bluuebunny/crossref_metadata_2025 huggingface dataset:
Gametic disequilibria between virulence genes in barley powdery mildew populations in relation to selection and recombination. I. Models
Authors: øSTERGÅRD HANNE , HOVMøLLER MOGENS S.
Published: April 2007

Two‐ and three‐locus models were developed to study the dynamics of gametic disequilibria (linkage disequilibria) between virulence genes in an aerial population of a haploid, biotrophic pathogen, illustrated by the fungus Erysiphe graminis f.sp. hordei. The models predicted that selection induced by two or three resistance genes in host varieties would generate gametic disequilibria between the corresponding virulence genes and that recombination taking place during sexual reproduction in most cases would reduce the amount of gametic disequilibrium attained by selection. The reduction of gametic disequilibrium during sexual reproduction depended on the recombination frequency multiplied by the proportion of spores produced by sexual reproduction and by the relative acreage of varieties on which the virulence genes considered were‘unnecessary'. The dynamics of gametic disequilibria in the aerial population were shown to be different depending on the use of resistance genes in host varieties. Two resistance genes present mainly in different varieties would generate negative gametic disequilibrium between the corresponding virulence genes, whereas two resistance genes present mainly in the same variety would generate positive gametic disequilibrium between the corresponding virulence genes. In both cases, the signs would be maintained as predicted until the virulence genes became fixed. However, if the disequilibrium initially was non‐zero with a sign opposite to that predicted from the distribution of the varieties, then the sign of the disequilibrium would not change instantaneously. These general results were valid for sexual as well as asexual reproduction. The predictions of the models were largely in accordance with those observed in Danish barley powdery mildew populations (Hovmøller & østergard, 1991). The results of the models were complemented by numerical studies to illustrate the dynamics of gametic disequilibria in specific cases, and to demonstrate the different evolutionary patterns of the virulence gene and genotype frequencies under different selection regimes.
Protection of potato from Rhizoctonia canker with binucleate Rhizoctonia fungi
Authors: ESCANDE A. R. , ECHANDI E.
Published: April 2007

Fifteen isolates of binucleate Rhizoctonia fungi (BNR) were studied as potential biocontrol agents for protection of potato from Rhizoctonia canker in artificially infested greenhouse soil and potato fields naturally infested with Rhizoctonia solani (AG‐3). Eight of the BNR reduced incidence and severity of Rhizoctonia stem canker in greenhouse experiments by an average of 78 and 85%, respectively. In a field naturally infested with R. solani, selected isolates of BNR and the fungicide Tops 2.5D (thiophanate‐methyl) were equally protective of potato from Rhizoctonia stem canker. BNR isolates gave protection of potato from Rhizoctonia stolon canker similar to PCNB and superior to Tops 2.5D. Cultivars Atlantic, Irish Cobbler, Kennebec, Norchip, Russet Burbank, and Superior were protected equally from Rhizoctonia stem canker by selected isolates of BNR under field conditions. Isolates of BNR show potential as biocontrol agents for protection of potato from Rhizoctonia canker.
Development and evaluation of a technique for screening watercress (Rorippa nasturtium‐aquaticum) for resistance to watercress yellow spot virus and crook‐root fungus (Spongospora subterranea f. sp. nasturtii)
Authors: WALSH J. A. , PHELPS K.
Published: April 2007

A technique was developed for screening large numbers of watercress plants for resistance to watercress yellow spot virus (WYSV) and the crook‐root fungus. Plants were raised in modules containing sand in the glasshouse, transported and placed in experimental watercress beds, recovered after 5 or 6 weeks, examined visually for crook‐root infection and tested by ELISA for infection by WYSV. High incidences of crook root (98.8%) and virus (88.9%) were obtained in a watercress line known to be susceptible to both pathogens. Evaluation of the technique using 10 different watercress lines showed that it was capable of revealing a range of responses from very susceptible to very resistant, with UK lines being most susceptible to both diseases. When grown on, the very resistant line was different morphologically from UK watercress and was identified as early winter‐cress (Barbarea verna). Results showed an association between crook root scores and ELISA values, providing further circumstantial evidence for the close relationship between the two pathogens. The implications of the results for watercress resistance screening are discussed.
Variation in the early development of Bremia lactucae on lettuce cultivars with different levels of field resistance
Authors: LEBEDA A. , REININK K.
Published: April 2007

The early stages of development of Bremia lactucae (lettuce downy mildew) were examined on lettuce cultivars possessing high (Iceberg and Regina di Maggio) and low (Great Lakes and Plenos) levels of field resistance. Germ tubes, appressoria, penetration, primary and secondary vesicles, intercellular hyphae and haustoria were observed 3. 6 and 24 h after inoculation of cotyledons and of leaf discs from adult plants. Differences were observed between cv. Iceberg and susceptible genotypes in the percentage of spores germinating and the incidence and speed of development of infection structures. Secondary vesicles were first observed 24 h and 6 h after inoculation in Iceberg and susceptible genotypes, respectively. The lowest incidence of secondary vesicle formation 24 h after inoculation (48 and/or 43%) was recorded in Iceberg and Regina di Maggio, and the highest incidence (68%) occurred in Plenos. The formation of intercellular hyphae and haustoria was not observed in cv. Iceberg some 24 h after inoculation. There were significant differences in the lengths of germ tubes formed on different cultivars. Those on cv. Iceberg were longer than those formed on susceptible cultivars. The results indicate that the field resistance of B. lactucae may result from mechanisms which are effective in the early stages of infection.
A microscopic study of the different classes of symptoms coded by the Vf gene in apple for resistance to scab (Venturia inaequalis)
Authors: CHEVALIER M. , LESPINASSE Y. , RENAUDIN S.
Published: April 2007

