Showing papers in "Plant Pathology in 1996"
TL;DR: During the sensitivity testing of different Brassica species and cultivars, it was found that Chinese cabbage showed a low percentage of infestation in two soils, whilst some oilseed rape and spring oilseed turnip rape cultivars showed high degrees ofinfestation in these soils.
Abstract: During 1986–87 the presence of clubroot in soils sampled from 190 fields was assessed using a bioassay method, based on baiting the soils with Brassica campestris spp. pekinesis cv. Granaat. Clubroot was detected in 148 (72%) of the fields investigated and, on average, 49.2% of the plants were infected according to the bioassay. Subsequent testing of fields in 1990 and 1992 (54 and 81 fields, respectively) where no further Brassica crops had been grown indicated a significant decrease in the degree of infestation to 7.1% in 1992. Clay soils showed, on average, the highest degree of infestation, and high infestation was recorded for a wide range of pH values (5.2–6.6). The highest degree of infestation was recorded on fields where oilseeds were grown five times during the period 1965–85. The results presented show that, in a field with 100% infestation, the level of infestation declined to below the detection level after a period of 17.3 years. The half-life of the spore inoculum was determined to be 3.6 years. During the sensitivity testing of different Brassica species and cultivars, it was found that Chinese cabbage showed a low percentage of infestation in two soils, whilst some oilseed rape and spring oilseed turnip rape cultivars showed high degrees of infestation in these soils.
238 citations
TL;DR: Random amplified polymorphic DNA assays were carried out on a range of isolates of F. poae to identify markers common to all isolates, offering the potential to determine the presence of F., poae in wheat and avoid problems commonly associated with conventional diagnosis of the disease and isolation of the pathogen.
Abstract: Random amplified polymorphic DNA assays were carried out on a range of isolates of F. poae to identify markers common to all isolates. Two fragments were isolated, cloned and used to probe Southern blots of DNA from F. poae and isolates from a range of wheat seed and stem base pathogens. One fragment, which hybridized preferentially to DNA of F. poae was partially sequenced and two pairs of primers (Fp8 F/R and Fp82 F/R) were generated for use in the polymerase chain reaction (PCR). Amplification of target DNA occurred following PCR of all isolates of F. poae but not from any of a range of other fungal species associated with diseases of cereal ears and seed. The primer pair Fp82 F/R was used to detect F. poae in extracts from wheat seed samples contaminated with Fusarium species. This system offers the potential to determine the presence of F. poae in wheat and avoid problems commonly associated with conventional diagnosis of the disease and isolation of the pathogen.
221 citations
TL;DR: The species-specific primer (CaInt2) developed in this work could be used for the accurate identification of C. acutatum and its detection on other host plants.
Abstract: An oligonucleotide primer (CaInt 2) was synthesized from the variable internal transcribed spacer (ITS) 1 region of ribosomal DNA (rDNA) from Colletotrichum acutatum PCR with primers CaInt2 and ITS4 (from a conserved sequence of the rDNA) amplified a 490 bp fragment from several isolates of C acutatum but not from other members of the genus Colletotrichum Amplification of this fragment was achieved from 100 fg of fungal DNA These primers amplified a fragment of the same size from DNA extracted from strawberry tissues infected by C acutatum Southern hybridization analysis confirmed the 490 bp fragment from C acutatum DNA and infected strawberry to be identical The species-specific primer (CaInt2) developed in this work could be used for the accurate identification of C acutatum and its detection on other host plants
169 citations
TL;DR: Variation for virulence was examined amongst 20 field collections of Plasmodiophora brassicae from France and the fractionation of different individual pathotypes from one original spore suspension confirmed the genetic heterogeneity of field populations of P. Brassicae.
Abstract: Variation for virulence was examined amongst 20 field collections of Plasmodiophora brassicae from France. Out of the 10 brassica lines tested, seven reacted differentially to inoculation; of these, two oilseed rape cultivars exhibited previously unreported differential responses. Some of the differential lines used previously to classify pathotypes of P. brassicae were susceptible to all collections, suggesting that pathogen populations in France may be different from those reported elsewhere. Good pathotype discrimination was obtained using a set of three cultivars of Brassica napus. Five pathotypes, P 1 , P 2 , P 3 , P 4 and P 5 , were detected and their occurrence was unrelated to host type. Pathogenic variation amongst 17 single-spore isolates derived from three field collections was studied, and five pathotypes were identified. Four isolates were classified as pathotype P 1 , pathogenic on all three differential hosts, and eight as pathotype P 4 , pathogenic on none of the three differentials. The five other isolates were classified as pathotypes P 3 , P 6 and P 7 , the latter two expressing patterns of reaction not observed for field collections. The fractionation of different individual pathotypes from one original spore suspension confirmed the genetic heterogeneity of field populations of P. brassicae.
