Is the parasitization rate of<Emphasis Type="Italic">Diaeretiella rapae</Emphasis> influenced when<Emphasis Type="Italic">Brevicoryne brassicae</Emphasis> feeds on<Emphasis Type="Italic">brassica</Emphasis> plants?

The development time and parasitization rate ofDiaeretiella rapae (M’Intosh) onBrevicoryne brassicae (L.) feeding on differentBrassica cultivars was studied in the laboratory at 20°C. The shortest development time from egg to adult parasitoid was 11.6 days on cabbage cv. ‘Yalova 1’ and the longest was 12.1 days on turnip cv. ‘Antep’ and rapeseed cv. local variety. Females lived significantly longer than males on the host plants used in the study. Females and males had the shortest longevity on rapeseed at 11.1 and 5.1 days, respectively. The highest percent parasitism ofB. brassicae byD. rapae was found on cabbage (40.20%), and the lowest was recorded on turnip (32.64%). Our results demonstrate that parasitism rate could be influenced by the plant quality, probably due to the nutritional status of the aphids or to toxic compounds ingested through the plant. Cabbage, cauliflower and broccoli were found to be suitable plants for the parasitoid, considering the development time of pre-adults, and the parasitization rate ofD. rapae onB. brassicae. http://www.phytoparasitica.org posting Jan. 23, 2007.     

Indirect interactions between rust (<Emphasis Type="Italic">Melampsora epitea</Emphasis>) and leaf beetle (<Emphasis Type="Italic">Phratora vulgatissima</Emphasis>) damage on<Emphasis Type="Italic">salix</Emphasis>

Willows (Salix spp.) are beneficial as a potential source of renewable energy, riparian barriers and riverbank control, yet are considered invasive weeds when they clog watercourses and lead to erosion and flooding. Interactions between willow rustMelampsora epitea (Thüm.) (Uredinales: Melampsoraceae) and leaf beetlePhratora spp. (Coleoptera: Chrysomelidae) feeding damage have an impact on effective pest management and biological control. The present study investigated the effects of(a) prior mechanical leaf damage on rust development, and(b) rust infection on beetle feeding under laboratory conditions for different time intervals and levels of damage. Willow rust infection significantly reduced the amount of leaf area consumed by beetles. The result was similar when a compatible or an incompatible rust pathotype was sprayed ontoSalix viminalis (L.) ‘Mullatin’ plants. There were no overall significant effects of mechanical damage on rust development, although the lowest level of rust infection was found with the incremental damage treatment. There were, however, differences of significance for leaf position and damage status, with damaged leaves at all positions having fewer pustules and a smaller pustule area than the corresponding undamaged leaves. There was no detectable effect of possible volatile emissions from crushed willow leaves on rust infection and development, although the volatile compoundcis-3-hexenyl acetate significantly reduced pustule diameter and overall pustule area. The results are discussed in terms of the implications for pest management and biological control. Corresponding author http://www.phytoparasitica.org posting April 6, 2003.     

The susceptibility of herbal willow to <Emphasis Type="Italic">Melampsora</Emphasis> rust and herbivores

Eight Dark-leaved willow (Salix myrsinifolia) clones and two naturally hybridised clones (S. myrsinifolia × S. phylicifolia), that are considered to be suitable for herbal production, were cultivated for 2 years in Luikonlahti and Punkaharju. Both experimental sites are located in eastern Finland and the distance between the sites is 140 km. Different cultivation methods were used, including combinations of soil tillage, plastic mulch and fertilisation, with the aim of comparing the growth and susceptibility of plants to pathogens and willow-eating herbivores amongst the clones cultivated by different methods. In both study years 2001 and 2002, Melampsora rust-infected willows occurred in Luikonlahti and in Punkaharju. The extent of rust severity varied greatly between the years and experimental areas and amongst the clones. In 2002, fertilisation increased rust severity in Luikonlahti, but the effect was the converse in Punkaharju. Mulch effect on rust severity was clone-dependent. Vole feeding was observed in 56% of the plants in Luikonlahti during the winter 2001–2002 and the frequency of damaged plants was nearly twice as much amongst the willows grown in unmulched soil as those with plastic mulch. Cultivation method had no effect on feeding by leaf beetles or the abundance of aphids. Of the cultivation methods tested here, plastic mulch seems to have the most important influence on willow cultivation, particularly by improving willow growth and also by decreasing winter-feeding by voles.     

