-inhibiting herbicides. However, recently many growers in northern and central Greece have experienced reduced sterile oat control after the application of fenoxaprop-p-ethyl or clodinafop-propargyl. Preliminary screening indicated that less than 80% of the treated plants in 36 and 43 out of 125 sterile oat accessions were killed (R, resistant) by the application of the four times the recommended dose of fenoxaprop-p-ethyl (R-Fx) or clodinafop-propargyl (R–Cd), whereas less than 80% of the treated plants in 29 of them were killed by both herbicides applied at four times the recommended dose. Also, more than 80% of the treated plants in 42 and 35 accessions were killed by the four times the recommended dose (r, partially resistant) of fenoxaprop-p-ethyl (r-Fx) or clodinafop-propargyl (r-Cd), respectively. Finally, more than 80% of the treated plants in 47 and 47 accessions were killed (S, susceptible) by the recommended dose of fenoxaprop-p-ethyl (S-Fx) or clodinafop-propargyl (S-Cd), respectively, whereas more than 80% of the treated plants in 36 of these accessions were killed by the recommended dose of both herbicides. The whole-plant dose-response method used to evaluate 17 R-Fx and five S-Fx accessions showed that 16, 17, 11, and two R-Fx accessions were cross-resistant to clodinafop-propargyl, diclofop-methyl, tralkoxydim, and pinoxaden, respectively. In particular, more than 50% of the treated plants in 12, 12, 17, and three accessions survived the respective application of the four times the recommended dose of fenoxaprop-p-ethyl, clodinafop-propargyl, diclofop-methyl, and tralkoxydim. Regarding pinoxaden, 3, 17 and 20% of the treated plants in three accessions survived its application at four times the recommended dose. All R and the S accessions were effectively controlled by the recommended dose of imazamox and mesosulfuron-methyl + iodosulfuron methyl-sodium. These results strongly suggest that cross-resistance to ACCase-inhibiting herbicides has been developed for sterile oat. Therefore, measures should be taken for crop rotation when possible or rotational use of the available ALS- and ACCase-inhibiting herbicides in order to reduce the potential for development of sterile oat multiple resistance. Regarding the accessions with cross-resistance to APP, CHD and PPZ, these could be effectively controlled by the application of mesosulfuron-methyl + iodosulfuron methyl-sodium.

Effect of Plant Density on Wild Oat Competition with Competitive and Non-Competitive Wheat Cultivars

Abstract Wild oat (Avena ludoviciana) is one of the worst weeds in wheat fields. The effect of wheat density on wild oat competition with more or less competitive wheat cultivars (Triticum aestivum) was investigated at the Experimental Farm of Plant Pests and Diseases Research Institute, Karaj, Iran. The experiment was established as a factorial combination of wheat varieties Roshan as non-competitive and Niknejad as competitive; three wheat densities (recommended density, recommended+25%, and recommended+50%, corresponding respectively with 300, 375, and 450 plants m- 2 for Roshan and 400, 500, and 600 plants nr 2 for Niknejad) and four wild oat densities (0, 25, 50, and 75 plants nr 2) were selected for this experiment. Hyperbolic equations were used to describe relationship between yield and weed density. Increase in wheat density reduced wild oat biomass. Maximum wild oat biomass was achieved at the highest density of the wild oat together with the lowest wheat density. The results showed that higher densities of wheat are able to suppress wild oat dry matter production. Inter-specific competition in Niknejad was 1.7 times more than that in Roshan. Maximum yields of Niknejad and Roshan in the presence of wild oat were obtained at recommended density+25% and recommended density, respectively. Increase in wheat density leads to a decrease in wheat yield due to an increase in intra-specific competition. Increase in wild oat density results in the reduction of wheat yield through decrease in fertile tiller per plant and spike nr 2.

Bulk method selection with grazing treatments on segregating populations from Avena sativa × A. sterilis hybrids

Seven F5 populations derived from Avena sativa × Avena sterilis hybrids were bulked and then sheep grazing and simulated grazing (lawn mowing) were imposed from F5 up to F10. After the 5 year, on 1100 F10 plants per treatment, determinations were performed on the following traits: growth habit at early and late tillering; number of tillers and height of young plants; height of headed plants; number of headed tillers per plant. After five generations under grazing treatments the frequency of prostrate young plants was significantly higher in sheep grazing and in lawn-mowing treatments compared to the control. Moreover, plants from both grazing treatments were found to be lower and more tillered. These results were confirmed at the end of tillering. No difference in final plant height and number of headed tillers was observed. In only five generations, sheep grazing changed the frequency of the different plant habits. A similar result, but to a lesser extent, was induced by simulated grazing. Both treatments heavily penalized the erect habit. Through this bulk method repeated for five generations integrated with two types of grazing we were able to split the start population into nine lots of F11 seeds corresponding to the 3 × 3 combinations of treatment (sheep grazing, simulated grazing, control) and juvenile growth habit (prostrate, erect, intermediate).