155 citations
TL;DR: The results indicate that the effectiveness of fungal suppression by Brassica crops will depend upon the species, age and type of Brassica tissue, which influence the type and concentration of isothiocyanates evolved, and the sensitivity of the pathogen.
Abstract: The superior growth of wheat following Brassica crops compared to that following non-Brassica crops may be due to the suppression of soilborne fungal pathogens by volatile isothiocyanates (ITCs) released in the soil during hydrolysis of glucosinolates contained in Brassica tissues. We investigated the effects of volatile compounds released from the root, shoot and seed meal tissues of canola (Brassica napus) and Indian mustard (Brassica juncea) on the mycelial growth of five soilborne pathogens of cereals—Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Pythium irregulare and Bipolaris sorokiniana. Three isolates of each species, originally collected from the roots of wheat (Triticum aestivum) and barley grass (Hordeum leporinum ) in southern Australia, were exposed to volatiles released in vitro when sterile water was added to freeze-dried Brassica tissues. The root and shoot tissues of both Brassica species were more suppressive at flowering than maturity and mustard tissues were generally more suppressive than canola. The degree of fungal suppression by the various Brassica tissues was related to the concentration and type of isothiocyanates released, which varied with Brassica species, tissue age and tissue type. There were significant differences in the sensitivity of the fungal species and among isolates of each species. Gaeumannomyces and Rhizoctonia were generally the most sensitive to the volatiles released, Pythium and Bipolaris the least. The results indicate that the effectiveness of fungal suppression by Brassica crops will depend upon the species, age and type of Brassica tissue, which influence the type and concentration of isothiocyanates evolved, and the sensitivity of the pathogen.
155 citations
TL;DR: All strains which degraded resveratrol and pterostilbene were highly or moderately pathogenic to in vitro cultures of grapevines after inoculation with agar disks containing mycelium, while those which were unable to degrade phytoalexins were non-pathogenic.
Abstract: The ability of eight isolates of Botrytis cinerea to degrade the stilbene phytoalexins, resveratrol and pterostilbene, was compared with their pathogenicity to grapevines. All strains which degraded resveratrol and pterostilbene were highly or moderately pathogenic to in vitro cultures of grapevines (Vitis rupestris) after inoculation with agar disks containing mycelium, while those which were unable to degrade phytoalexins were non-pathogenic. In all cases, the hydroxystilbene-degrading activity was related to the presence of laccase activity in the culture filtrates, as shown by using syringaldazine as substrate. The role of laccase-mediated degradation of phytoalexins in relation to pathogenicity of B. cinerea to grapevines in discussed.
117 citations
TL;DR: Nine races of Pseudomonas syringae p.s. pv.
Abstract: Isolates of Pseudomonas syringae pv. phaseolicola from Africa and other bean growing areas were categorized into nine races on the basis of their reactions to eight differential cultivars following artificial inoculation. Eight hundred and ninety-three isolates representing 303 disease occurrences were initially identified as P.s. pv. phaseolicola by their pathogenicity to bean, cultural and serological characteristics and phage sensitivity. These tests also served to distinguish P.s. pv. phaseolicola from the closely related pathovars P.s. pv. glycinea and P.s. pv. syringae. Detailed race determinations were carried out on 175 selected isolates of p.s. pv. phaseolicola representative of the different geographical regions and hosts in which the pathogen was found and nine races were identified. A number of races (1,2,5,6 and 7) were distributed worldwide with race 6 predominant. Other races were found mainly in Africa; races 3 and 4 in East/Central Africa and races 8 and 9 in Southern Africa. Most isolates were obtained from the major host, Phaseolus vulgaris. Alternative natural hosts included 10 legume species representative of seven different genera (Cajanus cajan, Desmodium sp., Lablab purpureus, Macroptilium atropurpureum, Neonotonia wightii, Phaseolus acutifolius, P. coccineus, P. lunatus, Vigna angularis and V. radiata). Of these, Desmodium sp. constitutes a new host record.