The different symptoms of resistance coded by the VF gene were studied on glasshouse‐grown apple seedlings after inoculation with a conidial suspension of Venturia inaequalis. Scanning‐electron microscopy and histological observations defined precisely three different classes of symptoms. Class 1 corresponds to a hypersensitive reaction. Class 2 is characterized by considerable tissue modifications but no sporulation whatsoever occurs; this reaction defines resistant hybrids. In class 3 the fungus remains active and can sometimes sporulate abundantly; this class was divided into two subgroups, one comprising all the hybrids of weak resistance, the other the hybrids of weak susceptibility.
Infection of sugar beet by Polymyxa betae in relation to soil temperature
Authors: BLUNT S. J. , ASHER M. J. C. , GILLIGAN C. A.
Published: April 2007

The effects of soil temperature on infection of sugar‐beet roots by the soil‐borne fungus Polymyxa betae were investigated in controlled environments. Pre‐germinated seeds were sown in pots of naturally infested soil and seedlings sampled at frequent intervals over a period of several weeks. Within the range 10‐30°C, the optimum soil temperature for infection was c. 25°C; the time between sowing and the first detectable infection was shortest and the subsequent rate of infection most rapid at this temperature. No infection was observed over 80 days at 10°C.

Both root and shoot dry weight were reduced on plants growing in infested soil at 15, 20 and 25 C compared with those growing in uninfested soil. In general, root growth was more severely affected than shoot growth and the effects were most pronounced at 20°C. These results were confirmed in a subsequent experiment in which P. betae‐infected root material was used as the inoculum. In addition to its role as the vector of beet necrotic yellow vein virus (the cause of Rhizomania disease), the significance of P. betae as a plant pathogen in its own right is discussed.
Comparison of responses to floret and seedling inoculation in wheat‐ Ustilago tritici and barley‐ U. nuda combinations
Authors: JONES P. , DHITAPHICHIT P.
Published: April 2007

The floret and seedling methods for inoculating wheat and barley with Ustilago tritici and U. nuda, respectively, were optimized and then compared. Floret inoculation gave higher infection levels than did seedling inoculation, especially for wheat, but problems of plant mortality and standardization of tissue age were greater with the former method. Floret inoculation identified wheat and barley varieties exhibiting embryo resistance (class 1 or 2 resistance) or seedling resistance (class 3), but only class 3 wheat and barley varieties (and one class 2 wheat variety) were resistant to seedling inoculation. Using near‐isogenic wheat lines expressing different levels of partial resistance to floret inoculation, positive correlations were obtained between percentage infection levels resulting from the two inoculation protocols. In addition, race‐specific host‐pathogen interactions such as hypersensitivity and partial smutting were expressed following both floret and seedling inoculation, indicating that these interactions operate in the developing plant. The potential value of the seedling inoculation method in fundamental and applied research into the cereal‐loose smut interaction is discussed.
Heat treatment for the control of rose and carnation grey mould (Botrytis cinerea)
Authors: ELAD Y. , VOLPIN HANNE
Published: April 2007

Thermal dip treatment for flower heads was found effective against grey mould of roses. Dipping of flowers in tap‐water at 50 C for 20 40 s was more effective than at higher or lower temperatures and than for longer treatments. The treatment was effective in five out of six rose cultivars and in one carnation cultivar. A 60, reduction in disease severity following the treatment was observed in flowers naturally infected by Botrytis cinerea, whereas the treatment was not effective against artificial infection of flowers. Conditioning of flowers for 2 days at 10 C and high relative humidity before incubation under grey‐mould‐conducive conditions (15 C. high humidity) increased the efficacy of the thermal treatment. Moreover, the combination of heat with 3 mM catechol was additively more efficient in reducing grey mould. The surfactant Tween 20 (001 %) improved the effect of treatment at 45 C for 20 s. A combination of heat with the fungicide chlorothalonil did not improve effectiveness, whereas when polyoxin B was combined with heat it was better than either treatment alone. The temperature of petals at the edge and centre of flower reached 37 and 27 C, respectively, at the end of a 20‐s incubation period in 50 C water. Conidia treated at 34‐40 C for 10‐20 s incited 60% less severe disease than conidia treated at 25 C in water.
Dry rot of potatoes caused by Gliocladium roseum
Authors: THERON D. J. , HOLZ G.
Published: April 2007

The pathogenicity of Gliocladium roseum, isolated in a previous study as the sole organism from potato tubers with typical Fusarium dry‐rot lesions, was investigated. Although usually considered to be a saprophyte, G. roseum was pathogenic to potato tubers (cv. Up‐to‐date) after artificial inoculation. It caused raised dry‐rot lesions from which it was consistently reisolated. This is the first report that dry rot of potato tubers is caused by G. roseum.
The role of soil, seed‐potato tuber and haulm in the transmission of Phoma foveata from infected seed to daughter tubers
Authors: CARNEGIE S. F. , CAMERON A. M.
Published: April 2007

Infestation of ridge soil by Phoma foveata was examined over 4 years by collecting soil samples at various distances from seed tubers which were either naturally infected or inoculated with a distinctive strain. Samples were taken on four occasions during each growing season. Infestation was greatest immediately around the tuber and was largely confined to a distance of 15 cm until haulm destruction, after which P. foveata was detected more widely in the soil.

