Pathogenicity of <Emphasis Type="Italic">Beauveria bassiana</Emphasis> and <Emphasis Type="Italic">Metarhizium anisopliae</Emphasis> against <Emphasis Type="Italic">Galleria mellonella</Emphasis>

The greater wax moth Galleria mellonella L. (Lepidoptera: Pyralidae) is occasionally found in beehives and is a major pest of stored wax. Entomopathogenic fungi have recently received attention as possible biocontrol elements for certain insect pests. In this study, 90 isolates of Beauveria bassiana and 15 isolates of Metarhizium anisopliae were screened for proteases and lipases production. The results showed significant variations in the enzymatic action between the isolates. In the bioassay, the selected isolates evinced high virulence against the 4th instar of the G. mellonella larvae. The isolates BbaAUMC3076, BbaAUMC3263 and ManAUMC3085 realized 100% mortality at concentrations of 5.5 × 10⁶ conidia ml⁻¹, 5.86 × 10⁵ conidia ml⁻¹, and 4.8 × 10⁶ conidia ml⁻¹, respectively. Strong enzymatic activities in vitro did not necessarily indicate high virulence against the tested insect pest. The cuticle of the infected larvae became dark and black-spotted, indicating direct attack of fungus on the defense system of the insects. The LC₅₀ values were 1.43 × 10³, 1.04 × 10⁵ and 5.06 × 10⁴ for Bba3263AUMC, Bba3076AUMC and Man3085AUMC, respectively, and their slopes were determined by computerized probit analysis program as 0.738 ± 0.008, 0.635 ± 0.007 and 1.120 ± 0.024, respectively.     

Effects of garlic (<Emphasis Type="Italic">Allium sativum</Emphasis>) juice containing allicin on <Emphasis Type="Italic">Phytophthora infestans</Emphasis> and downy mildew of cucumber caused by <Emphasis Type="Italic">Pseudoperonospora cubensis</Emphasis>

The volatile antimicrobial substance allicin (diallylthiosulphinate) is produced in garlic when the tissues are damaged and the substrate allicin (S-allyl-l-cysteine sulphoxide) mixes with the enzyme alliin-lyase (E.C.4.4.1.4). Allicin undergoes thiol-disulphide exchange reactions with free thiol groups in proteins and it is thought that this is the basis of its antimicrobial action. At 50 μg ml^-1, allicin in garlic juice inhibited the germination of sporangia and cysts and subsequent germ tube growth by Phytophthora infestans both in vitro and in vivo on the leaf surface. Disease severity in P. infestans-infected tomato seedlings was also reduced by spraying leaves with garlic juice containing allicin over the range tested (55–110 μg ml⁻¹) with an effectiveness ranging from approximately 45–100%. Similarly, in growth room experiments at concentrations from 50–1,000 μg ml⁻¹, allicin in garlic juice reduced the severity of cucumber downy mildew caused by Pseudoperonospora cubensis by approximately 50–100%. These results suggest a potential for developing preparations from garlic for use in specialised aspects of organic farming, e.g. for reducing pathogen inoculum potential and perhaps for use under glass in horticulture.     

Effect of azadirachtin on the control of <Emphasis Type="Italic">Anticarsia gemmatalis</Emphasis> and its impact on <Emphasis Type="Italic">Trichogramma pretiosum</Emphasis>

Anticarsia gemmatalis has great potential to reduce soybean productivity, and the egg parasitoid Trichogramma pretiosum is a major agent in the biological control of this pest in soybean fields. We show that azadirachtin is able to control velvetbean caterpillars in soybean plants and also that it has no effects on the parasitoid T. pretiosum. Soybean plants were sprayed with solutions containing control (water), 50 and 100 mg.l ⁻¹ of azadirachtin (AzaMax™ 12 g a.i.l ⁻¹). Third instar larvae of A. gemmatalis were confined to soybean plants after the spraying. Leaf consumption and larval mortality of A. gemmatalis were evaluated. Newly emerged females of T. pretiosum were placed for 24 h in tubes with a piece of cardboard containing the same doses of azadirachtin used against velvetbean caterpillars. After 24 h, cardboard with 20 eggs of A. gemmatalis was offered for parasitism during 24 h, and the emergence and sex ratio of progenies were determined. Azadirachtin at 50 or 100 mg.l ⁻¹ reduced leaf consumption and caused 100% mortality in A. gemmatalis larva. Azadirachtin did not negatively affect the parasitism, emergence or sex ratio of the progeny. This indicates that the product can be used with mass release of T. pretiosum to control A. gemmatalis.     

