Leaf Disc Bioassay Research Articles

Generating Susceptible Strain and Resistance Status of Field Populations of Spodoptera exigua (Lepidoptera: Noctuidae) Against Some Conventional and New Chemistry Insecticides in Pakistan

Two field populations of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) from Dera Ghazi Khan (D. G. Khan) and Multan, Pakistan, were tested for resistance to the 10 most commonly used insecticides in Pakistan by using a standard leaf disc bioassay on the F1 progeny. For comparison, a susceptible strain was generated from the Multan strain, which displayed lower LC50 values for most of the insecticides, by either mass rearing without exposure to insecticides or single-pair crosses against selected insecticides. The single-pair crosses generated a more susceptible strain than mass rearing. The D. G. Khan field strain was highly resistant to cypermethrin, profenofos, spinosad, abamectin, and chlorpyrifos and moderately resistant to deltamethrin, indoxacarb, and methoxyfenozide compared with susceptible lab strain. The Multan strain was highly resistant to profenofos and indoxacarb. Both field populations were susceptible to emamectin benzoate and lufenuron. Rotating these two insecticides with others that show very low, low, or moderate levels of resistance and have different modes of action may be useful for the effective management of this pest.

Antifeedant effect of commercial chemicals and plant extracts againstSchistocerca americana(Orthoptera: Acrididae) andDiaprepes abbreviatus(Coleoptera: Curculionidae)

The deterrent effect of seven commercial compounds against Schistocerca americana (Drury) nymphs and Diaprepes abbreviatus L. adults, two important citrus pests, was investigated. Extracts of three Florida local plants were also tested as deterrents. Antifeedant activity was assayed using a leaf-disk bioassay in choice and no-choice tests. The residual activity of the commercial compounds that significantly deterred the insects was assayed by applying them to foliage of citrus plants exposed to three time intervals of sunlight. Sabadilla, azadirachtin and ryanodine effectively deterred S. americana, whereas rotenone, sabadilla and ryanodine reduced the feeding activity of D. abbreviatus. Methanol and methylene chloride extracts of Ceratiola ericoides Michaux deterred D. abbreviatus, but only methylene chloride extract dissuaded S. americana. Methanol extract of Ardisia crenata Sims functioned as a feeding deterrent against both S. americana and D. abbreviatus, but only the latter species was deterred by methylene chloride extract of A. crenata. Extracts of Illicium parviflorum Michaux ex Ventetat only dissuaded the insects in choice bioassays. Rapid loss of effectiveness was observed under field conditions. Sabadilla was the only compound that maintained its antifeedant properties in the field, but only against S. americana. Based on their deterrency, some of the commercial products and plant extracts have potential for use as substitute crop protectants against the pest species tested.

Expression of the zga agglutinin gene in tobacco can enhance its anti-pest ability for peach-potato aphid (Myzus persica)

The effect of introduction of the Zephyranthes grandiflora agglutinin gene (zga) to tobacco on its anti-pest ability for peach-potato aphids was investigated. PCR analysis confirmed that the zga gene was integrated into the plant genome. The results from semi-quantitative RT-PCR and real-time PCR assays revealed that the zga gene was expressed at various levels in the transgenic plants. A bioassay with aphids indicated that transgenic plants conferred enhanced resistance to aphids. Compared with the controls, the average number of aphids fed with transgenic plants during a 20-day assay evidently decreased by 70.4% in leaf disc bioassay and 77.9% in whole plant bioassay. The average number of nymph was significantly reduced by 36.4% on zga-expressing plants in leaf disc bioassay and 35.6% in whole plant bioassay. The report indicated that the introduction of zga gene to tobacco plants is a useful method to improve its anti-pest ability for aphids.