The effects of various seed‐tuber and haulm treatments on the incidence of daughter‐tuber contamination by P. foveata were assessed over 5 years. Removing the infected seed tuber at emergence significantly (P < 0.05) reduced the contamination at haulm destruction of daughter tubers harvested by hand in only 1 out of 4years. In all years, tuber contamination was much less(P < 0.05) when stems were pulled and removed than when they were desiccated by applying diquat dibromide. The incidence of tuber contamination did not increase between haulm destruction and harvest when stems were pulled and removed. In another experiment, tuber contamination increased linearly with the delay in pulling haulms after applying diquat dibromide. Cutting and removal of stems tended to reduce gangrene contamination, relative to the desiccation of stems by applying diquat dibromide, in all years where the seed tuber had been removed, but in only 2 out of 5 years where the tuber was present.
Control of bottom rot disease of lettuce (Rhizoctonia solani) using preparations of Trichoderma viride, T. harzianum or tolclofos‐methyl
Authors: COLEY‐SMITH J. R. , RIDOUT C. J. , MITCHELL CHRISTINE M. , LYNCH J. M.
Published: April 2007

Isolates of Trichoderma were tested for their ability to control Rhizoctonia solani in lettuce seedlings and mature plants in a glasshouse. The best isolate (T. viride IMI 298375) was tested for its ability to control bottom rot disease in mature lettuce plants grown in polythene tunnels. Control was achieved in several trials but there was little evidence for much improvement after repeated applications and control with Trichoderma was poorer than that obtained with a standard treatment with tolclofos‐methyl. Evidence was obtained to suggest however that enhanced growth of lettuce plants following treatment with Trichoderma can produce marketable yields of lettuce similar to those obtained with tolclofos‐methyl.
Observations of apothecia of Tapesia yallundae and the cultural phenotypes of their progeny
Authors: KING A. C.
Published: April 2007

Apothecia of Tapesia yallundae, the perfect state of Pseudocercosporella herpotrichoides, were found in March 1989 on wheat and barley stubble at nine of 37 naturally infested sites and also at one inoculated site in Germany. Apothecia were found on about 1% of stems collected from the naturally infested sites and 5% of stems from the inoculated site, and were first found in the second week of March, coinciding with the onset of an early spring. Apothecia were not found on either green plants or second‐year stubble. In total, 143 single‐spore isolates were obtained from the apothecia and described according to a range of morphological characteristics. The majority were ascribed to P. herpotrichoides var. herpotrichoides. This is the first isolation of this variety from T. yallundae apothecia in Germany. Even single‐conidial isolates from one apothecium showed much variation, some being like var. herpotrichoides, some like var. acuformis and some intermediate.
Methods for assessing the resistance of raspberry genotypes to Phytophthora root rot
Authors: KENNEDY D. M. , DUNCAN J. M.
Published: April 2007

Zoospores and agar discs of the fungus causing raspberry root rot (Phytophthora fragariae) were used as inoculum to assess the resistance of raspberry genotypes in pot experiments. High zoospore concentrations resulted in more severe symptoms than low concentrations. There was a strong correlation between the results obtained with zoospores and with colonized agar discs as inocula. although disease was generally more severe with the former. The severity of root rot was correlated with a number of other symptoms, in particular growth reductions, stem lesions and leaf wilting, and some of these symptoms could possibly be used as measures of resistance. There was no evidence of physiological specialization with the range of isolates and raspberry genotypes used. Pot tests in general gave results which matched reported resistances in the field, and could be used to assess the resistance of breeding lines and selections.
Synthesis of the solanapyrone phytotoxins by Ascochyta rabiei in response to metal cations and development of a defined medium for toxin production
Authors: CHEN Y. M. , STRANGE R. N.
Published: April 2007

The phytotoxic solanapyrones A, B and C, were produced by Ascochyta rabiei when grown on Czapek Dox nutrients supplemented with an aqueous extract of chickpea seed but not when grown on Czapek Dox nutrients alone. Addition of amino acids, vitamins and inorganic ions to Czapek Dox nutrients allowed the production of the solanapyrones in good yields. By systematic elimination of components of this medium, the constituents essential for toxin production were identified as the divalent metal cations Zn2+, Ca2+, Mn2+ and Cu2+. When these ions were added to Czapek Dox nutrients in the same concentrations as those found in chickpea extract, concentrations of the phytotoxins similar to those found in Czapek Dox nutrients supplemented with chickpea extract were detected. Removal of cations from the chickpea supplement by a cation exchange resin did not affect the growth of the fungus compared with growth on the medium containing the complete supplement, but no toxin was produced. A defined medium and cultural conditions suitable for solanapyrone production by A. rabiei are described.
Plasmids in Pseudomonas syringae pv. pisi carry genes for pathogenicity
Authors: MAZAREI M. , KERR A.
Published: April 2007