<Emphasis Type="Italic">Pythium</Emphasis> and <Emphasis Type="Italic">Phytophthora</Emphasis> species associated with root and stem rots of kalanchoe

Pythium and Phytophthora species were isolated from kalanchoe plants with root and stem rots. Phytophthora isolates were identified as Phytophthora nicotianae on the basis of morphological characteristics and restriction fragment length polymorphism (RFLP) analysis of the rDNA-internal transcribed spacer regions. Similarly, the Pythium isolates were identified as Pythium myriotylum and Pythium helicoides. In pathogenicity tests, isolates of the three species caused root and stem rots. Disease severity caused by the Pythium spp. and Ph. nicotianae was the greatest at 35°–40°C and 30°–40°C, respectively. Ph. nicotianae induced stem rot at two different relative humidities (60% and >95%) at 30°C. P. myriotylum and P. helicoides caused root and stem rots at high humidity (>95%), but only root rot at low humidity (60%).     

Use of sclerotia of<Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> for efficient production of conidia of<Emphasis Type="Italic">Coniothyrium minitans</Emphasis> in liquid culture

A study was conducted to determine the feasibility of using sclerotia ofSclerotinia sclerotiorum for producing conidia ofConiothyrium minitans in liquid culture. The medium (SST) was made of water containing 2.0, 1.5, 1.0 or 0.5% (w/v) ground sclerotia ofS. sclerotiorum and 100 μgl ⁻¹ thiamine hydrochloride (HCl). One ml of conidial suspension (2 × 10⁷ conidia ml⁻¹) ofC. minitans LRC 2534 was inoculated into 100 ml of SST medium or control (thiamine HCl in water) and incubated at 20 ± 2°C on a shaker at 200 rpm. Subsamples were removed periodically and examined under a compound microscope. Conidia in the SST media germinated within 24 h, developed into branched hyphae within 48 h, produced pycnidia after 3–4 days, and the pycnidia released mature conidia after 7 days. Production of conidia varied with the concentration of sclerotia in the SST medium. It was lower (3.6 × 10⁶ conidia ml⁻¹) at 0.5% but higher (1.2 × 10⁸ conidia ml⁻¹) at 2%. The new conidia were viable and the colonies developing from them showed the original morphological characteristics. It was concluded that using SST liquid medium as a substrate for mass production of conidia ofC. minitans has potential for use in commercial development of this mycoparasite as a biocontrol product. http:www.phytoparasitica.org posting Jan. 23, 2007.     

Thermal requirements and development of<Emphasis Type="Italic">Bracon vulgaris</Emphasis>, a parasitoid of the cotton boll weevil

The development ofBracon vulgaris Ashmead (Hymenoptera: Braconidae), parasitoid of the cotton boll weevilAnthonomus grandis Boheman (Coleoptera: Curculionidae), was studied at constant temperatures of 18, 20, 23, 25, 28, 30, 33, 35 and 38°C, 70±10% r.h. and 14:10 L:D period. The period from egg to adult ofB. vulgaris varied from 8.6 (35°C) to 32.9 days (18°C). The eggs of this parasitoid became desiccated and did not hatch at 38°C. The lower development threshold (LDT) and thermal constants for development varied withB. vulgaris instar, but they were similar for males and females. The duration of the 1st, 2nd, 3rd and 4thB. vulgaris instars required, respectively, 9.36 degree-days above one LDT=12.27°C, 13.48 degree-days above one LDT=6.83°C, 11.65 degree-days above one LDT=9.41°C, and 12.82 degree-days above one LDT=10.78°C.B. vulgaris has physiological adaptations to different threshold temperatures and it shows high potential to be used againstA. grandis in cotton crops.     