117 citations
TL;DR: A collection of 124 isolates of turnip mosaic virus was gathered from around the world, principally from European countries, and characterized by inoculation to four differential lines of Brassica napus, finding no pathotypes confined to any geographical area.
Abstract: A collection of 124 isolates of turnip mosaic virus was gathered from around the world, principally from European countries, and characterized by inoculation to four differential lines of Brassica napus (oilseed rape and swede). Three symptom phenotypes were induced-apparent immunity, local infection only, or systemic infection. Twelve distinct patterns, i.e. pathotypes, were observed. Three pathotypes were predominant in the collection: pathotype 1 isolates, which were the most common, did not overcome any of the most extreme sources of resistance in the differential lines. Of the other two, pathotype 3 isolates overcame one of the major sources of resistance and pathotype 4 isolates overcame all sources of resistance. The distribution of pathotypes within Europe was examined. No pathotype was confined to any geographical area, although pathotype 4 isolates were not found in southern Europe or Asia. Most isolates (90) originated from Brassica hosts, while others were from other cruciferae genera (19) or non-crucifers (5). The species of plant that the isolates originated from was not clearly related to the pathotype of the isolates. Resistance to pathotype 1 isolates is controlled by a dominant allele in one of the differential lines, and resistance sources are being examined in the other lines. Isolates belonging to pathotype 1 appeared to be able to mutate readily to overcome the resistance in one of the rape differential lines, but no isolates appeared to mutate to overcome the other major source of resistance in the differentials. The implications of the results for disease control strategies are discussed.
104 citations
TL;DR: Random amplified polymorphic DNA assays were carried out on a range of isolates of Rhizoctonia cerealis to identify markers common to all isolates, offering the potential to detect the presence of R. cerealis in cereals and avoid problems commonly associated with conventional diagnosis of this disease and isolation of the pathogen.
Abstract: Random amplified polymorphic DNA assays were carried out on a range of isolates of Rhizoctonia cerealis to identify markers common to all isolates. Two fragments were isolated, cloned and used to probe Southern blots of DNA from R. cerealis and isolates from a range of anastomosis groups of Rhizoctonia solani. The two fragments hybridized specifically to DNA of R. cerealis and not to DNA of any of the isolates of R. solani. Both fragments were partially sequenced and two pairs of primers were generated for use in the polymerase chain reaction (PCR). Amplification of a fragment of the anticipated size occurred following PCR of all isolates of R. cerealis and not from any of a range of fungal species associated with disease of the stem base of cereals. The primer pairs for R. cerealis were also used, along with those for Microdochium nivale and W and R-type of Pseudocercosporella herpotrichoides, to detect these pathogens in extracts from field-grown wheat plants exhibiting symptoms of sharp eyespot, eyespot, foot rot or a combination of the diseases. No relationship was found between visual disease assessment of sharp eyespot at growth stage 37 and the results of PCR. However, the results of PCR and visual disease assessment at growth stage 75 were similar, indicating that visual disease assessment may not be reliable until later growth stages. This system offers the potential to detect the presence of R. cerealis in cereals and avoid problems commonly associated with conventional diagnosis of this disease and isolation of the pathogen.
94 citations
TL;DR: The incidence of wilt was recorded in runner and fruiting crops of 13 strawberry cultivars at 72 locations in southern England in 1989 and 1990, and soil samples from the sites were analysed for Verticillium dahliae to estimate an inoculum concentration which corresponds to 5% wilt incidence for cv.
Abstract: The incidence of wilt was recorded in runner and fruiting crops of 13 strawberry cultivars at 72 locations in southern England in 1989 and 1990, and soil samples from the sites were analysed for Verticillium dahliae. Linear regressions of wilt incidence on inoculum concentration in soil for runner crops of the susceptible cv. Elsanta in both years were significant whilst that for runner crops of the susceptible cv. Hapil in 1989 approached significance; the regression for cv. Elsanta fruiting crops in 1990 was not significant. The inclusion of sand content of soil in the regression model improved the fit for the cv. Hapil data but not for the cv. Elsanta data; neither clay nor silt content of soil significantly improved the fit of the models for any data set. There were insufficient data in either year for regression analysis for other cultivars, but the levels of wilt generally corresponded with the degree of soil infestation and broadly reflected known field resistance. The data were used to estimate an inoculum concentration which corresponds to 5% wilt incidence (IC 5 ) for cv. Elsanta. It is suggested that this could be used as a yardstick for determining the risk of unacceptable levels of wilt in susceptible cultivars on the basis of pre-planting soil analysis. For the 44 sites where the cropping history over the 15 years prior to soil analysis was available there was no clear association between any crop and soil infestation levels at or above the IC 5 . However, V. dahliae was more common at sites with a history of vegetatively propagated crops than at sites which had only supported crops grown from true seed.