All strains tested which are pathogenic to peas and which react with antiserum to Pseudomonas syringae pv. pisi contain two to four plasmids; non‐pathogens contain none. Two plasmids from a pathogenic strain were transferred individually to a non‐pathogenic recipient strain of Pseudomonas syringae: both plasmids converted the recipient to a pathogen on peas and from hypersensitivity negative to hypersensitivity positive on tobacco. Neither plasmid encoded homoserine catabolism.
Interactions between Fusarium oxysporum f. sp. tracheiphilum and Meloidogyne spp. in Vigna unguiculata. 1. Effects of different inoculum densities on Fusarium wilt
Authors: HARRIS A. R. , FERRIS H.
Published: April 2007

The effects of chlamydosporesandconidia of Fusarium oxysporum f sp. tracheiphilum at different initial spore concentrations were compared in the wilt‐susceptible cowpea cultivar California Blackeye No. 5 (CB5). In glasshouse experiments with one inoculum density of either Meloidogyne incognita or M javanica, chlamydospores resulted in greater incidence and severity of Fusarium wilt than conidia at the same inoculum densities. Wilt symptoms also increased on wilt‐resistant cultivar CB3 as inoculum densities of M. javanica were increased. When three cultivars were infested with moderate or high densities of both F. o. tracheiphilum and M. javanica. only CB5 developed sere wilt at either inoculum density. The wilt‐tolerant cultivar Grant had mild wilt symptoms in most plants at moderate inoculum densities, and a tenfold increase in inoculum did not increase wilt ratings. CB3. however, had higher incidence and severity of Fusarium wilt symptoms at high inoculum densities, although 60% of the plants survived for 9 weeks.
First record of Sclerotinia sclerotiorum on tree tomato in Ecuador
Authors: VELASTEGUI J. R. , BALL S. F. L.
Published: April 2007

Cyphomandra betacea (tree tomato) plants with stem lesions were observed in an orchard in Tungurahua province, Ecuador. Dissection of the stems showed the presence of black sclerotia. Further laboratory analysis and pathogenicity tests revealed that Sclerotinia sclerotiorum was the causal agent of the disease. Although recorded in New Zealand, this is the first report of the pathogen affecting tree tomato in Ecuador.
Root rot of lavender caused by Phytophthora nicotianae
Authors: PUTNAM M.
Published: April 2007

Lavender (Lavandula angustifolia) plants with rotted roots and discoloured vascular systems consistently yielded cultures of fungi that were identified as Phytophthora nicotianae van Breda de Haan (=P. parasitica). Inoculation experiments using either zoospore suspensions or mycelial fragments were successful in reproducing symptoms originally observed on wilting and dying plants. Lavender is a new host for P. nicotianae.
The expression of components of seedling resistance to Erysiphe graminis f.sp. tritici in Triticum timopheevii and a hexaploid derivative
Authors: NASHAAT N. I. , MOORE K.
Published: April 2007

The expression of components of resistance to three isolates of Erysiphe graminis f.sp. tritici was investigated on the primary seedling leaves of nine lines of Triticum timopheevii, one line of the wild ancestor T. araraticum, and a hexaploid derivative from T. timopheevii, CI 12633. Wheat cv. Nord Desprez was used as a susceptible control with no known major gene for powdery mildew resistance. The expression of resistance was studied 72 h after inoculation to investigate the efficiency of a sequence of barriers to fungal penetration and development: cuticle; papilla; epidermal cell hypersensitivity; and the hypersensitive necrotic reaction of mesophyll cells underlying the infection area. Production of conidia was assessed after 10 days. Neither the profuse epidermal hairs nor the leaf cuticle contributed directly to resistance in T. timopheevii. However, both T. araraticum and T. timopheevii exhibited papilla‐mediated resistance and, in addition, T. araraticum showed epidermal hypersensitivity. T. timopheevii may possess a gene or genes for resistance additional to those already transferred to, and expressed in. the hexaploid derivative CI 12633, particularly those which condition the hypersensitive necrotic reaction of the mesophyll cells surrounding the infection area.
Effect of foliar diseases of wheat on the physiological processes affecting yield under semi‐arid conditions
Authors: SHTIENBERG D.
Published: April 2007

The effects of Septoria leaf blotch (caused by Mycosphaerella graminicola) and leaf rust (caused by Puccinia recondita) on the physiological processes affecting yield were studied under semi‐arid conditions in Israel. In the absence of water stress, photosynthetic activity decreased as the rate of disease development increased. Transpirational activity was reduced in leaves infected by Septoria leaf blotch, but intensified in leaves with slight leaf rust infections (0.5‐5% severity). When leaf rust seventy exceeded 5%, the transpirational water loss declined. Both diseases were associated with a decrease in the maximum rate of grain‐weight accumulation, but did not affect the duration of weight accumulation. An inverse linear relationship between disease and final grain weight was observed. Under conditions of water stress, photosynthetic activity of leaves slightly infected by Septoria leaf blotch (1‐5”., severity) was greater than that of uninfected leaves. Severely infected leaves (” 25% severity) photosynthesized the least. Both diseases were associated with a decrease in the maximum rate of grain weight accumulation, but the duration of accumulation was longer in tillers with a slow rate of disease development than in uninfected tillers. The relationship between disease and final grain weight in these cases was parabolic; yield was increased at low levels of infection. Theoretical explanations and practical implications of these phenomena are presented and discussed.
Studies on resistance to dichlofluanid and other fungicides in Botrytis cinerea
Authors: REWAL N. , COLEY‐SMITH J. R. , SEALY‐LEWIS H. M.
Published: April 2007

