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.     

Reduction in common milkweed (Asclepias syriaca) occurrence in Iowa cropland from 1999 to 2009

The role of common milkweed in the lifecycle of the monarch butterfly has increased interest in the presence of this weed in the north central United States. An initial survey conducted in 1999 found that low densities of common milkweed occurred in approximately 50% of Iowa corn and soybean fields. In 2009, common milkweed was present in only 8% of surveyed fields, and the area within infested fields occupied by common milkweed was reduced by approximately 90% compared to 1999. The widespread adoption of glyphosate resistant corn and soybean cultivars and the reliance on post-emergence applications of glyphosate for weed control in crop fields likely has contributed to the decline in common milkweed in agricultural fields.     

Screening of three sweet potato (Ipomoea batatas L.) cultivars for resistance to different virulence groups of root-knot nematodes (Meloidogyne spp.) under controlled conditions

The susceptibility of three sweet potato cultivars (Ipomoea batatas L.) C4, TIS 3290 and TIS 9162 was evaluated against 156 isolates of Meloidogyne spp. with the aim to include resistant/tolerant sweet potato cultivars in a crop rotation scheme for the management of root-knot nematodes. The nematode isolates corresponded to races 1, 2 and 3 of Meloidogyne arenaria (n = 7), races 1, 2, 3 and 4 of M. incognita (n = 131) and Meloidogyne javanica (n = 18). Also, the isolates of M. incognita were differentiated in virulence groups: Pepper (n = 35), Pepper-Mi (n = 25), Tomato (n = 41) and Tomato-Mi (n = 30), depending on their ability to parasitize resistant pepper and tomato cultivars. The tested isolates of M. javanica parasitized C4 and TIS 3290, but not TIS 9162, whereas M. arenaria parasitized C4 and TIS 9162, but not TIS 3290, and M. incognita was able to parasitize the three sweet potato cultivars tested. C4 was the most susceptible cultivar to all nematode species tested, especially M. incognita, TIS 3290 was the most resistant and TIS 9162 was in between (7.2, 62.9 and 26.9% of resistant plants, respectively). Susceptibility of the sweet potato cultivars showed slight variations depending on the race or virulence group of M. incognita. The results suggest that sweet potato cultivars TIS 3290 and TIS 9162 may be used as rotation crops in fields where root-knot nematodes are present, their selection depending on the Meloidogyne isolates present. The use of resistant sweet potato cultivars would be preferably combined with other management practices to avoid virulence selection in nematode isolates.     

Characterization of Solanum chomatophilum resistance to 2 aphid potato pests, Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer)

The aphids Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer) (Hemiptera: Aphididae) are responsible for yield reduction in potato (Solanum tuberosum) production by direct phloem feeding and by spreading viruses. Breeding resistant traits from Solanum chomatophilum into the potato germplasm provides alternative means to control aphid infestations. Integrated pest management strategy, using plant resistance, benefits from the characterization of the resistance and of its impact on aphid biology. Our objective was to characterize the resistance of S. chomatophilum by assessing the effects of accessions, plant parts on aphid performance, and by assessing the impact of the resistance factors on different aphid developmental stages and on alate morph production. Detailed aphid performance was obtained by measuring fecundity, survival, percentage of nymphs that reached adult moult, and population growth using whole plant and clip cage experimental designs. Accession and plant physiological age, but not aphid developmental stage, influenced all life-history parameters, except for alate morph production which was not induced on the resistant accessions. Plant part influence was independent of plant species and accession. Both experimental designs resulted in congruent resistance levels at the accession level for each of the two aphid species, supporting the use of any of them in S. chomatophilum resistance screening. PI243340 was resistant to both aphid species, while PI365324 and PI310990 were also resistant to M. euphorbiae and M. persicae, respectively.     