형질전환 고구마에 대한 Glufosinate-ammonium 저항성 간이진단법

Transgenic herbicide-resistant sweet potato plants (Ipomoea batatas (L.) Lam.) produced through a biolistic transformation were used in this study. The objective of this research was to find out a rapid and reliable assay method for confirming glufosinate-ammonium resistance. The techniques tested include whole-plant bioassay, one leaf bioassay, and leaf disk bioassay. Parameters investigated in this study were leaf injury and ammonium accumulation at 1 and 5 days after treatment of glufosinate-ammonium. In the leaf disk bioassay, leaf injury of the transgenic line 7171 was 1.9-fold less affected by glufosinate-ammonium than th e wild type. The leaf injury of 7171 in one leaf and whole-plant bioassays was 59- and 92-fold less affected by glufosinate-ammonium, respectively, compared with that of the wild type. Leaf disk, one leaf, and whole-plant bioassays showed that ammonium accumulation of the 7171 was 2 to 20-, 4 to 43-, and 6 to 115-fold less affected by 0.5-5 mM glufosinate-ammonium than that of the wild type. All three bioassays successfully distinguished the resistance from the transgenic lines, but one leaf bioassay is the simplest and quickest. Leaf injury and ammonium accumulation we re the same in leaves 1, 3, 5, 7, and 10 of 3 mM glufosinate-ammonium treated plants or nontreated plants. The one leaf bioassay was chosen as the standard procedure for future confirmation of resistance in transgenic sweet potato because it is a rapid and reliable assay.

Evaluation of Downy Mildew Resistance in Grapevine by Leaf Disc Bioassay with In Vitro- and Greenhouse-Grown Plants

Genetic improvement of grapevine for resistance against downy mildew ( Plasmopara viticola ) by biotechnological techniques requires reliable procedures to screen large populations of plants. In comparison with greenhouse-grown plants, in vitro plantlets are not often used in screening procedures, although they present some advantages such as compact size, the availability of a high number of replicates per genotype, and the potential to screen improved genotypes directly from in vitro plants. Leaf disc inoculation bioassay was used to evaluate grapevine resistance to downy mildew on Vitis vinifera Chardonnay, hybrid Seyval, and Vitis riparia Gloire de Montpellier (susceptible, moderately susceptible, and highly resistant, respectively) with both in vitro plantlets and greenhouse-grown plants. Disease symptoms and resistance reaction were evaluated for sporulation and necrosis in two independent experiments. For all parameters, leaves from in vitro plantlets appeared more resistant than leaves from greenhouse-grown plants, in particular for the intermediate and the susceptible genotypes. Necroses were not observed on leaf discs of the susceptible genotype, whereas necrotic spots appeared on leaf discs of both intermediate and resistant genotypes regardless of how the plants were grown. Based on sporulation and necrosis symptoms, ranking of genotypes differed according to growing conditions. Although the method based on in vitro grown plants is less reliable than that based on greenhouse plants, it can be used as a preliminary assay to eliminate the most susceptible plantlets obtained by biotechnology.

Efficacy of fungicides with various modes of action in controlling the early stages of an Erysiphe necator-induced epidemic

Limiting the use of fungicides is due to become an important issue in managing Erysiphe necator (Schwein) Burrill infections in vineyards. The authors determined how three fungicides currently used by vine growers could be managed to control the early stages of an E. necator-induced epidemic. Leaf-disc bioassays and field experiments suggested that the protectant quinoxyfen induced minor disruption in E. necator development, but compounds with protectant and curative properties (tebuconazole and trifloxystrobin) caused significant, although different, disruption during E. necator-induced epidemics. Bioassays showed that each of the antifungals were most effective at different stages of fungal development, tebuconazole before sporulation and trifloxystrobin after sporulation of the colonies. Results from the bioassay also highlighted likely occurrences in the field, where several stages of fungal development are encountered simultaneously. The present findings were complementary: leaf-disc tests showed when the fungicides were most effective at inhibiting E. necator infection cycles; the field trial provided results in terms of incidence and severity of disease on bunches without reference to the pathogenic cycle development. A protection strategy combining the different types of fungicide under study is suggested.