Amongst 489 mass isolates of Botrytis cinerea obtained from protected crops in Humberside during 1989, 67.3% were resistant to benomyl and 45.6% to vinclozolin. Two isolates resistant to dichlofluanid were obtained with EC50 values for mycelial growth on agar of 21.45 and 19.78 μg dichlofluanid/ml, respectively, compared with values of 1.55‐2.56 μg/ml for sensitive isolates. EC50 values for germ tube emergence and length were much lower than for mycelial growth. In population studies carried out on tomatoes in a polythene tunnel, resistance to dichlofluanid and to vinclozolin declined over a 12‐week period in the absence of fungicides. Field isolates of B. cinerea resistant to dichlofluanid were also resistant to vinclozolin. but strains resistant to dichlofluanid and sensitive to vinclozolin were obtained during population studies. Examination of a number of features of dichlofluanid‐resistant strains revealed a degree of lack of fitness as compared with sensitive strains. There is no indication that resistance to dichlofluanid poses a great threat to the control of grey mould in Humberside at present.
Pre‐ and post‐harvest control of mango anthracnose in the Philippines
Authors: DODD J. C. , BUGANTE R. , KOOMEN I. , JEFFRIES P. , JEGER M. J.
Published: April 2007

Two field trials were conducted in the Philippines in successive years to compare the effectiveness of different pre‐ and post‐harvest treatments on the development of anthracnose on mango fruits caused by Colletoirichum gloeosporioides. In one experiment, the pre‐harvest application of benomyl (250 mg/l a.i.) significantly reduced the number of conidia sampled in tree canopies compared with control trees (no fungicide) and was associated with a significant reduction in the post‐harvest development of anthracnose on fruits. A hot‐benomyl dip (850 mg/l a.i. at 52 55 C for 10 min) completely eradicated anthracnose on fruit treated on the day of harvest. In a second experiment, pre‐harvest applications of prochloraz (500 mg/l a.i.) either within a pre‐planned spray schedule or applied strategically (when 18 h or more continual leaf wetness was recorded by a sensor placed within a tree canopy) gave the best control of anthracnose on fruits. A hot‐benomyl dip was again the most effective post‐harvest treatment for fruit treated on the day of harvest and on the day after. There was no significant difference, however, between hot‐benomyl dips or prochloraz dips (500 mg/l a.i. for 10 min) at ambient temperatures when fruit were treated on the third day after harvest. The implications of these results for the production and treatment of Carabao mangoes in the Philippines are discussed.
Interacting effects of interrupted humid periods and light on infection of wheat leaves by Mycosphaerella graminicola (Septoria tritici)
Authors: SHAW M. W.
Published: April 2007

Septoria tritici conidia require a long period at 100%, humidity to infect wheat plants successfully. When periods of 100% relative humidity were interrupted by up to 48 h at 75% relative humidity, infection by S. tritici on winter wheat cvs Longbow and Avalon was only slightly reduced. Breaks at 50% relative humidity had larger effects, but still allowed infection to occur. Infection was reduced more by a break in humidity which began in the light, but previous and subsequent light regimes strongly influenced the final outcome in all cases. When a wet period was interrupted twice, the two dry breaks interacted strongly under some circumstances; in these cases the probability of infection was much greater than expected from multiplying the effects of the single breaks. Under other conditions two breaks had more nearly multiplicative effects, or interacted to reduce the probability of infection below that expected from the individual breaks. There was evidence that light reduced the probability of successful infection earls in the infection process, but stimulated it later. The data were consistent with infection occurring faster on cv. Longbow, but otherwise being affected in qualitatively similar ways on both cultivars. The complexity of the effects and their interactions suggests that it will not be possible to find a simple set of infection conditions to apply in the field. The data rule out a number of simple mathematical models of the infection process.
Effect of pokeweed antiviral protein (PAP) on the infection of plant viruses
Authors: CHEN Z. C , WHITE R. F. , ANTONIW J. F. , LIN Q.
Published: April 2007

At a concentration of 0.4 μg purified pokeweed antiviral protein (PAP)/ml, the formation of local lesions on tobacco leaves caused by tobacco mosaic virus infection was completely inhibited and at 25 ng PAP/ ml. 68% inhibition was still obtained. PAP protected plants from infection by viruses from seven virus groups‐five RNA viruses: tobacco mosaic virus, cucumber mosaic virus, alfalfa mosaic virus, potato virus X and potato virus Y; and two DNA viruses: African cassava mosaic virus (ssDNA) and cauliflower mosaic virus (dsDNA). Virus infection was probably blocked by PAP at a very early stage. PAP infiltrated into the intercellular spaces through the lower surfaces of leaves inhibited infection by virus inoculated on the upper leaf surface, and partially prevented PVY transmission by aphids. However. PAP did not show any activity against two bacterial and six fungal pathogens.
Albugo Candida, the cause of a serious white rust of Lunaria biennis in Crete
Authors: VAN ALOUNAKIS D. J.
Published: April 2007

Albugo Candida, the causal agent of white blister rust of Lunaria biennis and other crucifers. is recorded for the first time in Crete.
Incidence of Chalara elegans in groundnut seed samples and seed transmission of blackhull
Authors: LABUSCHAGNE N. , KOTZÉ J. M.
Published: April 2007