Host-pathogen interaction of root-knot nematode <Emphasis Type="Italic">Meloidogyne incognita</Emphasis> on pepper in the southeast of Spain

The feeder roots of pepper plants (cv. California Wonder) in Campo de Cartagena (southeast Spain) were found to be severely infected by Meloidogyne incognita. Morphometric traits, differential host test and DNA analysis based on PCR were used to characterize the nematode. Naturally and artificially infected pepper plants showed severe yellowing and stunting, with heavily deformed and damaged root systems. Root galls were spherical and commonly contained more than one female and egg masses with eggs. Typical giant cells with a granular cytoplasm and many hypertrophied nuclei were observed in histological preparations. The relationship between initial nematode population density (Pi) and pepper plant growth was tested in greenhouse experiments with inoculum levels that varied from 0 to 64 eggs and second-stage juveniles (J₂) ml⁻¹ soil. A Seinhorst model was fitted to plant height and top fresh weight data of inoculated and non-inoculated plants. The tolerance limit with respect to plant height and fresh top weight of pepper to M. incognita was estimated as 0.85 eggs and J₂ ml⁻¹ soil. The minimum relative values (m) for plant height and top fresh weight were 0.15 and 0.16, respectively, at Pi ≥ 64 eggs and J₂ ml⁻¹ soil. The maximum nematode reproduction rate (Pf/Pi) was 315.4 at an initial population density (Pi) of 4 eggs and J₂ ml⁻¹ soil. The obtained results could be used as a base to establish field experiments that allow strategies to prevent surpassing the threshold of nematodes in fields that are infested.     

Selection,characterization and genetic analysis of laboratory mutants of <Emphasis Type="Italic">Botryotinia fuckeliana</Emphasis> (<Emphasis Type="Italic">Botrytis cinerea</Emphasis>) resistant to the fungicide boscalid

Resistance to the fungicide boscalid in laboratory mutants of Botryotinia fuckeliana (Botrytis cinerea) was investigated. The baseline sensitivity to boscalid was evaluated in terms of colony growth (EC₅₀ = 0.3–3 μg ml⁻¹; MIC = 10–30 μg ml⁻¹) and conidial germination (EC₅₀ = 0.03–0.1 μg ml⁻¹; MIC = 1–3 μg ml⁻¹) tests. Mutants were selected in vitro from wild-type strains of the fungus on a fungicide-amended medium containing acetate as a carbon source. Mutants showed two different levels of resistance to boscalid, distinguishable through the conidial germination tests: low (EC₅₀ ∼ 0.3 μg ml⁻¹, ranging from 0.03 to 1 μg ml⁻¹; MIC > 100 μg ml⁻¹) and high (EC₅₀ > 100 μg ml⁻¹) resistance. Analysis of meiotic progeny from crosses between resistant mutants and sensitive reference strains showed that resistant phenotypes were due to mutations in single major gene(s) inherited in a Mendelian fashion, and linked with both the Daf1 and Mbc1 genes, responsible for resistance to dicarboximide and benzimidazole fungicides, respectively. Gene sequence analysis of the four sub-units of the boscalid-target protein, the succinate dehydrogenase enzyme, revealed that single or double point mutations in the highly conserved regions of the iron-sulphur protein (Ip) gene were associated with resistance. Mutations resulted in proline to leucine or phenylalanine replacements at position 225 (P225L or P225F) in high resistant mutants, and in a histidine to tyrosine replacement at position 272 (H272Y) in low resistant mutants. Sequences of the flavoprotein and the two transmembrane sub-units of succinate dehydrogenase were never affected.     