Application of plant growth regulators mitigates chlorotic foliar injury by the black pecan aphid (Hemiptera: Aphididae)

Black pecan aphid, Melanocallis caryaefoliae (Davis) (Hemiptera: Aphididae), feeding elicits localized chlorotic injury to pecan foliage [Carya illinoinensis (Wangenh.) K Koch] and apparent acceleration of leaf senescence and defoliation. The ability of certain plant growth regulators (PGRs) (forchlorfenuron, gibberellic acid and aviglycine) to prevent M. caryaefoliae from triggering pecan leaf chlorosis and senescence-like processes was evaluated on two dates in both 2006 and 2007. Treatments were applied to orchard foliage and used in laboratory leaf-disc bioassays to assess possible reduction in aphid-elicited chlorosis and concomitant effects on aphid mortality and development. Foliage pretreated with forchlorfenuron + gibberellic acid prior to being challenged with aphids resulted in significantly less aphid-elicited chlorosis than did control or aviglycine-treated leaf discs. No PGR affected aphid mortality; however, development time was increased by forchlorfenuron + gibberellic acid in 2006 and by aviglycine + gibberellic acid on one date in 2007. Certain PGRs possess the potential for usage on pecan to protect foliar canopies from M. caryaefoliae via changes in the susceptibility of the host leaf to senescence-like factors being introduced by feeding aphids. This protective effect on host foliage and the associated suppressive effect on development of feeding aphids might also be relevant to pest management programs on other aphid-crop systems in which aphid-elicited chlorosis and senescence-like processes can limit profitability.

VARIATION FOR FUNGICIDE RESISTANCE AMONG CUCURBIT POWDERY MILDEW POPULATIONS IN THE CZECH REPUBLIC

A total of 88 cucurbit powdery mildew (CPM) isolates, 52 of Golovinomyces cichoracearum (Gc) and 36 of Podosphaera xanthii (Px), collected in the period 2005-2007 in the Czech Republic (CR), were screened for tolerance and/or resistance to the three freauentlv used fungicides (fenarimol, dinocap, thiophanate-methyl). Benomvl served as ineffective control. Bv employing benomyl, which used to be the most frequently used fungicide for CPM protection in the CR, we could monitor and compare the development and changes in variation of tolerance/resistance among Czech CPM populations. A total of 36 (21 Gc, 14 Px) CPM isolates from 2007 were also tested for azoxystrobin tolerance and/or resistance. Fungicide sensitivity was determined by a modified leaf-disc bioassay with five concentrations. Significant differences among fungicides and even between the years 2005 and 2006-2007 were observed. Occurrence of resistant and/or tolerant isolates of both powdery mildew species in different locations was observed. Fenarimol (Rubigan 12 EC) and dinocap (Karathane LC) expressed decreasing efficacy in the year 2005, however in 2006-2007 these two fungicides showed a high level of effectiveness and all isolates of both CPMs were controlled by the recommended concentration (36 μg a.i./ml for fenarimol, 105 μg a.i./ml for dinocap). Benomyl (Fundazol 50 WP) and thiophanate-methyl (Topsin M 70 WP) were found to be ineffective, because the majority of screened isolates (96%) belonged to the highly resistant strains, with a resistant reaction at the recommended concentration (250 μg a.i./ml for benomyl, 525 μg a.i./ml for thiophanate-methyl) and tolerance or resistance at a higher concentration than recommended (500 μg a.i./mi for benomyl and 1050 μg a.i./ml for thiophanate-methyl). Some 89% of CPM isolates (except the year 2005 only 22% Px isolates tested on thiophanate-methyl) showed profuse or limited sporulation at a concentration 2× higher than recommended (1000 μg a.i./ml for benomyl and 2100 μg a.i./ml for thiophanate-methyl). Azoxystrobin (Ortiva) expressed decreasing efficacy, with 37% of isolates (Gc and Px) tolerant or resistant to the recommended concentration (500 μg a.i./ml) and also to higher concentrations (1000 μg a.i./ml and 2000 μg a.i./ml).

Lethal and Sublethal Effects of an Insect Growth Regulator, Pyriproxyfen, on Obliquebanded Leafroller (Lepidoptera: Tortricidae)