The incidence of Chalara elegans (=Thielaviopsis basicola) was determined in seven commercial groundnut seed samples in the Vaalharts area of South Africa. It was demonstrated that 1.19% of the seeds were infected by the pathogen. Surface contamination of visually healthy seeds from the same samples was confirmed by testing seed washings for viable propagules of C. elegans on raw carrot dises. Naturally infected seed planted in autoclaved soil in the greenhouse resulted in 34.4% severely diseased plants. These results indicate that seed‐borne inoculum may be an important factor in the transmission of this disease.
Differentiation of pathotypes of lettuce mosaic virus
Authors: PINK D. A. C. , KOSTOVA D. , WALKEY D. G. A.
Published: April 2007

Differential interactions between six crisphead cultivars of lettuce and five isolates of lettuce mosaic virus (LMV) have identified a novel resistance factor in the cultivar Ithaca. The LMV isolates could be grouped into three pathotypes, two of which have not been identified previously. One of these, pathotype III represented by an isolate from Spain, was virulent on all of the cultivars. There were differences in aggressiveness between isolates of the same pathotype.
Resistance responses of albino leaves of Triticum timopheevii mutants inoculated with Erysiphe graminis f.sp. tritici
Authors: NASHAAT N. I. , MOORE K.
Published: April 2007

Albino chlorophyll mutants induced in Triticum timopheevii by γ‐irradiation were used to study the influence of photosynthetic activity on the development of two isolates of Erysiphe graminis f.sp. tritici, and host resistance responses, 72 h after inoculation. The results showed that the percentage of pathogenic units inhibited by papilla response and epidermal cell hypersensitivity was lower on albino primary seedling leaves than on normal green seedling leaves. On albino leaves, the development of pathogenic units which established a successful penetration was either decreased or retarded, depending on the isolate used. This effect could be attributed to the limited supply of assimilates from the albino host due to the lack of photosynthetic activity.
A comparison of dust and acetone infusion applications of tolclofos‐methyl to bean seeds for the control of Rhizoctonia solani
Authors: PHILLIPS A. J. L.
Published: April 2007

Treatment of bean (Phaseolus vulgaris) seeds with tolelofos‐methyl reduced pre‐emergence mortality and hypocotyl rot caused by Rhizoctonia solani. The fungicide was effective when applied either as a dust treatment or by infusion in acetone solution. Neither the acetone infusion nor the dust application had any adverse effects on the germination of seeds sown in uninfested soil. Acetone infusion was effective after short periods of immersion (1 min) and with repeated use of the same solution, thus resulting in more efficient use of the fungicide. Both methods of fungicide application effectively controlled rhizoctonia diseases in the field.
Reduced sensitivity of Botrytis cinerea to two sterol biosynthesis‐inhibiting fungicides: fenetrazole and fenethanil
Authors: ELAD Y.
Published: April 2007

Isolates of Botrytis cinerea having reduced sensitivity to the sterol biosynthesis‐inhibiting (SBI) fungicides fenetrazole and fenethanil were obtained from one out of four sites from which isolates were tested. Reduced sensitivity was associated with poor disease control by fenetrazole, which had been applied with dichlofluanid. Conidial germination and hyphal growth of B. cinerea from the four sites were tested in vitro on media amended with the fungicides. Following fenetrazole or fenethanil treatment, at the site where control had failed, populations of B. cinerea were detected with higher EC50 and EC90 values than at the three other sites. Germination of conidia of B. cinerea was markedly inhibited by 1.0 μg/ml fenetrazole and 0.5–1.0 μg/ml fenethanil. The frequency of isolates insensitive to 1.0 μg/ml fenetrazole or to 0.5 μg/ml fenethanil was 3.4 and 1.8 times higher, respectively, at the site where control had failed, compared with another site where SBI fungicides had never been applied to control grey mould. Grey mould caused by selected isolates of B. cinerea which expressed the phenotype of low sensitivity to SBI fungicides in leaves of tomato, pepper and Senecio cineraria was not controlled by either fenetrazole or fenethanil (1 .5–3.0 μg/ml). However, up to 100% disease reduction was obtained when leaves infected by sensitive isolates were treated with the fungicides.
Phytotoxicity of destruxin B and its possible role in the pathogenesis of Alternaria brassicae
Authors: BUCHWALDT L. , GREEN H.
Published: April 2007

Destruxin B, the major phytotoxin produced by Allernaria brassicae, was not host‐specific on 30 different plant species, causing necrotic and chlorotic symptoms both on host and non‐host plants. Isolation of sufficient amounts of two minor destruxins, homodestruxin B and destruxin B2, made it possible for the first time to show that they were both phytotoxic to leaves of Brassica napus. There were significant differences between taxonomic plant groups in their sensitivity to destruxin B. Brassica species were most sensitive to the toxin, and sensitivity decreased as relatedness of plant groups became more distant: thus it is suggested that destruxin B is host‐selective in nature. The dilution‐end‐point of destruxin B was 0.2–3.8 μg/ml for the most sensitive host species, and 15 120 μg/ml for the least sensitive. The sensitivity of non‐host species was between 15 and 750 μg/ml. The symptoms caused by destruxin B appear to be light‐dependent. There were significant differences between taxonomic plant groups in their sensitivity to destruxin B. Brassica species were most sensitive to the toxin, and sensitivity decreased as relatedness of plant groups became more distant, thus it is suggested that destruxin B is host‐selective in nature. Destruxin B appears to be a virulence factor, contributing to the aggressiveness of A. brassicae by conditioning the host tissue and thereby determining the susceptibility of the host.
The host range of Polymyxa betae in Britain
Authors: BARR K. J. , ASHER M. J. C.
Published: April 2007