83 citations
TL;DR: A critical review of available historical and scientific evidence related to the question of the origin of the first European populations of the potato late blight pathogen Phytophthora infestans shows that the bases for the current theories of a direct introduction of the fungus into Europe and North America from either a Mexican or an Andean centre of origin and of diversity are questionable.
Abstract: A critical review of available historical and scientific evidence related to the question of the origin of the first European populations of the potato late blight pathogen Phytophthora infestans is presented. It shows that the bases for the current theories of a direct introduction of the fungus into Europe and North America from either a Mexican or an Andean centre of origin and of diversity are questionable. An alternative theory, involving a three-step process: (i) migration from central Mexico to South America several centuries ago; (ii) migration from South America to the US in 1841-1842, and (iii) migration to Europe from either South America, US or both in 1843-1844, is in good agreement with both the historical records and with the genetics and structure of current populations.
TL;DR: The hypothesis that the effect of heating on the decay of apples caused by P. expansum is not only the result of direct inhibition of fungal germination and growth by high temperature, but is also partly due to the formation of an inhibitory substance in the heated peel is supported.
Abstract: Penicillium expansum is one of the main postharvest pathogens of apples in Israel. Heating apple fruit inoculated with P. expansum for 96 h at 38°C completely inhibited decay development. Fruit held for 24 h at 42°C or 12 h at 46°C had significantly reduced decay after an additional 14 days incubation at 20°C, compared with unheated inoculated control fruit. Mycelial growth and percentage spore germination in vitro were inversely proportional to length of time of exposure to various temperatures. The ET50 for spore germination was 42, 34 and 20 h at 38, 42 and 46°C, respectively, while the ET50 for mycelial growth was 48, 44 and 36 h at those temperatures. When Penicillium spores were incubated on crude extract prepared from the peel of apple fruits held 4 days at 38°C, germ tube elongation was significantly reduced, while the walls of the tubes were thicker, compared with germ tubes from spores incubated on crude extract prepared from peel of non-heated fruit. The evidence presented here supports the hypothesis that the effect of heating on the decay of apples caused by P. expansum is not only the result of direct inhibition of fungal germination and growth by high temperature, but is also partly due to the formation of an inhibitory substance in the heated peel.
TL;DR: Cultural methods are described which aid rapid differentiation of four distinct taxa of T. harzianum and five other species from the mushroom environment and some difficulties are discussed concerning differentiation of Trichoderma spp.
Abstract: A major epidemic of green mould in mushroom compost in Northern Ireland lasting for 6 months was caused by a single taxon of Trichoderma harzianum (Th2). This has continued to be the principal taxon causing losses in Ireland over a 9-year period. A separate taxon, Th4, is responsible for a more recent epidemic in North America. Cultural methods are described which aid rapid differentiation of four distinct taxa of T. harzianum and five other species from the mushroom environment. After developing a standard cultural method, isolates were grouped visually by features such as growth rate at 27°C, amount of aerial mycelium, and the effect of light on sporulation form and timing. Most of these groups of isolates had significantly different growth rate ratios from each other at 27°C/17°C. Furthermore isolates within groups which had been separated by culture features alone were closely homologous as regards their microscopic morphological features such as phialides and phialospores. Two factors underlining the importance of using a standard cultural method in aiding identification were that culture morphology was shown to vary widely on different media and spore size was found to vary significantly with incubation temperature. Some difficulties are discussed concerning differentiation of Trichoderma spp. using classical microscopic features alone.
TL;DR: It is suggested that, since levels of resistance already available within P. vulgaris provide adequate protection, bean breeders should resist the temptation to incorporate new complete resistance from P. acutifolius as it would risk destabilizing the host–bacterium relationship by introducing race-specific resistance that is more likely to prove transient in agriculture.