<Emphasis Type="Italic">Paecilomyces lilacinus</Emphasis>, a potential biocontrol agent on apple rust mite <Emphasis Type="Italic">Aculus,schlechtendali</Emphasis> and interactions with some fungicides <Emphasis Type="Italic">in vitro</Emphasis>

The apple rust mite Aculus schlechtendali (Nal.) (Acari: Eriophyidae), is a main pest in apple-growing areas in Ankara, Turkey, and chemical control applications have some limitations. Entomopathogenic fungi have a potential for biological control of mites. In this study, an entomopathogenic fungus, Paecilomyces lilacinus (Thom) Samson (Deuteromycota: Hyphomycetes), was first isolated from the mite cadavers on Japanese crab apple leaves and pathogenicity of the fungus was observed in different inoculum densities and relative humidities. The pathogen caused up to 98.22% mortality of the mite population. The effects of some fungicides on the entomopathogenic fungus were determined in in vitro studies. Carbendazim, penconazole and tebuconazole were the most effective fungicides on mycelial growth of P. lilacinus, with EC₅₀ values under 3 μg ml⁻¹. In spore germination tests, captan, mancozeb, propineb were the most effective fungicides, followed by tebuconazole, penconazole, nuarimol and chlorothalonil. Sulphur could not inhibit the conidia germination totally at 5,000 μg ml⁻¹. Copper oxychloride and fosetyl-al prevented conidia formation at concentrations above 1,000 μg ml⁻¹.     

Note: Evaluation of antibacterial activity of essential oil of<Emphasis Type="Italic">Rosa damascena</Emphasis> on<Emphasis Type="Italic">Erwinia amylovora</Emphasis>

Significant antibacterial activity was observed in the essential oil (E.O.) ofRosa damascena Mill. and the Minimum Bactericidal Concentration (MBC) of the E.O. was determined as 1386.5 μg ml⁻¹ forErwinia amylovora, the causal agent of fire blight disease. Ooze formation on immature pears and lesion formation in artificially inoculated shoots were completely (100%) prevented by the essential oil ofR. damascena (1500 μg ml⁻¹), essential oil ofThymbra spicata var.spicata (500 μg ml⁻¹) and streptomycin (100 μg ml⁻¹). Copper oxychloride plus maneb significantly reduced ooze formation and lesion formation, but the control was less than that obtained with the essential oils or streptomycin. The essential oil ofR. damascena may be a useful natural bactericide for the control of the fire blight pathogen,E. amylovora. http://www.phytoparasitica.org posting July 14, 2004.     

Fumigant toxicity of<Emphasis Type="Italic">Eucalyptus blakelyi</Emphasis> and<Emphasis Type="Italic">Melaleuca fulgens</Emphasis> essential oils and 1,8-cineole against different development stages of the rice weevil<Emphasis Type="Italic">Sitophilus oryzae</Emphasis>

Essential oils extracted fromEucalyptus blakelyi (1,8-cineole, 77.5%),Melaleuca fulgens (1,8-cineole, 56.9%) and 1,8-cineole were shown to have fumigant toxicity against different development stages ofSitophilus oryzae. The eggs ofS. oryzae were the most tolerant, followed by pupae, larvae and adults in that order.M. fulgens oil,E. blakelyi oil and 1,8-cineole at 100 μl per liter of air gave, respectively, LT₅₀ values of 16.2, 17.4 and 9.1 h for adults, 31.1, 19.3 and 17.5 h for larvae, 55.6, 75.2 and 39.7 h for pupae, and required >7 days for eggs. Only 1,8-cineole (200 μl ⁻¹ air) gave a significant egg kill by 7 days and the LT₉₅ was 134.5 h. 1,8-Cineole could be a useful new fumigant. http://www.phytoparasitica.org posting Oct. 3, 2004.     

Survival of <Emphasis Type="Italic">Ralstonia solanacearum</Emphasis> in infected tissues of <Emphasis Type="Italic">Capsicum annuum</Emphasis> and in soils of the state of Pernambuco,Brazil

The survival of Ralstonia solanacearum A1-9^Rif race 1 phylotype I was studied in ten different soil types in the absence of the host plant as well as in infected tissues of the stem and root of bell peppers buried in the soil at 0, 5, and 15 cm. The survival time of R. solanacearum A1-9^Rif in the ten soil types ranged from 42 up to 77 days. Among the chemical and physical characteristics of the soil, clay content, residual moisture, and available water were positively correlated, and pH was negatively correlated, with survival time, population size at 42 days, and area under the population curve. The pathogen survival differed significantly in relation to the plant tissues, but not with respect to the incorporation depth of the infected tissues. The root tissue of bell pepper supported a larger bacterial population at 7 and 21 days (5 × 10⁴ and 3.1 × 10⁴ CFU g⁻¹ tissue, respectively) compared with the stem tissue (0.35 × 10⁴ and 0.48 × 10⁴ CFU g⁻¹ tissue, respectively) and also had a larger area under the population curve. On the other hand, the stem tissues presented a greater decomposition rate and pH compared with the roots. In conclusion, the different types of studied soils as well as the infected bell pepper tissues were considered potential primary sources of R. solanacearum inocula, but only for a short period.     