Environmental persistence of<Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> products tested under natural conditions against<Emphasis Type="Italic">Thaumetopoea wilkinsoni</Emphasis>

Information on persistence ofBacillus thuringiensis (Bt) is needed to improve the microbial pest management programs against the pine processionary mothThaumetopoea wilkinsoni in pine forests in Israel. The persistence of the microbe under natural conditions of rain and sunlight was evaluated and is documented here for the first time. Pine saplings were sprayed with three commercialBt products, Foray 48B, Delfin WG and Dipel DF, all used at 32,000 IU mg⁻¹ in a formulation with 1% (w/v) of condensed milk. In experiments conducted in November and December of 2004, the saplings were either exposed to rain and sunlight or were sheltered to avoid these environmental factors. The lowest rainfall recorded in the 8-day experiments was 16.5 mm (test 2) and the heaviest was 71.1 mm (test 1). Solar irradiation ranged from 9.4 to 10.9 MJ m⁻². The minimum temperature was close to 10°C and the maximum was less than 23°C. Needles of the treated saplings and their controls were sampled after 0, 1, 5 and 8 days, and were fed to 1 ^st or 2 ^nd instar larvae. Dipel DF persisted better than Delfin WG and still retained its initial activity of 80–100% mortality on day 8 at low rainfall (test 2). Dripping ofBt from upper to lower branches was quantified with the larval bioassays. The milk formulation proved to be an effective rain-fasting adjuvant. http://www.phytoparasitica.org posting May 4, 2007.     

Mechanisms of resistance in<Emphasis Type="Italic">Lycopersicon</Emphasis> germplasm to the whitefly<Emphasis Type="Italic">Bemisia argentifolii</Emphasis>

The silverleaf whiteflyBemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) [also known as strain B of the sweetpotato whiteflyB. tabaci (Gennadius)] is a major pest of tomatoes due to both feeding damage and transmission of plant viruses. Certain wild species ofLycopersicon have demonstrated high levels of resistance to the pest. Greenhouse studies were undertaken to quantify the effects on whitefly behavior and mortality of individual, resistant plants selected from three accessions ofL. pennellii (Corr.) D’Arcy (LA 1340, LA 1674 and LA 2560), five accessions ofL. hirsutum f.typicum Humb. & Bonpl. (LA 386, LA 1353, LA 1777, PI 127826 and PI 127827) and one accession ofL. hirsutum f.glabratum C.H. Mull. (PI 126449). In no-choice experiments, fewer adults settled on leaflets of the wild species and deposited 75–100% fewer eggs compared to the cultivated tomato,L. esculentum Mill. Adult mortality ranged from 77–100% on wild accessions but was only 1% onL. esculentum. Most dead adults were trapped in glandular trichome exudates. The effects of these resistant accessions onB. argentifolii were mechanically transferable by appressing the trichome exudates onto the leaves of the susceptible tomato, indicating an association between the factors mediating the resistance and the glandular trichomes. Laboratory studies evaluated the repellent, fumigant and residual toxic effects of representative constituents of trichome exudates onB. argentifolii adults by using selected concentrations and probit analyses. RC₅₀ values (estimated concentration to repel 50% of the adults) and LC₅₀ values for fumigant and residual toxicity indicated that 2-tridecanone had low levels of repellent and residual toxicity activity; that 2-undecanone had high levels of repellent and fumigant activity; and that ginger oil (composed, in part, of sesquiterpene hydrocarbons) had high levels of repellent and residual toxicity activity. These studies suggest that multi-factor resistance exists in wild tomato germplasm. By combining genetically the observed chemical constituents of resistance into a single germplasm, the resulting resistance may be more difficult forB. argentifolii to overcome. http://www.phytoparasitica.org     