Abstract: A total of 93 isolates of Xanthomonas campestris pv. phaseoli was collected from five countries over a wide range of altitude (500–2320 m above sea level) in eastern Africa. Collections were made from a range of cultivars of the common bean (Phaseolus vulgaris), including known sources of resistance, as well as from wild alternative but symptomless hosts including Senna (Cassia) hirsuta and Digitaria scalarum. From these, 30 isolates were selected for detailed characterization. Although variation was found in parameters including phage type, the production of brown pigment in culture media and in-vitro growth rate, there was little consistent relationship between these characters and pathogenic variation.
Studies of pathogenic variation among the 30 isolates on 20 genotypes of P. vulgaris revealed quantitative host non-specific differences (aggressiveness). On tepary bean (P. acutifolius), however, the 30 isolates interacted differentially with the genotypes and eight distinct physiologic races of X. c. pv. phaseoli were defined, suggestive of an underlying gene-for-gene relationship.
Despite this apparent gene-for-gene interaction, resistance to common bacterial blight in P. vulgaris, derived from earlier use of P. acutifolius, has apparently remained non-specific and essentially durable. It is suggested that, since levels of resistance already available within P. vulgaris provide adequate protection, bean breeders should resist the temptation to incorporate new complete resistance from P. acutifolius as it would risk destabilizing the host–bacterium relationship by introducing race-specific resistance that is more likely to prove transient in agriculture.
TL;DR: Using a qualitative imprint method, it was confirmed that the more resistant the genotype, the lower the bacterial colonization of the stem, which formed the basis for a model for predicting the degree and stability of resistance in tomato.
Abstract: Different criteria were compared for assessing bacterial wilt resistance in 13 tomato genotypes varying in disease susceptibility. Wilt severity and bacterial invasiveness at collar and midstem were compared in the field under cooler (March to May, 20-28°C) and warmer months (June to August, 23-29°C), which were unfavourable and favourable to wilt symptom expression, respectively. A model was proposed for determining resistance regardless of climatic conditions prevalent during field experimentation. This model was based on an estimate of bacterial invasiveness termed the colonization index. Using a qualitative imprint method we confirmed that the more resistant the genotype, the lower the bacterial colonization of the stem. The colonization index accounted both for wilted plants and for infected asymptomatic plants in which Pseudomonas solanacearum populations failed to produce wilt. The colonization index at midstem was the more useful indicator of resistance under favourable conditions. When environmental conditions were unfavourable to wilt, colonization index at collar level discerned resistant genotypes more clearly. The results formed the basis for a model for predicting the degree and stability of resistance in tomato.
TL;DR: The extent of isozyme and RAPD-PCR polymorphisms found in Fusarium strains potentially provides a method for identifying the fungi both at strain and species level.
Abstract: Differences in isozyme and RAPD-PCR polymorphisms amongst 33 isolates of Fusarium avenaceum were compared using native polyacrylamide gel electrophoresis and agarose gel electrophoresis. The isolates were collected from different regions of Finland. Amongst eight enzymes analysed clear isozyme polymorphism was detected in five enzymes which could be grouped into 20 different electrophoretic phenotypes and three main groups at the similarity level of 70% in unweighted pair group method with arithmetic average (UPGMA) analysis. RAPD-PCR analysis differentiated all F. avenaceum strains from each other. The phenotypes resulting from RAPD-PCR analysis were grouped into five main groups by UPGMA analysis at the similarity level of 55%. These main groups had several similarities with the main groups from isozyme analysis. RAPD-PCR patterns of 16 isolates of Fusarium graminearum, F. culmorum, F. equiseti, F. oxysporum and F. redolens were also studied and strains from each Fusarium species formed individual groups in UPGMA and principal components analyses. Thus, the extent of isozyme and RAPD-PCR polymorphisms found in Fusarium strains potentially provides a method for identifying the fungi both at strain and species level.
TL;DR: Higher temperature increased infection of flowers but reduced infection of stem wounds, and whilst flower infection increases, this is counteracted by increased flower production and a decrease in the proportion of infections reaching the peduncle and stem.