Analysis of QTL for resistance to head smut (Sporisorium reiliana) in maize

Head smut of maize, caused by the fungus Sporisorium reiliana, is an important disease in the temperate maize-growing areas worldwide. In this study, we mapped and characterized quantitative trait loci (QTL) conferring resistance to S. reiliana using a F_2:3 population of 184 families derived from a cross between Mo17 (resistant parent) and Huangzao4 (susceptible). The population was evaluated for resistance in replicated field trials with artificial inoculation of S. reiliana chlamydospores in Gongzhuling of Jilin Province and Harbin of Heilongjiang Province of China, two hot spots of head smut incidence, in 2003 and 2004. Genotypic and G × E variances for disease incidence were highly significant in the population. Heritability estimates for percentage disease incidence in the 2-location and 2-year evaluation ranged from 0.62 to 0.70. Composite interval mapping on a linkage map (1956.1 cM distance; 9.34 cM average interval) constructed with 84 SSR and 135 AFLP markers, identified five QTL, one each on chromosomes 1, 3 and 8 and two on chromosome 2, accounting for 5.0–43.7% of the phenotypic variance across four environments. One major QTL on chromosome 2 explaining up to 43.7% of the phenotypic variance can potentially be used in molecular marker-assisted selection for head smut resistance in maize.     

Weed control and sensitivity of oats (Avena sativa) with various doses of saflufenacil

Saflufenacil is a new herbicide being developed by BASF for broadleaved weed control in maize, soybean and other crops prior to crop emergence. Six field studies were conducted in Ontario, Canada over a three year period (2008-2010) to evaluate the potential of saflufenacil applied pre-emergence (PRE) at various doses for broadleaved weed control in oats. Saflufenacil applied PRE caused minimal visible injury at 1, 2 and 4 weeks after emergence (WAE) in oats. At 4 WAE, the dose of saflufenacil required to provide 95% control of Ambrosia artemisiifolia (common ragweed), Chenopodium album (common lambsquarters), Polygonum convolvulus (wild buckwheat), Polygonum scabrum (green smartweed) and Sinapsis arvensis (wild mustard) was 72 to >100, >100, 74, 58 and >100 g ai ha⁻¹, respectively. Generally, similar saflufenacil dose-response trends were seen at 8 WAE. The doses of saflufenacil required to provide 95% reduction in density and dry weight ranged from 95 to >100 and 42 to >100 g ai ha⁻¹ respectively for A. artemisiifolia, C. album, P. convolvulus, P. scabrum and S. arvensis. Oat yield showed no sensitivity to saflufenacil at the doses evaluated. Based on this study, saflufenacil applied PRE can be safely used in spring planted oats for the control of some troublesome annual broadleaved weeds.     

Improvement of bacterial blight resistance of hybrid rice in China using the Xa23 gene derived from wild rice (Oryza rufipogon)

A novel bacterial blight (BB) resistance gene, Xa23, identified from Oryza rufipogon was introgressed into three popular restorer lines (Minghui63, YR293 and Y1671) for wild abortive cytoplasmic male sterility by marker-assisted backcross breeding approach in combination with artificial inoculation and stringent phenotypic selections. The three derived BB resistant restorer lines (Minghui63-Xa23, YR293-Xa23 and Y1671-Xa23) and their hybrid combinations with Zhenshan97A (Shanyou63-Xa23), NongfengA (Fengyou293-Xa23) and Zhong9A (Zhongyou1671-Xa23) demonstrated similar BB resistance spectrum as the donor parent, CBB23 (B). The newly developed BB resistant restorers and their derived hybrids were identical to their respective original versions for agronomic traits especially under disease free condition. However, under severe disease condition, the three BB resistant restorer lines exhibited significantly higher grain weight and spikelet fertility as compared to the respective original restorer lines thus further resulting in BB resistant hybrids with significantly higher grain yields than their respective popular original hybrids. The results indicated that the Xa23 gene could completely express its dominant broad spectrum resistance in different backgrounds of both restorer and male sterile lines across different growth stages, suggesting its immense breeding value in BB resistance improvement for hybrid rice. Moreover, a reasonable utilization and deployment of Xa23 gene for efficient control of BB disease in hybrid rice production was recommended.     