The obliquebanded leafroller, Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae), is one of the most destructive pests of tree fruit in Washington. The development of insecticide resistance in C. rosaceana has led us to explore new management tactics. The use of very low doses of insecticides that have strong sublethal effects represents an environmentally friendly option to improve existing integrated pest management (IPM) strategies. We tested the insect growth regulator pyriproxyfen to determine its lethal and sublethal effects on growth and development of C. rosaceana. A leaf-disk bioassay was used to test seven concentrations of pyriproxyfen ranging from 0 to 30 ppm on fifth-instar C. rosaceana. Male and female larvae were assessed separately for mortality as well as other parameters of growth and development. The LC, values for males and females were 2.4 and 4.8 ppm, respectively. The response to pyriproxyfen was concentration-dependent: only 5-6% of the larvae treated with the highest concentration emerged as morphologically normal adults compared with 86% emergence in the controls. The pupation and adult emergence was significantly delayed at concentrations higher than 1 ppm. The weights of C. rosaceana pupae and adults were significantly increased, whereas fecundity and fertility were significantly reduced at a sublethal concentration of 0.3 ppm. We conclude that both lethal and sublethal effects might exhibit significant impacts on the population dynamics of C. rosaceana in tree fruit orchards treated with low concentrations of pyriproxyfen.

Assessment of oviposition site quality by aphidophagous hoverflies: reaction to conspecific larvae

Aphidophagous predators adapt their foraging behaviour to the presence of conspecific and heterospecific larvae. We studied the effect of the presence of conspecific larvae and their tracks on the oviposition site selection of an aphid-specific predator, Episyrphus balteatus DeGeer (Diptera: Syrphidae), in two-choice experiments using a leaf disc bioassay. Gas chromatography – mass spectrometry analysis was used to identify the volatile chemicals released from odour extracts of E. balteatus larval tracks. The behavioural effects of these volatile substances on hoverfly females were also evaluated. Our experiments demonstrated that E. balteatus females were deterred from ovipositing when presented with a Vicia faba leaf with aphids and conspecific larvae. The oviposition-deterring stimulus was also active when females were presented with a leaf that contained conspecific larval tracks. A mixture of chemical compounds was found in the volatile pattern of odour extracts of larval tracks. The main volatile chemicals were 3-methylbutanoic acid, 2-methylbutanoic acid, 2-methylpropanoic acid, 3-hydroxy-2-butanone, hexanoic acid and phenol. Females also laid significantly fewer eggs in response to odorant volatiles emitted from larval extracts. These results highlight that predatory hoverfly females avoid ovipositing in aphid colonies in which conspecific larvae or their tracks are already present, suggesting that this behaviour constitutes a strategy that enables females to optimize their oviposition site and reduce competition suffered by their offspring.

TOXIC AND ANTIFEEDANT ACTIVITIES OF STERCULIA FOETIDA (L.) SEED CRUDE EXTRACT AGAINST SPODOPTERA LITURA (F.) AND ACHAEA JANATA (L.)

ABSTRACT Toxic and antifeedant effects of the Wild Indian almond, Sterculia foetida L. seed crude extract on the two important lepidopteran pests- castor semilooper, Achaea janata L. (Noctuidae: Lepidoptera) and the Asian armyworm, Spodoptera litura (Fab.) (Noctuidae: Lepidoptera) on castor (Ricinus communis L.) plants were studied by leaf disc bioassay method. The acetone extract applied to the leaf disc showed toxicity to the S. litura and feeding deterrency to the A. janata. Insecticidal and feeding deterrent activities of the crude extract acted in a dose dependent manner against the test insects. At a higher dose of 2.5 mg/cm2, 3rd instar larvae of S. litura died within 24h of treatment. The treated larvae were dehydrated and shrunken before death. Lower doses (0.5mg/cm2) also exhibited toxic effects to S. litura but with time delayed process. However, the same compound has produced 100 % feeding deterrent activity to A. janata at 4.0 mg/cm2. At lower doses employed, i.e. 2.0 mg/cm2 a moderate percentage (20 %) of feeding deterrent action was found after 24h treatment. It is interesting to find that the seed extract of S. foetida acted as insecticide to S. litura and as an antifeedant to the semilooper, thus indicating the dual mode of action of the compound against the different pest larvae treated. However, a study on the mechanism of action is presumed to reveal interesting observations on the mode of action of this botanical pesticide.