绿柳突瓣叶蜂生物学及防治

绿柳突瓣叶蜂(Nematus ruyanus Wei)是发生在甘肃天水柳树上的一种新害虫。严重危害旱柳、垂柳、金丝柳等多种柳树,1年发生6代,非越冬茧结于叶背,越冬茧结于1-5 cm表土中,以预蛹在越冬茧内越冬。幼虫食叶危害,每年3月下旬至10月下旬是该虫的猖獗危害期。用25%灭幼脲,1.8%阿维菌素乳油,40%氧乐果乳油2000倍液防治3龄前幼虫防效达95%以上;用2.5%溴氰菊酯乳油,3%高渗苯氧威乳油4000倍液喷冠防治3龄后大幼虫防效可达90%以上。     

Effect of a biocontrol agent (<Emphasis Type="Italic">Bacillus subtilis</Emphasis>) and modified atmosphere packaging on postharvest decay control and quality retention of litchi during storage

The efficacy of biological control and two types of modified atmosphere packaging (MAP) alone and in combinations was evaluated under cold storage as well as simulated market-shelf conditions to control decay and pericarp browning on litchi cv. ‘McLean’s Red’. Fruits were dipped for 2 min at 15°C inBacillus subtilis or prochloraz separately, packed in MAP [low density polyethylene (LDPE) or polypropylene (PP)], heat sealed and stored at 2°C and 90% r.h. for 18 days followed by 2 days at 14°C and 75% r.h. to simulate market-shelf conditions. A commercially adopted sulfur dioxide treatment was included as a comparative control. Fruits treated withB. subtilis + PP or prochloraz + PP and stand-alone PP treatment did not show decay or browning at 2°C. Decay and browning were controlled significantly after 2 days at 14°C inB. subtilis + PP or prochloraz + PP treatments. However, the prochloraz + PP affected the natural pinkish-red color of the pericarp and gave higher h° (hue angle) values. The stand-alone PP treatment (∼14% O₂, ∼5% CO₂) showed 11.3% decay due mainly toAlternaria alternata andCladosporium spp. at 14°C. The effectiveness of the MAP was improved at 14°C whenB. subtilis was combined with PP, controlling decay and pericarp browning and retaining the fruit color and quality.B. subtilis survived in PP at 2° and 14°C, but not in LDPE. Stand-alone LDPE (∼3% O₂, ∼10% CO₂) and combination treatmentsB. subtilis + LDPE or prochloraz + LDPE failed to control decay and pericarp browning. Higher yeast populations were observed in LDPE orB. subtilis + LDPE at both 2° and 14°C.Candida, Cryptococcus andZygosaccharomyces spp. were the predominant yeasts in all LDPE treatments. Reprints ofS. Afr. Litchi Growers’ Assoc. Yearb. references can be obtained from D. Sivakumar.     

Isolation of pathogenicity- and gregatin-deficient mutants of <Emphasis Type="Italic">Phialophora gregata</Emphasis> f. sp. <Emphasis Type="Italic">adzukicola</Emphasis> through <Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis>-mediated transformation