Diclofop resistance in sterile wild oat ( Avena sterilis L.) in wheat fields in Greece and its management by other post-emergence herbicides

In 2009, a survey was conducted of cereal fields in five prefectures of Greece to establish the frequency and distribution of herbicide-resistant sterile wild oat ( Avena sterilis L.). In total, 104 sterile wild oat accessions were collected and screened in a field experiment (conducted in 2009 and repeated in 2010) with several herbicides commonly used to control this weed. Most of the sterile wild oat accessions (89%) were classed as resistant (or developing resistant) to the ACCase-inhibiting herbicide diclofop, while resistance to other ACCase-inhibiting herbicides was markedly lower. The results of the pot experiments showed that some of the sterile wild oat accessions were found to have a very high level of diclofop resistance (resistance index up to 28.6), while cross-resistance with other herbicides was common. The levels of resistance and cross-resistance patterns varied among biotypes with different amount and time of selection pressure, indicating either more than one mechanism of resistance or different resistance mutations in these sterile wild oat biotypes. LA14, which had the highest diclofop resistance level (28.6 resistance index), showed resistance to all APP herbicides applied and non-ACCase inhibitors. Alternative ACCase-inhibiting herbicides, namely tralkoxydim and pinoxaden remain effective on 86 and 92% of the tested sterile wild oat populations, respectively. For the ALS-inhibiting herbicide mesosulfuron + iodosulfuron, nearly all the sterile wild oat accessions were susceptible (97%), with only 3 of them classed as developing resistance. Therefore, there is an opportunity to effectively control sterile wild oat by selecting from a wide range of herbicides and other cultural practices. Early post-emergence herbicide application is strongly suggested, since it could decrease the number of resistant seeds in the field and slow down the dispersal of this major problem.

Discrimination of sterile oat ( Avena sterilis) in winter barley ( Hordeum vulgare) using QuickBird satellite images

Site-specific weed management implies detecting the location of weeds in order to generate maps of their spatial distribution. This information facilitates a more accurate application of herbicides, spraying them in the exact areas of weed growth and in the required doses. In order to explore the potential of commercial satellites to discriminate and map weeds, we used the information contained in high spatial resolution images acquired by the QuickBird satellite to assess the density of sterile oat ( Avena sterilis) present in a winter barley field at two different dates (March and June). Our results confirmed the potential of using satellite images in the spectral discrimination of weed patches in infested fields. The results of binary logistic regressions showed that the best matches in the classification of three categories (low, medium, or high sterile oat densities) corresponded to the March image. QuickBird’s March image provided reliable estimates of sterile oat patches in barley crops when weed density was relatively high (between 86% and 94% of agreement between predicted and observed densities). However, when weed densities were lower than 10 plants/m 2 there were serious difficulties to distinguish them from weed-free zones (between 72 and 75% of global agreement in the classification) with large underestimation of medium density weed patches (10 plants/m 2). This is a potential limitation considering than the thresholds used for herbicide application decisions are generally close to this density. However, the information obtained may still be useful for producing field maps to describe the spatial distribution of this weed. Moreover, these studies have provided valuable information on the best spectral regions and/or vegetation indices for approaching discrimination between sterile oat and cereal crops and the most suitable period for it.