Effect of <Emphasis Type="Italic">Beauveria bassiana</Emphasis> mixed with diatomaceous earth on mortality,mycosis and sporulation of <Emphasis Type="Italic">Rhyzopertha dominica</Emphasis> on stored wheat

A laboratory assay was designed to determine the insecticidal efficacy of Beauveria bassiana (Balsamo) Vuillemin (Hyphomycetes: Moniliales) and diatomaceous earth (Diafil 610) against Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae). The fungus B. bassiana was applied at 2.23 × 10⁷, 2.23 × 10⁸ and 2.23 × 10⁹ conidia kg⁻¹ of wheat individually as well as mixed with 200 and 400 ppm of Diafil 610. The conditions for the trials were 30 ± 2^oC with 55% r.h. and the counts for mortality were made after 8, 16 and 24 d. All the dead adults were removed after each count and the vials were kept for the next 60 d to assess the emergence of the F₁ generation. The findings from these studies proved that the extended exposure interval and the highest combined dose rate of the entomopathogenic fungus and the diatomaceous earth gave the maximum mortality of the beetles. The emergence of the progeny was also highly suppressed where the maximum dose rate of the synergized treatments was applied. The rate of mycosis and sporulation in the cadavers of R. dominica was maximum where the low dose rates of B. bassiana were applied.     

Shift of sensitivity of <Emphasis Type="Italic">Botrytis cinerea</Emphasis> to azoxystrobin in greenhouse vegetables before and after exposure to the fungicide

From 2004 to 2006, 213 isolates of Botrytis cinerea never exposed to Q_O center inhibitors (Q_OIs) were collected to determine the baseline sensitivity to azoxystrobin. In the absence of salicylhydroxamic acid (SHAM), the mean EC₅₀ values were 10.49 ± 13.12 and 0.36 ± 0.48 mg l⁻¹ for inhibiting mycelial growth and conidium germination, respectively. In the presence of SHAM, the mean EC₅₀ values were 2.24 ± 1.29 and 0.22 ± 0.11 mg l⁻¹. In 2010, five azoxystrobin-resistant isolates were detected with the resistance frequency of 2.25% in greenhouse tomatoes after 4 years of continuous exposure. These resistant isolates showed cross-resistance to other Q_OIs but not to boscalid. In addition, these resistant isolates had comparable growth, sporulation and pathogenicity ability as sensitive isolates and maintained resistance in plants and the presence of SHAM. The G143A point mutation predicted to cause a change from glycine to alanine at codon 143 of cyt b gene was found in all resistant isolates.     

Pathogenicity of morphologically different isolates of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> with <Emphasis Type="Italic">Brassica napus</Emphasis> and <Emphasis Type="Italic">B. juncea</Emphasis> genotypes

Sclerotinia stem rot caused by Sclerotinia sclerotiorum is a serious threat to oilseed production in Australia. Eight isolates of S. sclerotiorum were collected from Mount Barker and Walkway regions of Western Australia in 2004. Comparisons of colony characteristics on potato dextrose agar (PDA) as well as pathogenicity studies of these isolates were conducted on selected genotypes of Brassica napus and B. juncea. Three darkly-pigmented isolates (WW-1, WW-2 and WW-4) were identified and this is the first report of the occurrence of such isolates in Australia. There was, however, no correlation between pigmentation or colony diameter on PDA with the pathogenicity of different isolates of this pathogen as measured by diameter of cotyledon lesion on the host genotypes. Significant differences were observed between different isolates (P ≤ 0.001) in two separate experiments in relation to pathogenicity. Differences were also observed between the different Brassica genotypes (P ≤ 0.001) in their responses to different isolates of S. sclerotiorum and there was also a significant host × pathogen interaction (P ≤ 0.001) in both experiments. Responses between the two experiments were significantly correlated in relation to diameter of cotyledon lesions caused by selected isolates (r = 0.79; P < 0.001, n = 48). Responses of some genotypes (e.g., cv. Charlton) were relatively consistent irrespective of the isolates of the pathogen tested, whereas highly variable responses were observed in some other genotypes (e.g., Zhongyou-ang No. 4, Purler) against the same isolates. Results indicate that, ideally, more than one S. sclerotiorum isolate should be included in any screening programme to identify host resistance. Unique genotypes which show relatively consistent resistant reactions (e.g., cv. Charlton) across different isolates are the best for commercial exploitation of this resistance in oilseed Brassica breeding programmes.     