Phialophora gregata f. sp. adzukicola, a causal agent of brown stem rot in adzuki beans, produces phytotoxic compounds: gregatins A, B, C, D, and E. Gregatins A, C, and D cause wilting and vascular browning in adzuki beans, which resemble the disease symptoms. Thus, gregatins are considered to be involved in pathogenicity. However, molecular analyses have not been conducted, and little is known about other pathogenic factors. We sought to isolate nonpathogenic and gregatin-deficient mutants through Agrobacterium tumefaciens-mediated transformation (ATMT) for cloning of pathogenicity-related genes. The co-cultivation of P. gregata and A. tumefaciens for 48 h at 20°C with 200 μM acetosyringone resulted in approximately 80 transformants per 10⁶ conidia. The presence of acetosyringone in the A. tumefaciens pre-cultivation period led to an increase in T-DNA copy number per genome. Of 420 and 110 transformants tested for their pathogenicity and productivity of gregatins, one nonpathogenic and three gregatin-deficient mutants were obtained, respectively. The nonpathogenic mutant produced gregatins, whereas the gregatin-deficient mutants had pathogenicity comparable to the wild-type strain. This is the first report of ATMT of P. gregata. Further analysis of these mutants will help reveal the nature of the pathogenicity of this fungus including the role of gregatin in pathogenesis.     

Relating plant and pathogen development to optimise fungicide control of phoma stem canker (<Emphasis Type="Italic">Leptosphaeria maculans</Emphasis>) on winter oilseed rape (<Emphasis Type="Italic">Brassica</Emphasis><Emphasis Type="Italic">napus</Emphasis>)

In winter oilseed rape experiments at Rothamsted in 2000/01 to 2002/03 growing seasons, the severity of phoma stem canker epidemics in summer depended on the timing of phoma leaf spot epidemics in the previous autumn, and hence on the timing of Leptosphaeria maculans ascospore release. The first major release of L. maculans ascospores was earlier in 2000 (26 September) and 2001 (18 September) than in 2002 (21 October). Consequently, the autumn phoma leaf spot epidemic was also earlier in 2000 and 2001 than in 2002. The resulting stem canker epidemics were severe by harvest (July) in 2001 and 2002 but not in 2003. No correlation was found between the severity or duration of phoma leaf spotting (lesion days or lesion °C-days) and the subsequent severity of phoma stem canker epidemics. Rates of leaf production and loss were similar in the three growing seasons. Out of ca. 25 leaves produced on plants during each season, leaf numbers 10–14 generally remained on plants for the longest. Treatment with flusilazole + carbendazim in autumn decreased the severity of phoma leaf spotting for several weeks after treatment, decreased the severity of stem canker the following summer and increased yield significantly in 2001 and 2002 but not in 2003. The most effective timings for flusilazole + carbendazim application were when leaves 7–11 were present on most plants and at least 10% of plants were affected by phoma leaf spot. Two half-dose applications of fungicide reduced phoma stem canker and increased yield more than a single full dose application when phoma leaf spot epidemics were early (<800 °C-days after sowing).     

A New Species of Ectoparasite,<Emphasis Type="Italic">Erythraeus ankaraicus</Emphasis> Saboori,Cobanoglu and Bayram,on<Emphasis Type="Italic">Asymmetrasca (=Empoasca) decedens</Emphasis> in Cotton

The infestation intensity of the ectoparasiteErythraeus ankaraicus Saboori, Cobanoglu and Bayram (Acari: Erythraeidae) onAsymmetrasca (=Empoasca) decedens (Paoli) (Homoptera: cicadellidae),Aphis gossypii Glov. (Homoptera: Aphididae) andThrips tabaci Lind. (Thysanoptera: Thripidae) in cotton was investigated during the 2004 and 2005 growing seasons in Aydin Province, TurkeyE. ankaraicus was recorded for the first time onA. decedens leafhopper nymphs during the early growth stage of cotton. Parasitism ofE. ankaraicus on leafhopper nymphs during the first week of July 2004 was 68.9%, whereas in the middle of July 2005 — when mite populations peaked — it reached only 15.1%. Parasitism of the leafhopper was not observed from the middle of the season onward (August to October).     