Insecticide resistance traits differ among and within host races in Aphis gossypii

The polyphagous cotton-melon aphid Aphis gossypii Glover is structured into geographically widespread host races comprising a few clones specialised on Cucurbitaceae, cotton, eggplant or pepper. To assess insecticide resistance among and within host races, leaf disc bioassays were conducted on aphid clones collected from Cucurbitaceae (genotypes C4 and C9), cotton (genotypes Burk and Ivo), eggplant (genotype Auber) and pepper (genotype PsP4). Molecular diagnostic (PCR-RFLP) and enzyme assays were also performed to detect the basic mechanisms underlying insecticide resistance. All six clones were susceptible to acetamiprid (neonicotinoid) or carbosulfan (carbamate). Conversely, all clones were resistant to dimethoate (organophosphate) (RF = 4.1-38.1) and carried mutation S431F in the acetylcholinesterase gene. Auber, PsP4 and Burk also carried mutation A302S in this gene, which possibly conferred moderate resistance (RF = 3.7-6.8) to profenofos and monocrotophos (organophosphates). Auber and Burk were highly resistant (RF = 41.2 and 473 respectively) to cypermethrin (pyrethroid). This resistance was likely associated with point mutation super-kdr (M918L) in the voltage-gated sodium channel gene (para gene) or metabolic detoxification mediated by esterase and oxidase enzymes. Multiple resistance to a broad range of insecticides and multiple mechanisms of resistance in some clones could explain to some extent the low genetic diversity observed within A. gossypii host races.

Assessment of resistance components of bigleaf hydrangeas (Hydrangea macrophylla) to Erysiphe polygoni in vitro

Powdery mildew caused by Erysiphe polygoni is one of the major foliar diseases on bigleaf hydrangeas (Hydrangea macrophylla). To determine resistance components of bigleaf hydrangea to powdery mildew, fungal development and host necrotic cells were compared on six bigleaf hydrangea cultivars using a detached leaf disk bioassay. Germination percentages of conidia were not significantly different among cultivars. However, percent geminated conidia with secondary germ tubes (GCSGT), percent necrotic host cells, infection efficiency, latent period, and sporulation were significantly different among cultivars. In general, ‘Veitchii’ was resistant, ‘Nikko Blue’ was susceptible, and ‘Madame Emile Mouillere’, ‘Forever Pink’, ‘Lilacina’, and ‘Holstein’ were intermediate. Necrotic cells were macroscopically visible in all cultivars regardless of assigned resistance levels. Significantly more necrotic cells were detected in resistant cultivar ‘Veitchii’ compared with other cultivars. These results suggest that hypersensitive reaction is not a qualitative trait of resistance, but the frequency of necrotic cells could be one of several resistance components that contribute to restrain fungal growth and colony development. Additionally, percent GCSGT, infection efficiency, latent period, and sporulation could be used to evaluate partial resistance in bigleaf hydrangea to powdery mildew using a detached leaf disk assay.

Effect of botanical extracts on the biological activity of granulosis virus against Pieris brassicae

A granulosis virus strain infecting Pieris brassicae (PbGV) was isolated from the dry temper- ate region of northwestern Himalayas as a potential microbial agent for its management. The effect of different botanicals (having insecticidal action against P. brassicae) on the bioefficacy of PbGV was evaluated under laboratory conditions using leaf disc bioassays on cabbage for improving the insecticidal performance of the PbGV. The synergistic action of different botanical extracts was evident in terms of reduction in LC50 values against different botanical extracts. Among different extracts, petroleum-ether extract of neem seed kernel (NSK) when combined with PbGV resulted in maximum reduction of LC50 value (4.39×10 2 occlusion bodies (OBs) ml �1 ) fol- lowed by methanolic extract (7.38×10 2 OBs ml �1 ) and aqueous extract (9.36×10 3 OBs ml �1 ) as com- pared with PbGV alone (1.85×10 4 OBs ml �1 ) for 2nd instar larvae of the test insect. These trends were found analogous in cases of 3rd and 4th instars of P. brassicae with different solvent extracts of NSK. The other botanicals evaluated, viz., Eupatorium and Artemesia, also resulted in reduction of LC50 values for 2nd, 3rd and 4th instars as compared with PbGV alone when different extracts were combined with virus for bioassays. The studies suggest that the PbGV in combination with botanical pesticides could be more useful as a bio-pesticide against cabbage butterfly (P. brassicae) in IPM programs.