Monitoring resistance of field populations of diamondback moth Plutella xylostella L. (Lepidoptera: Yponomeutidae) to five insecticides in South China: A ten-year case study

Diamondback moth Plutella xylostella (L.) is a cosmopolitan pest of crucifers and it is particularly notorious for its resistance to commonly used insecticides. To provide a basis for future resistance management strategies, this study evaluated the resistance status of this pest to five insecticides, namely abamectin, β-cypermethrin, fipronil, monosultap and phoxim, in South China from 1999 to 2009 with 4-6 populations tested each year. Laboratory bioassays using a standard leaf-dip method were conducted on 3rd instar larvae which were the progeny of field collected insects. LC₅₀ values were estimated by probit analysis and resistance factors were calculated by comparing the field populations with a fully insecticide-susceptible lab population. The results showed that the diamondback moth populations generally had low resistance to abamectin from 1999 to 2005, but that resistance increased significantly during 2007-2009 with resistance factors as high as 122.4. Resistance factors for β-cypermethrin in 35 populations tested were high to very high in all 10 years with values ranging from 32.2 to 683.6. Resistance to fipronil had a progressive increase and increased markedly from 2007 to 2009 with one resistance factor of 56.1. Resistance to monosultap was low in 2004, but increased progressively from 2007 to 2009 with one resistance factor of 129.1. Resistance to phoxim was low to moderate from 2004 to 2008, but was moderate and high in 2009 with resistance factors ranging from 58.7 to 129.1. These results indicate that resistance of diamondback moth populations to most commonly used insecticides in South China has increased and is now generally high.     

Cross-resistance, inheritance and stability of resistance to acetamiprid in cotton whitefly, Bemisia tabaci Genn (Hemiptera: Aleyrodidae)

The cotton whitefly Bemisia tabaci, (Genn.) is an important pest of field crops, vegetables and ornamentals worldwide. Neonicotinoids are considered an important group of insecticides being used against B. tabaci for several years. B. tabaci has developed resistance to some of the compounds of the group. This study was designed to investigate if the selection of B. tabaci with acetamiprid would give a broad-spectrum of cross-resistance and to genetically classify the resistance. At G₁ a low level of resistance to acetamiprid, imidacloprid, thiamethoxam, thiacloprid and nitenpyram was observed with resistance ratios of 3-fold, 8-, 9-, 6- and 5-fold, respectively, compared with a laboratory susceptible population. After selection for eight generations with acetamiprid, resistance to acetamiprid increased to 118-fold compared with the laboratory susceptible population. Selection also increased resistance to imidacloprid, thiamethoxam, thiacloprid, nitenpyram, endosulfan and bifenthrin but no change in susceptibility to fipronil was observed. Furthermore resistance in a field population was stable in the absence of acetamiprid selection pressure. Genetic crosses between resistant and susceptible populations indicated autosomal and incompletely recessive resistance. Further genetic analysis suggested that resistance could be controlled by a single factor. The high level of cross-resistance and stability of incomplete resistance in the field population is of some concern. However, lack of cross-resistance between acetamiprid and fipronil or unstable resistance in the resistant population could provide options to use alternative products which could reduce acetamiprid selection pressure.     