<Emphasis Type="Italic">Ulocladium atrum</Emphasis> as a biological control agent for white mold of bean caused by<Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis>

Field studies were conducted near Lethbridge, Alberta, Canada, in 2001, 2004 and 2005 to determine the efficacy of the antagonistic fungusUlocladium atrum for control of white mold of bean caused bySclerotinia sclerotiorum. Results of the 3 years of field trials showed that, compared with the untreated control, foliar application of a spore suspension ofU. atrum (300 ml m⁻² of 10⁶ spores ml⁻¹ suspension) significantly reduced incidence and severity of white mold, increased seed yield and reduced contamination of bean seed by sclerotia ofS. sclerotiorum. The level of control of white mold observed in the treatment ofU. atrum was similar to that of the mycoparasitic fungusConiothyrium minitans, but lower than the fungicide treatments of Ronilan (vinclozolin) at the rate of 1200 g ha⁻¹ per application in 2001, or Lance (boscalid) at the rate of 750 g ha⁻¹ per application in 2004 and 2005. The potential for use ofU. atrum as a biological control agent for sclerotinia diseases is discussed. http://www.phytoparasitica.org posting Nov. 12, 2006.     

Behavior and mutation of <Emphasis Type="Italic">Ralstonia solanacearum</Emphasis> in <Emphasis Type="Italic">Solanum toxicarium</Emphasis> grown in aseptic culture

To study the behavior and mutation of Ralstonia solanacearum in Solanum toxicarium, which is resistant to bacterial wilt, S. toxicarium was grown in aseptic culture and inoculated with R. solanacearum. Although 60%–80% of the inoculated plants were wilting after 2 to 3 days, most wilted plants had recovered by 20 days after inoculation. The pathogen was reisolated from over 98% of inoculated plant stems, but the percentage of recovery decreased the closer the isolation sites were toward the upper stem sections. Three colony types, characterized as fluidal white, nonfluidal red, and a mixture of fluidal white and nonfluidal red, were reisolated from the stems. Nonfluidal red colonies were less virulent on tomato plants than fluidal white colonies.     

Persistence of <Emphasis Type="Italic">Xanthomonas axonopodis</Emphasis> pv. <Emphasis Type="Italic">vignicola</Emphasis> in weeds and crop debris and identification of <Emphasis Type="Italic">Sphenostylis stenocarpa</Emphasis> as a potential new host

The survival of Xanthomonas axonopodis pv. vignicola, incitant of cowpea bacterial blight and pustule, in residues of infested cowpea leaves was studied in the field in the forest savanna transition zone of South Benin and under variable controlled conditions. The pathogen survived for up to 60 days when placed on the soil surface, and up to 45 days buried at depths of 10 and 20 cm. In the glasshouse, bacteria survived in residue mixed with soil for at least 2 months in dry soil and less than 2 months in moist soil. The pathogen survived at least 30 days in the field after spray-inoculation on the weed species Euphorbia heterophylla, Digitaria horizontalis and Synedrella nodiflora; 20 days on Panicum subalbidum; 10 days on Euphorbia hirta; and 5 days on Talinum triangulare. After leaf-infiltration under glasshouse conditions, the pathogen was detected after 90 days in D. horizontalis; 75 days in T. triangulare, P. subalbidum and S. nodiflora; 60 days in E. hirta, and 30 days in E. heterophylla. Among 12 legume species tested as alternative hosts of X. axonopodis pv. vignicola, only Sphenostylis stenocarpa (African yam bean) showed typical symptoms of cowpea bacterial blight in a glasshouse experiment following artificial inoculation. This is the first time this legume species has been identified as a potential host of X. axonopodis pv.vignicola. Crop residue and weeds are likely sources of primary inoculum when planting two consecutive cowpea crops per year and they probably play a role in dissemination of the pathogen during the cropping season. The alternate host may form a bridge for primary inoculum between cropping seasons.     