Remarkable changes of weed species in Spanish cereal fields from 1976 to 2007

Management practices, geographical gradients and climatic factors are factors explaining weed species composition and richness in cereal fields from Northern and Central Europe. In the Mediterranean area, the precise factors responsible for weed distribution are less known due to the lack of data and surveys. The existence of weed survey data of year 1976 in the Zaragoza province of the Aragon region, Spain, offered us the opportunity to compare present weed species with weed species growing 30 years ago. No detailed comparison of changes in weed species composition in cereal fields in that period of time has been conducted in the Mediterranean area. Here a survey was conducted in the Aragon region from 2005 to 2007. Weeds were surveyed in 138 winter cereal fields in ten survey areas where winter cereals are the main crops, using the same methodology applied 30 years ago. In the Zaragoza province, 36 fields were chosen in the same municipalities than in the previous survey. Several management, geographic and climatic variables of each field were recorded and related to weed species with multivariate analysis. Diversity index were calculated and related to survey area and altitude. Our results show that out of the 175 species only 26 species were found in more than 10% of the surveyed fields. The main species were Papaver rhoeas, Lolium rigidum, Avena sterilis and Convolvulus arvensis found in more than half of the surveyed fields. L. rigidum was related to dryland, while the other species were found overall. Furthermore, we found that management, geographical and climatic factors were significantly related to weed species distribution. In particular altitude, survey areas, irrigation and herbicide use in post-emergence were the most driving factors explaining weed species distribution. Species richness was higher in survey areas with extensive management practices and increased with altitude excepting a very productive area with intensive management practices at high altitude where richness was as low as in the irrigated lowlands. The main differences found between the 1976 and the 2005–2007 surveys were (1) the striking increase of grass weeds, (2) the high decrease of mean weed species number found in each field declining from 9 to 3 and (3) the frequency decrease of many weed species probably caused by agriculture intensification in that period of time. The growing importance of other weed species is probably related to their adaptation to minimum tillage, which is a widespread technique nowadays.

Yellow Stunt, a Tobacco Disease Caused by Pythium dissotocum, in Southern Parts of Brazil.

Yellow stunt, an emerging disease of tobacco (Nicotiana tabacum), has become increasingly prevalent in tobacco-growing regions in southern Brazil. Major symptoms are moderate to severe stunting, yellowing of leaves, severe wilting, darkened roots, necrosis of stem tissue directly above the soil line, and plant death. Phytophthora glovera was first proposed in 1999 as the primary causal agent of yellow stunt (1), but since then, there has been no data or completion of Koch's postulates to support this. Fifty-six isolates of fungi and fungus-like organisms were obtained from stem and root samples of tobacco plants with typical symptoms of yellow stunt in the Brazilian States of Rio Grande do Sul, Santa Catarina, and Paraná during the growing seasons of 2004/05 to 2006/07. They were identified to species level by analysis of the morphological characteristics (2) and sequence of rDNA internal transcribed spacer regions 1 and 2 (ITS1 and ITS2) (3). The isolates were identified as Pythium dissotocum (29), Fusarium oxysporum (10), P. graminicola (5), Rhizoctonia solani (4), F. solani (3), P. ultimum (3), P. deliense (1), and P. inflatum (1). The ITS sequences of the 29 isolates identified as P. dissotocum were identical. The nucleotide sequence of one isolate, LFM27/2005, has been deposited in GenBank (GQ495982). Analysis of ITS sequences alone was not sufficient to differentiate this isolate from other species in the Pythium subclade B2, such as P. coloratum, P. lutarium, P. marinum, or P. diclinum. However, the combination of morphological and cultural characteristics (2) and sequence data support our identification of LFM27/2005 and similar isolates as P. dissotocum. Colonies of LFM27/2005 on cornmeal agar had filamentous sporangia and formed slightly inflated, dendroid structures. Zoospores formed at 5°C. Daily growth rate on potato carrot agar was 13 mm at 25°C. The oogonia (22 μm in diameter) were nonornamented and either intercalary or terminal. Antheridia, commonly 1 to 2 per oogonium, were sessile, born on unbranched stalks, and either monoclinous or diclinous. Aplerotic or nearly plerotic oospores measured 20 μm in diameter with a smooth wall 2.5 μm thick. Pathogenicity tests for each pathogen were performed in a greenhouse at ~24°C in pots filled with pine bark substrate infested with inoculum at the time Burley tobacco plants showed five expanded leaves. Each test consisted of five plants and was repeated three times. Inoculum for one to three isolates representative of each pathogen was prepared by growing 2-month-old cultures at 28°C in the dark for 7 days on potato dextrose agar medium overlaid with three sterile oat kernels. Noninfested oat kernels were used for control plants. Forty days after inoculation, only plants inoculated with isolates of P. dissotocum exhibited all symptoms associated with yellow stunt. P. inflatum and R. solani did not induce yellow stunt symptoms and the others induced only wilting and root rot. P. dissotocum was recovered from an inoculated, symptomatic plant, fulfilling Koch's postulates. Its morphology was identical to isolates obtained from original field samples. The results demonstrate the association of isolates of P. dissotocum with tobacco yellow stunt in Brazil.