Neonate Silkworm (Bombyx mori) Larvae Are Attracted to Mulberry (Morus alba) Leaves with Conspecific Feeding Damage

The silkworm Bombyx mori is a molecular genetic model for the Lepidoptera. Its odorant receptor genes have been described, and preliminary studies suggest that several are expressed specifically in the larval caterpillar stage. This study was undertaken to identify olfactory behaviors specific to the larvae. A two-choice leaf disk bioassay with naive neonate larvae was used to evaluate the attractiveness of three types of mulberry leaf (Morus alba): newly flushed leaves from branch tips, mature leaves, and mature leaves with feeding damage caused by conspecific larvae. Mature leaves with feeding damage were the most attractive, newly flushed leaves were moderately favored, and undamaged mature leaves were the least attractive. Volatile odors collected from whole mulberry leaves by using solid-phase microextraction fibers were analyzed by gas chromatography-mass spectrometry. The volatile profile of newly flushed leaves and mature leaves damaged by conspecific larvae was more complex compared to undamaged mature leaves. By comparing the volatile makeup of each leaf type, a list of 22 candidate odors responsible for attracting the neonate larvae was generated; alpha-farnesene was particularly notable as a herbivore-induced volatile. These odors will be used in future in vitro studies to determine whether they activate larval-specific odorant receptors.

Dissemination of the entomopathogenic fungi, Lecanicillium longisporum and L. muscarium, by the predatory bug, Orius laevigatus, to provide concurrent control of Myzus persicae, Frankliniella occidentalis and Bemisia tabaci

The simultaneous use of two biocontrol agents for the concurrent control of three pest species was investigated. Leaf disc bioassays were conducted to establish a suitable method (surface dosing) for the dissemination of an entomopathogenic fungus ( Lecanicillium longisporum or L. muscarium) by the predatory bug Orius laevigatus. Predatory bugs surface dosed with fungal conidia successfully disseminated conidia onto sweet pepper leaf discs. Most (98%) of the peach-potato aphids ( Myzus persicae) that were subsequently maintained on the leaf discs became infected with the pathogen and died within 5 days. However, fungal conidia disseminated by surface dosed predatory bugs did not infect and kill the western flower thrips ( Frankliniella occidentalis) or the sweetpotato whitefly ( Bemisia tabaci). Plant trials were performed to assess the efficacy of using predatory bugs surface dosed with L. longisporum as an effective means of controlling M. persicae and F. occidentalis populations. The results indicated that the number of aphids and thrips were significantly lower (66% and 95%, respectively) on the plants where surface dosed predatory bugs were used as a control measure compared with plants where the fungal pathogen alone was used and were statistically comparable to the numbers on plants where the predatory bug alone was used. The potential for using this dual approach is discussed in the context of improved biological control of glasshouse pests.

BIOLOGICAL POTENCY OF CERTAIN PLANT EXTRACTS IN MANAGEMENT OF TWO LEPIDOPTERAN PESTS OF RICINUS COMMUNIS L.

ABSTRACT Antifeedant and toxic effects of leaf extracts of Syzygium cumini L., Ocimum basilicum L., Luffa aegyptica Mill., Eichhornia crassipes Mart., Tamarindus indica L., Terminalia catappa L., Limonia acidissima L., Murraya koenigii L., Breynia retusa (Dennst.)., Jatropha curcas L., Tectona grandis L., Momordica charantia L., Mangifera indica L., Ricinus communis L., and the seed extract of Madhuca indica Gmelin., were evaluated in the laboratory and the results were reported. The crude acetone extracts were tested against the third instar larvae of two lepidopteran pests i.e. the tobacco cut worm, Spodoptera litura Fab. and the castor semilooper, Achaea janata L. (Noctuidae: Lepidoptera). Antifeedant activity of the extracts was assessed through leaf disc choice tests. The leaf extracts of M. charantia, T. grandis, M. indica and T. indica exhibited strong antifeedant activity (> 85%) in leaf disc bioassays at a dosage of 100-mg/ 21cm2. Test extracts of T. grandis, M. indica and M. charantia produced higher toxicity (> 80%) to S. litura and A. janata at 20-100 mg/21 cm²/ larva doses were applied, and LD50 values of (47.85- 72.60 mg/21 cm²). The acetone extracts of M. charantia, T. grandis and M. indica were found to exhibit both feeding deterrent and toxic activities. The other plants which were tested had moderate effects towards these pests. Based on their feeding deterrent and toxic properties, some of these plant extracts have potential for use as alternative crop protectants against certain pest species.