Control of<Emphasis Type="Italic">Penicillium digitatum</Emphasis> on citrus fruit using two plant extracts and study of their mode of action

Extracts from two plants from Ethiopia,Withania somnifera andAcacia seyal, were evaluated for their potential as natural biopesticides and to study their mode of action. Methanolic extracts of these plants were testedin vivo on citrus fruit for their efficacy to controlPenicillium digitatum when applied on wounded or unwounded fruit surfaces. Relative to the control, 70% and 75% of wound — inoculated fruit did not develop decay symptoms for up to 21 days of storage at 25°C and >85% r.h. An increase in cell wall-bound phenolics was evident in wounded fruit treated with plant extracts and inoculated with a spore suspension ofP. digitatum. Scanning electron microscopy revealed deposition of crystalline plant material sticking to the pathogen and around the wound site. The application of the plant extracts increased the epiphytic background total microbial population but decreased diversity. http://www.phytoparasitica.org posting May 7, 2007.     

Note: First record of the eucalyptus gall wasp<Emphasis Type="Italic">Ophelimus maskelli</Emphasis> and its parasitoid,<Emphasis Type="Italic">Closterocerus chamaeleon</Emphasis>, in Turkey

The Eucalyptus gall waspOphelimus maskelli (Ashmead) (Hymenoptera: Chalcidoidea: Eulophidae: Ophelimini) and its introduced parasitoidClosterocerus chamaeleon (Hymenoptera: Chalcidoidea: Eulophidae: Entedoninae) are recorded for the first time in the Mediterranean and Aegean Regions of Turkey. Some morphological characters of adult and larva ofO. maskelli and larva ofC. chamaeleon are described and illustrated. Distributions of the two wasps collected in Turkey are given. Results indicate that the biological control agent,C. chamaeleon, has spread over 1,300 km in the 16 months since its initial release.     

Mechanical transmission of <Emphasis Type="Italic">Potato spindle tuber viroid</Emphasis> between plants of <Emphasis Type="Italic">Brugmansia suaveoles,Solanum jasminoides</Emphasis> and potatoes and tomatoes

Potato spindle tuber viroid (PSTVd) has been recently found in many solanaceous ornamental plant species. This study reports on the effectiveness of mechanical transmission between Brugmansia suaveolens, Solanum jasminoides, potato and tomato. Inoculation with ‘infected’ plant sap diluted in water, rubbing with contaminated finger tips and cutting with contaminated razor blades all resulted in transmission of PSTVd. Temperature, plant species and source of inoculum were found to be critical factors. An average temperature of 15°C only resulted in a few infections, whereas transmission at 20 and 25°C was more successful. Tomatoes were more susceptible to PSTVd than B. suaveolens, S. jasminoides and potatoes. Furthermore, S. jasminoides was a better source of inoculum than B. suaveolens. No transmission was obtained after repeated addition of inocula to tomato roots. These results indicate that PSTVd can be transmitted between plant species in practice by crop handling.     

Occurrence of <Emphasis Type="Italic">Rhexocercosporidium carotae</Emphasis> on cold stored carrot roots in the Netherlands

Winter carrot for the fresh market is an important cash crop for many organic arable farms in the Netherlands. In recent years carrot roots from cold stores have been affected by superficial dark brown to black spots. To gain insight into the pathogens causing the blemish and the effect of agronomic practices on their occurrence, surveys were carried out among crops harvested in 2001 and 2002. In addition carrots harvested in 2003 were screened for root spotting pathogens. Rhexocercosporidium carotae (syn. Acrothecium carotae and Pseudocercosporidium carotae) was the dominant pathogen in blackish spots on carrots harvested in 2001. On carrots harvested in 2002 and 2003 Alternaria radicina was detected more frequently. Multiple regression analysis indicated that a higher occurrence of the blemish may be linked with harvest conditions and presence of umbelliferous plants. The effect of the temperature on conidial germination, mycelial growth and pathogenicity of R. carotae was studied. The estimated optimum and maximum temperature for growth of R. carotae was 19 and 29°C, respectively. Inoculation experiments demonstrated that wounds are good invasion routes. Infection occurred at 3, 10 and 20°C, but not at 30°C. Penetration into wounds was greatest at 20°C.