Aggressiveness and mycotoxin production of eight isolates each of <Emphasis Type="Italic">Fusarium graminearum</Emphasis> and <Emphasis Type="Italic">Fusarium verticillioides</Emphasis> for ear rot on susceptible and resistant early maize inbred lines

Fusarium graminearum and F. verticillioides are among the most important pathogens causing ear rot of maize in Central Europe. Our objectives were to (1) compare eight isolates of each species on two susceptible inbred lines for their variation in ear rot rating and mycotoxin production across 3 years, and (2) analyse two susceptible and three resistant inbred lines for potential isolate x line interactions across 2 years by silk-channel inoculation. Ear rot rating, zearalenone (ZEA) and deoxynivalenol (DON) concentrations were evaluated for all F. graminearum isolates. In addition, nivalenol (NIV) concentrations were analysed for two NIV producers. Fumonisin (FUM) concentrations were measured for all F. verticillioides isolates. Mean ear rot severity was highest for DON producers of F. graminearum (62.9% of the ear covered by mycelium), followed by NIV producers of the same species (24.2%) and lowest for F. verticillioides isolates (9.8%). For the latter species, ear rot severities differed highly among years (2006: 24%, 2007: 3%, 2008: 7%). Mycotoxin concentrations among isolates showed a broad range (DON: 100–284 mg kg⁻¹, NIV: 15–38 mg kg⁻¹, ZEA: 1.1–49.5 mg kg⁻¹, FUM: 14.5–57.5 mg kg⁻¹). Genotypic variances were significant for isolates and inbred lines in all traits and for both species. Isolate x line interactions were significant only for ear rot rating (P < 0.01) and DON concentration (P < 0.05) of the F. graminearum isolates, but no rank reversals occurred. Most isolates were capable of differentiating the susceptible from the resistant lines for ear rot severity. For resistance screening, a sufficiently aggressive isolate should be used to warrant maximal differentiation among inbred lines. With respect to F. verticillioides infections, high FUM concentrations were found in grains from ears with minimal disease symptoms.     

Vascular blackening of wasabi rhizomes caused by <Emphasis Type="Italic">Pectobacterium carotovorum</Emphasis> subsp. <Emphasis Type="Italic">carotovorum</Emphasis>

Wasabi (Wasabia japonica) is grown for its highly-valued rhizome which is used as a condiment in Japanese food. Symptoms of vascular blackening in the rhizome were first observed in 2005 in plants grown in British Columbia, Canada. Microscopic observations and microbial isolation from infected tissues revealed that most of the xylem tracheid cells were blackened and bacteria were consistently associated with symptomatic plants. The bacterium most frequently recovered was identified as Pectobacterium carotovorum subsp. carotovorum (Pcc) using BioLog™ and sequencing of a specific ~510 bp IGS region. Pathogen-free plants obtained using meristem-tip micropropagation were inoculated with a wasabi isolate of Pcc. Vascular blackening symptoms developed in the rhizome after 8 weeks when the rhizome was first wounded by stabbing or cutting, or if the roots were pre-inoculated with Pythium species isolated from rhizome epidermal tissues, followed by inoculation with Pcc at 1 × 10⁸ cells ml⁻¹. Xylem tracheid cells were blackened and Pcc was reisolated from all diseased tissues. The highest frequency of rhizome vascular blackening occurred at 22°C and 27°C and these tissues occasionally succumbed to soft rot at higher temperatures, but not when inoculated tissues were incubated at 10°C. The rooting medium used by growers for vegetative propagation of wasabi was shown to contain Pcc but the pathogen was not recovered from the irrigation water. Entry of Pcc through wounds on wasabi rhizomes and the host tissue response result in symptoms of vascular blackening.     