Resistance toMelampsora larici-epitea leaf rust inSalix: analyses of quantitative trait loci

Quantitative resistance of Salix to Melampsora larici-epitea leaf rust was studied in 2 Salix mapping populations. One population was a backcross between a S. schwerinii x S. viminalis hybrid and S. viminalis, and the other was an F2 population between S. viminalis and S. dasyclados. A leaf disc bioassay was used to study the components of quantitative resistance (latent period, uredinia number, and uredinia size) to 3 isolates of the leaf rust. The analysis of quantitative trait loci (QTLs) revealed 9 genomic regions in the backcross population and 7 genomic regions in the F2 population that were important for rust resistance, with QTLs explaining 8-26% of the phenotypic variation. An important genomic region was identified for the backcross population in linkage group 2, where QTLs were identified for all resistance components for 2 of the rust isolates. Four of the QTLs had overlapping mapping intervals, demonstrating a common genetic background for latent period, uredinia diameter, and uredinia number. QTLs specific to some rust isolates and to some resistance components were also found, indicating a combination of common and specific mechanisms involved in the various resistance components. Breeding implications in relation to these findings are discussed.

Monitoring of acaricide resistance of Tetranychus urticae (Acari: Tetranychidae) from Korean apple orchards

Tetranychus urticae populations were collected from ten commercial apple orchards and their susceptibilities to 12 acaricides were tested using a leaf disc bioassay. The resistance of each T. urticae population was reported as the LC 50 value, the resistance ratio (RR) and the slope of the probit–concentration regression. Cross resistances of T. urticae populations were estimated using the Spearman's rank correlation coefficient. Most local populations showed low resistance levels (RR ≤ 10). Development of resistance to METI and pyrethroid acaricides differed among local populations. The highest RR value (154.6) was found in the Uiseong population to tebufenpyrad. The Geochang population was highly resistant, especially to METI and pyrethroid acaricides. T. urticae populations collected from Suwon, Chungju, Yeongju and Geochang showed moderate resistance (10 < RR ≤ 40) to more than two acaricides. Resistance ratios to abamectin, chlorfenapyr, fenbutatin-oxide and milbemectin were low (RR ≤ 10) in all populations. The LC 50 values of abamectin, chlorfenapyr, fenbutatin-oxide and milbemectin ranged from 0.06 to 0.2 mg/l, from 0.67 to 3.38 mg/l, from 10.12 to 40.85 mg/l and from 0.47 to 3.01 mg/l, respectively. We discuss possible cross-resistance to acaricides using Spearman's rank correlation coefficient.

PCR-based monitoring of recent isolates of tobacco blue mold from Europe reveals the presence of two genetically distinct phenotypes differing in fungicide sensitivity

Bioassays testing the fungicide sensitivity against metalaxyl of Peronospora tabacina isolates collected in German tobacco fields in 2005 revealed the presence of two phenotypes, resistant and sensitive. DNA fingerprints using SSR and minisatellite primers allowed separation of the samples into two groups. The differences in amplification patterns coincided with the sensitive and resistant reaction of the isolates in metalaxyl bioassays. New primers were developed which allowed PCR-based detection of P. tabacina and differentiation of the metalaxyl-sensitive and the metalaxyl-resistant phenotype, respectively. Screening of recent blue mold isolates from Germany and other European countries for metalaxyl sensitivity with leaf disk bioassays coincided completely with the PCR-based identification of the two phenotypes. In Germany, exclusively resistant isolates were found between 2002 and 2004. These still dominate. Since 2005 the co-occurrence of sensitive isolates has been shown. No similar monitoring has been done in any other European country. However, we found the resistant phenotype reaction in a French isolate of 2004 and in two out of three Italian isolates in 2007. Two isolates from Poland and Bulgaria in 2007 were sensitive to metalaxyl. For all 58 isolates tested since 2002 the metalaxyl bioassay-based resistance type and the PCR-based tests for sensitive and resistant genotype coincided. Using genotype-specific primers for future population studies may help to trace the sources of the pathogen from which yearly propagation starts.