The host range of Polymyxa betae on common arable weed species in Britain was determined by growing plants in naturally infested soil and examining their root systems for the presence of resting spores (cystosori). Of the 24 species tested, only Atriplex patula and Chenopodium album of the Chenopodiaceae, and Silene alba of the Caryophyllaceae, were found to be heavily infected. S. alba is a newly recorded host species for Polymyxa. The host specificity of isolates of P. betae from Beta vulgaris, C. album and A. patula was investigated by observing which of 11 test plants could be infected by the isolates obtained from this soil. Three main biotypes of P. betae appeared to be distinguishable: one which was able to infect all chenopodiaceous species; one which had a narrower host range; and one which was able to infect S. alba. The role of weed species in the epidemiology of rhizomania is discussed.
A recessive resistance gene of alfalfa to alfalfa mosaic virus and impact of high ambient temperature on virus resistance
Authors: IWAI H. , HORIKAWA Y. , FIGUEIREDO G. , HIRUKI C.
Published: April 2007

Twenty plants of alfalfa cv. Beaver were screened for resistance to alfalfa mosaic virus (AIMV) severe strain A‐515. ELISA screening on both inoculated and apical leaves at fixed temperature (20°C day, 16°C night) suggested the following three types of clonal response to AIMV infection: extremely resistant, AIMV not normally detected from either inoculated or apical leaves; resistant, AIMV detected from inoculated leaves only and did not spread systemically; susceptible, AIMV detected from both inoculated and apical leaves. When plants in the second category were maintained at high temperature (30°C), AIMV was detected from inoculated and apical leaves 6 and 12 days after inoculation, but was not detectable in the apical leaves thereafter. Plants in the first category remained extremely resistant at all temperatures tested. The results of comparative tests using progeny plants of extremely resistant, resistant and susceptible plants, and of their hybrids, suggested that the resistance to AIMV A‐515 was controlled by a temperature‐dependent recessive gene.
Stem and bulb nematode, Ditylenchus dipsaci, associated with a dry rot of potato tubers
Authors: COTTEN J. , HOOPER D. J. , FOLEY M. F. , HANCOCK MARY
Published: April 2007

About 10% of the ware‐sized tubers from a 1990 potato crop were found to have dry rot. The affected tubers had well defined circular patches of dry rot, either with the tuber surface depressed and cracked, or with deep, open cavities where the tuber surface had disintegrated. The rotted tissues contained large numbers of the stem and bulb nematode, Ditylenchus dipsaci. This is the first recorded case in the UK; dry rot of potato tubers is usually caused by the potato tuber nematode D. destructor.
Mitochondrial plasmids do not determine host range in crucifer‐infecting strains of Fusarium oxysporum
Authors: MOMOL E. A. , KISTLER H. C.
Published: April 2007

The role of mitochondrial plasmids in determining host range in Fusarium oxysporum was tested by using protoplast fusion and fluorescence‐assisted cell sorting to produce hybrid cells between two strains (ATCC 9990 and ATCC 16601) of the fungus which differ in host specialization to cabbage and radish. Three nuclear restriction fragment length polymorphism‐defined loci and mitochondrial chromosomal RFLP markers were identified which distinguish the parental strains. In addition, parents were differentiated by mitochondrial plasmids, one unique to each strain and previously correlated to host specialization. In single‐spored cultures derived from cell fusions (‘fusion cultures’), only parental combinations of the three nuclear genetic markers were detected. However, non‐parental combinations of nuclear and cytoplasmic markers were recovered. One mitochondrial plasmid type was not found in fusion cultures; the other plasmid type was detected in association with both parental mitochondrial chromosome types. Several fusion cultures lacked detectable plasmid sequences of either type. Chromosome‐sized DNAs from parent strains and fusion cultures were separated using contour‐clamped homogeneous electric field (CHEF) electrophoresis. Electrophoretic karyotypes of the parent strains differed greatly, with eight and 11 chromosome‐sized bands detected for ATCC 9990 and ATCC 16601, respectively. Electrophoretic karyotypes of fusion cultures generally were indistinguishable from one or the other parental types. Host specialization was not correlated with mitochondrial plasmid type, but rather with nuclear genotype.
Eastern and southern African strains of Xanthomonas campestris pv. vasculorum are distinguishable by restriction fragment length polymorphism of DNA and polyacrylamide gel electrophoresis of membrane proteins
Authors: QHOBELA M. , CLAFLIN L. E.
Published: April 2007

Thirty strains of Xanthomonas campestris pv. vasculorum were examined using restriction fragment length polymorphism (RFLP) of genomic DNA and polyacrylamide gel electrophoresis (PAGE) of membrane proteins. Previous reports suggested the existence of two geographical strains of this pathogen, occurring primarily in eastern and southern Africa. Seventeen southern African strains from South Africa and Zimbabwe formed an apparent clonal population with distinct and unique RFLP patterns and membrane protein profiles. Similarly, 11 eastern African strains from Mauritius and Reunion and two from Australia were also related, indicating the possible existence of two distinct organisms separated by geographical locality but otherwise involved in the same disease. Our data confirm the long‐held hypothesis that eastern African strains of X. c. pv. vasculorurn are distinct from the southern African strains. The fact that these two distinct strains cause the same disease, gumming disease of sugar cane, suggests the possible independent development of the pathogen in these two localities.
Tomato yellow leaf curl virus: host range and virus‐vector relationships
Authors: MANSOUR A. , AL‐MUSA A.
Published: April 2007