Monitoring for MACE resistance in the tobacco-adapted form of the green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae) in the eastern United States

Organophosphate and carbamate insecticides exert their neurotoxic effects by inhibiting acetylcholinesterase (AChE), thereby, prolonging the action of acetylcholine at cholinergic synapses, resulting in neuronal hyperexcitation. Mutations at the AChE target site confer modified acetylcholinesterase (MACE) phenotypes. Target-site insensitivity of AChE was characterized in field-collected, tobacco-adapted forms of the green peach aphid, Myzus persicae (Sulzer), from nine different states in the eastern United States from 2004 to 2007. The specific activity of the AChE among the 65 aphid colonies screened by Ellman's assay ranged from 0.017–0.259 U/min/mg protein. Eight colonies, with a wide range of specific activities were chosen to study the inhibition of AChE in the presence of two carbamate insecticides, methomyl and pirimicarb. IC₅₀ values for methomyl ranged from 0.35 to 2.4 μM, while six out of eight colonies had lower values that ranged from 0.16 to 0.30 μM for pirimicarb. Two colonies that were inhibited by methomyl had very high IC₅₀ values for pirimicarb, 40.4 and 98.6 μM respectively. The target-site insensitivity in these two colonies that are resistant to pirimicarb could be due to an ace2 gene mutation. This is the first instance where MACE phenotypes in M. persicae from the United States were studied and confirmed. The results indicate that the possible insensitivity due to MACE resistance in some colonies may render selected carbamate insecticides ineffective. Concerns of MACE resistance in managing the tobacco-adapted form of the green peach aphid on tobacco in the United States are discussed.     

Saccharin-induced systemic acquired resistance against rust (Phakopsora pachyrhizi) infection in soybean: Effects on growth and development

We examined the effect of saccharin on the systemic acquired resistance (SAR) response of soybean to the fungus Phakopsora pachyrhizi, the causal agent of soybean rust. Plants were grown hydroponically in half-strength Hoagland’s solution and were challenged with the pathogen 1, 5, 10 and 15 d after treatment with 3 mM saccharin applied either as a foliar spray or a root drench at the 2nd trifoliate (V3) and early reproductive (R1) stages. Plants were destructively harvested and assessed for visible rust symptoms 2 wk after inoculation. Mode of saccharin application was a significant factor influencing the severity of rust infection. Saccharin applied as a root drench was more effective than the foliar spray treatment at inducing SAR, with increased resistance observed 1 d after application. Systemic protection against rust infection was still apparent 15 d after application of saccharin as a root drench. In contrast, foliar treatment with saccharin did not increase systemic protection until 15 d after treatment. When systemic protection was induced by the application of saccharin in either manner, there was no significant reduction of plant growth, except when plants were inoculated 15 d after the saccharin application as a root drench at the R1 stage of development.     

Use of two PGPR strains in the integrated management of blast disease in rice (Oryza sativa) in Southern Spain

Biocontrol capacity of two plant growth-promoting rhizobacteria (PGPR) strains, against blast disease in rice paddy fields in Southern Spain was studied in three cropping seasons. Both strains (Pseudomonas fluorescens Aur 6 and Chryseobacterium balustinum Aur 9) had already shown biocontrol capacity against pathogens, ability to induce systemic resistance against leaf pathogens and against salt stress in different plant species. Bacterial treatments were carried out on seeds and/or on leaves. Strains were inoculated individually and in combination. Protection against natural disease incidence was evaluated, and rice production and quality measured in 2005 and 2006 trials. In 2004, natural disease incidence was low (between 0.1% and 0.35% of damaged leaf surface) due to environmental conditions; under these conditions, both strains significantly protected plants against rice blast. In 2005, disease incidence was higher than in 2004, reaching higher values of affected leaf surface in controls. In these conditions, each strain individually protected rice against rice blast, although the combination of both strains was the most effective treatment. All three treatments (Aur 6, Aur 9 and Aur 6 + Aur 9) reached 50% protection in panicles, with Aur 9 being the most effective. In 2006, the most effective treatment was the combination of both strains on leaves in three physiological stages, suggesting a biocontrol mediated protection. On the other hand, when bacteria were applied to seeds, disease incidence decreased up to 50%, suggesting induction of systemic resistance. Finally, a direct relation between protection mediated by the PGPR and the increase in rice productivity (mT/ha) and quality (weight of 1000 seeds and number of intact grains after milling) was found.