Effect of botanical extracts on the biological activity of granulosis virus against <Emphasis Type="Italic">Pieris brassicae</Emphasis>

A granulosis virus strain infecting Pieris brassicae (PbGV) was isolated from the dry temperate 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 × 10² occlusion bodies [OBs] ml⁻¹) followed by methanolic extract (7.38 × 10² OBs ml⁻¹) and aqueous extract (9.36 × 10³ OBs ml⁻¹) as compared with PbGV alone (1.85 × 10⁴ 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.     

Effects of some fungicides on <Emphasis Type="Italic">Isaria farinosa</Emphasis>, and <Emphasis Type="Italic">in vitro</Emphasis> growth and infection rate on <Emphasis Type="Italic">Planococcus citri</Emphasis>

The effects of some fungicides used against citrus diseases, on mycelial growth and conidial germination of Isaria farinosa (Holmsk.) Fries [Sordariomycetes: Hypocreales] and also on the pathogenicity of the fungus on citrus mealybug, Planococcus citri (Risso), were determined. Systemic fungicides such as tebuconazole, penconazole and nuarimol were the most effective as regards both conidial germination and mycelial growth. Protective fungicides such as captan, chlorothalonil, mancozeb and propineb inhibited conidial germination at between 1 and 5 μg ml⁻¹ concentration, but captan, chlorothalonil and propineb did not inhibit the mycelial growth at 5,000 μg ml⁻¹. Mancozeb inhibited mycelial growth between 2,500 and 5,000 μg ml⁻¹. Sulphur and copper oxychloride did not inhibit the fungus even at very high concentrations. Sulphur, copper oxychloride, fosetyl-al, chlorothalonil and carbendazim did not decrease the mortality percentage caused by I. farinosa. Tebuconazole, penconazole and mancozeb were the most effective and respectively reduced the mortality from 83% to 33%, 28% and 30% in the ovisacs, from 81% to 29%, 27% and 29% in the 1st instar larvae, and from 84% to 34% in the adult females.     

Rapid screening of <Emphasis Type="Italic">Musa</Emphasis> species for resistance to Fusarium wilt in an <Emphasis Type="Italic">in vitro</Emphasis> bioassay

In order to accelerate breeding and selection for disease resistance to Fusarium wilt, it is important to develop bioassays which can differentiate between resistant and susceptible cultivars efficiently. Currently, the most commonly used early bioassay for screening Musa genotypes against Fusarium oxysporum f. sp. cubense (Foc) is a pot system, followed by a hydroponic system. This paper investigated the utility of in vitro inoculation of rooted banana plantlets grown on modified medium as a reliable and rapid bioassay for resistance to Foc. Using a scale of 0 to 6 for disease severity measurement, the mean final disease severities of cultivars expressing different levels of disease reaction were significantly different (P ≤ 0.05). Twenty-four days after inoculation with Foc tropical race 4 at 10⁶ conidia ml⁻¹, the plantlets of two susceptible cultivars had higher final disease severities than that of four resistant cultivars. Compared with ‘Guangfen No.1’, ‘Brazil Xiangjiao’ is highly susceptible to tropical race 4 and its mean final disease severity was the highest (5.27). The plantlets of moderately resistant cultivar ‘Formosana’ had a mean final disease severity (3.53) lower than that of ‘Guangfen No.1’ (4.33) but higher than that of resistant cultivars: ‘Nongke No.1’, GCTCV-119, and ‘Dongguan Dajiao’ (1.87, 1.73, and1.53, respectively). Promising resistant clones acquired through non-conventional breeding techniques such as in vitro selection, genetic transformation, and protoplast fusion could be screened by the in vitro bioassay directly. Since there is no acclimatization stage for plantlets used in the bioassay, it helps to improve banana breeding efficiency.     

Pathogenicity and reproductive fitness of <Emphasis Type="Italic">Pratylenchus coffeae</Emphasis> and <Emphasis Type="Italic">Radopholus arabocoffeae</Emphasis> on Arabica coffee seedlings (<Emphasis Type="Italic">Coffea arabica</Emphasis> cv. Catimor) in Vietnam