The Jordanian isolate of tomato yellow leaf curl virus has a narrow host range restricted to a few solanaceous plants. Severe symptoms developed on tomatoes and Datura stramonium, whereas Nicotiana glutinosa and N. tabacum cvs Samsun and Havana 423 were infected without showing symptoms. The whitefly Bemisa tabaci is an efficient vector; a single whitefly was able to transmit the virus. The minimum acquisition and inoculation feeding periods were 60 and 30 min, respectively, and the latent period was 20–24 h. The virus was retained by Bemisia tabaci for 11 days. The host range and virus vector relationships of the Jordanian isolate are reported for the first time.
Variation for pathogenicity on tomato among parasexual recombinants of Verticillium dahliae
Authors: O'GARRO L. W. , CLARKSON J. M.
Published: April 2007

Ten haploid prototrophic recombinant isolates (HPRs) were obtained from each of 15 parasexual crosses between complementary autotrophs derived from nine tomato isolates and one eggplant isolate of V. dahliae, including those identified as race 1 and race 2. These HPRs were tested for pathogenicity to the tomato cultivar Roma which is susceptible to both races. HPRs from a ‘selfed’ race 2 × race 2 cross were as pathogenic as the wild‐type parent. The pathogenicity of HPRs derived from all other crosses was variable, and generally lower than the parental mean. A particularly marked reduction in pathogenicity, compared with the parental mean, was observed for HPRs recovered from two crosses between isolates belonging to different heterokaryon compatibility groups. The results suggest that pathogenicity to cv. Roma is controlled by complex interactions between genes at numerous loci.
Production and germination of sclerotia of Sclerotinia sclerotiorum
Authors: SANSFORD C. E. , COLEY‐SMITH J. R.
Published: April 2007

Uniform and non‐aggregated sclerotia of Sclerotinia sclerotiorum were produced in large numbers in a medium consisting of wheat grain and Pcerlite. Sclerotia of five out of seven isolates germinated and produced fertile apothecia when incubated in moist Perlite under near‐UV radiation.
The development of different pathotype groups of Mycosphaerella pinocles in susceptible and partially resistant pea leaves
Authors: NASIR M. , HOPPE H.‐H. , EBRAHIM‐NESBAT F.
Published: April 2007

The progress of infection by high‐ and low‐virulence isolates of Mycosphaerella pinodes was examined in susceptible and partially resistant pea leaves. Conidia germinated with one or more germ tubes which frequently branched and formed appressorium‐like structures on the leaf surface. Penetration occurred through the epidermal walls. A structure similar to an infection vesicle was formed, lying partly in the epidermal wail and partly in the cell lumen. From this structure, a penetration hypha was derived which initiated the development of the intra‐ and intercellularly‐growing fungal colony. Infections led to rapid tissue collapse in both susceptible and resistant interactions. In resistant interactions, the formation of infection vesicles and penetration hyphae was reduced, and the development and spread of lesions was retarded.
First record of Peronospora sparsa on Prunus laurocerasus
Authors: HALL G. , COOK R. T. A. , BRADSHAW N. J.
Published: April 2007

Peronospora sparsa Berkeley was identified on cherry laurel (Prunus laurocerasus) which was being propagated in four nurseries across England and Wales in 1987. This is the first record of a downy mildew on Prunus laurocerasus.
Serological and biological variation between and within subgroup I and II strains of cucumber mosaic virus
Authors: WAHYUNI W. S. , DIETZGEN R. G. , HANADA K. , FRANCKI R. I. B.
Published: April 2007

Fourteen strains of cucumber mosaic virus (CMV) from Australia have been characterized by their host range and symptomatology. They were classified as subgroup I or II strains by a dot‐blot molecular hybridization assay between their total viral RNAs and selected cDNAs. The strains FNY and LNy, both from the USA, were used as the subgroup I‐ and subgroup II‐type strains, respectively. A range of serological tests was used to compare these isolates. Gel immunodiffusion tests, with standard antigens homologous to the antisera prepared against glutaraldehyde‐fixed virus of 11 strains, showed that they could be divided into three serogroups on the basis of spur formation in heterologous reactions. Two of the serogroups included either subgroup I or subgroup II isolates, whereas the third serogroup consisted of only one strain (YWA) which was homologous to all the strains tested. Use of heterologous standard antigens in this test failed to show further subgrouping of the antigens. Double‐antibody sandwich (DAS) ELISA using polyclonal antibodies to distinct virus strains also placed the 14 strains in the same three serogroups. When eight different monoclonal antibodies (MAbs) were used in indirect ELISA, one of them distinguished subgroup‐I strains and another distinguished subgroup‐II strains; the YWA strain fell into subgroup II. Other MAbs showed narrower or broader specificity. Thus both molecular hybridization with total RNA and specific MAbs may be useful for separating isolates of CMV into subgroups I and II. Spur formation using heterologous standard antigens to the antisera, as well as being more difficult to interpret, was not a reliable criterion for classification.