The pathogenicity and reproductive fitness of Pratylenchus coffeae and Radopholus arabocoffeae from Vietnam on coffee (Coffea arabica) seedlings cv. Catimor were evaluated in greenhouse experiments. The effect of initial population densities (Pi = 0, 1, 2, 4, 8, 16, 32, 64, 128, and 256 nematodes per cm³ soil) was studied for both species at different days after inoculation (dai). The data were adjusted to the Seinhorst damage model Y = m + (1-m).z^Pi-T. Tolerance limit (T) for P. coffeae was zero for the height and the diameter of the coffee plants. For the diameter, the T-value for R. arabocoffeae was 25.6 for 30 and 60 dai and 12.8 for 90 and 120 dai. After 4 months T was zero. The low tolerance limits indicate that Arabica coffee is highly intolerant to both nematode species. At the end of the experiment (180 dai), all plants were infected and most were dead when inoculated with R. arabocoffeae at initial densities of 32, 64, 128 and 256 nematodes/cm³ soil. For P. coffeae plant death was already observed at the lowest inoculation densities. Growth of coffee was reduced at all inoculation levels for both species. Pratylenchus coffeae and R. arabocoffeae caused intense darkening of the roots, leaf chlorosis and a strong reduction of root and shoot growth. It was observed that P. coffeae mainly destroyed lateral roots rather than tap roots, whereas R. arabocoffeae reduced tap root length rather than the lateral roots. At the lowest inoculum densities, the reproduction factor of P. coffeae was 2.38 and 2.01 for R. arabocoffeae, indicating that arabica coffee is a host for both species. Plant growth as expressed by shoot height and shoot and root weight measured 60 dai was negatively correlated with nematode (both species) density as expressed by the geometric mean of nematode numbers at 30 and 60 dai.     

A molecular mechanism of resistance to streptomycin in <Emphasis Type="Italic">Xanthomonas oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzicola</Emphasis>

Xanthomonas oryzae pv. oryzicola, the causal agent of rice leaf streak disease, was found to be sensitive to streptomycin (an aminocyclitol glycoside antibiotic), by inhibition of protein synthesis resulting from interference with translational proofreading. This study aimed to determine the molecular resistance mechanism of X. oryzae pv. oryzicola to streptomycin. Seven streptomycin-resistant mutants were obtained by UV induction or streptomycin selection. These mutants can grow at 100 μg ml⁻¹ of streptomycin while the wild-type strain (RS105) cannot grow at 5 μg ml⁻¹. Sequencing indicated that the rpsL gene encoding ribosomal protein S12 has 375 bp encoding 125 amino acid residues. In all resistant strains, a mutation in which AAG was substituted for AGG (Lys→Arg) occurred either at codon 43 or 88. Two plasmids, pUFRRS and pUFRRX, were constructed by ligating the rpsL gene into the cosmid pUFR034. The plasmids pUFRRS and pUFRRX containing the Lys→Arg mutation of the rpsL gene conferred streptomycin resistance to the sensitive wild-type strain by electroporation. Both transformants, RS1 and RS2, could grow in the medium containing 50 μg ml⁻¹ of streptomycin. A mutation at codon 43 or 88 in rpsL can result in resistance of Xanthomonas oryzae pv. oryzicola to streptomycin.     

Effect of seed priming with <Emphasis Type="Italic">Serratia plymuthica</Emphasis> and <Emphasis Type="Italic">Pseudomonas chlororaphis</Emphasis> to control <Emphasis Type="Italic">Leptosphaeria maculans</Emphasis> in different oilseed rape cultivars

The efficacy of a seed treatment of oilseed rape (OSR) (Brassica napus) with the rhizobacteria Serratia plymuthica (strain HRO-C48) and Pseudomonas chlororaphis (strain MA 342) applied alone or in combination against the blackleg disease caused by Leptosphaeria maculans was tested with different cultivars. Seeds were soaked in bacterial suspensions (bio-priming) to obtain log₁₀ 6–7 CFU seed⁻¹. Cotyledons were inoculated with a 10 ul droplet of L. maculans spore suspension of log₁₀ 7 spores ml⁻¹ and the disease index (size of lesions) was evaluated 14 days later. A mean disease reduction of 71.6% was recorded for S. plymuthica and of 54% for P. chlororaphis. The combined treatment was not superior to the treatment with S. plymuthica alone. The reduction of the disease caused by S. plymuthica was independent of the cultivar’s susceptibility, whereas the control effect recorded with P. chlororaphis increased with decreasing cultivar resistance to blackleg disease. The bacterial colonization of OSR was restricted to the roots and hypocotyl. No significant difference in bacterial colonization of the rhizosphere was observed between different cultivars, nor between single or combined bacterial seed treatments.