Abstract: Botrytis cinerea causes serious crop losses in greenhouse tomato crops through infection of flowers and stem wounds. Experiments were carried out to determine the effects of inoculum concentration, relative humidity (RH), and temperature at these two infection sites. Infection of permanent flower parts increased as a function of inoculum concentration and both length of exposure to high RH (approximately 100% for 0–36 h) and specified continuous RH (56–100%). A low level of infection was still evident under continuous 56% RH. Interruption of periods of high RH with breaks of low RH did not reduce infection. Infection of stem wounds was less dependent on inoculum concentration or RH. Factorial combinations of inoculum concentration, RH, and temperature produced significant interactions. Higher temperature increased infection of flowers but reduced infection of stem wounds. The main implications for control in commercial crops are as follows. Lowering the aerial spore concentration by maintaining the disease at a low level will reduce flower infection. Lowering RH will reduce but not eliminate flower infection but will have only a small effect on stem infection. Raising the temperature (from 15 to 25°C) will reduce stem infection, and whilst flower infection increases, this is counteracted by increased flower production and a decrease in the proportion of infections reaching the peduncle and stem.
TL;DR: The extent of pathogenic variation encountered amongst sexually reproducing rusts suggests that more pathotypes probably exist and will arise in future in response to selection given by long-term clonal willow plantings.
Abstract: Eighteen isolates of the rust fungus Melampsora from different locations in England were tested for pathogenicity to a large range of willow clones (Salix spp.) in experiments involving inoculation of leaf discs. Seventeen of the isolates were of leaf-infecting M. epitea var. epitea, 16 of which represented forms which alternated on Larix and one of which alternated on Ribes. The remaining isolate was of uncertain identity. Two experiments were carried out. In the first experiment, 24 willow clones consisting of 20 Salix species and interspecific hybrids were inoculated with eight isolates from clones of S. viminalis. In the other experiment, 77 clones from 57 species or hybrids were inoculated with 10 isolates from several Salix spp. The M. epitea var. epitea isolates from S. viminalis clones were all similarly pathogenic, whilst most of the other isolates expressed distinct host specificity. Eight distinct pathotypes were recognized within M. epitea var. epitea. All except one of these alternated on Larix and could be assigned to three formae speciales which had been reported previously in Europe: four pathotypes to f.sp. larici-epitea typica, two to f.sp. larici-retusae, and one to f.sp. larici-daphnoides. The Ribes-alternating pathotype of M. epitea var. epitea infected only S. purpurea. Nine differential willow hosts are proposed as reference clones to distinguish between the larch-alternating pathotypes, the Ribes-alternating rust and the 'stem-infecting' form. The extent of pathogenic variation encountered amongst sexually reproducing rusts suggests that more pathotypes probably exist and will arise in future in response to selection given by long-term clonal willow plantings.
TL;DR: A likelihood-based method for fitting spatio-temporal stochastic models for the spread of a plant disease to experimental observations is proposed and it is demonstrated that a model in which this relationship is a power-law is superior to one which uses a negative exponential.
Abstract: We propose and illustrate a likelihood-based method for fitting spatio-temporal stochastic models for the spread of a plant disease to experimental observations. The models considered are individual-based, with members of the population occupying discrete sites on a two-dimensional lattice. The disease is assumed to be characterized by presence/absence, and infection of susceptible individuals by infected individuals is represented as a stochastic process. The method described can be applied to estimate parameters in models of this kind when observations consisting of temporal sequences of disease maps are available. The use of measures of spatial aggregation as measured from simulated and real epidemics is proposed as a means of assessing the relative merits of alternative models for the spread of a disease. To illustrate the technique we fit and compare two models, which differ in the relationship between infective pressure and distance, to observations of an epidemic of citrus tristeza virus (CTV). It is demonstrated that a model in which this relationship is a power-law is superior to one which uses a negative exponential and the importance of model choice for the design of control strategies is discussed briefly.
TL;DR: The present study assayed the effect of six isothiocyanates (ITCs), produced by the enzymatic hydrolysis of glucosinolates, on fungal pathogens of pear, finding the highest effectiveness came from glucoraphenin, which also afforded pathogen control at the highest level of pathogen concentration.
Abstract: The present study assayed the effect of six isothiocyanates (ITCs), produced by the enzymatic hydrolysis of glucosinolates, on fungal pathogens of pear. Sample pear fruits were artificially inoculated through induced wounds with conidial suspensions of Botrytis cinerea, Rhizopus stolonifer, Monilinia laxa, Mucor piriformis or Penicillium expansum and were then treated with ITCs. Of the six ITCs tested, the ITC from glucoraphenin showed the highest effectiveness after 6 days at 20°C, against M. laxa, B. cinerea and M. piriformis. The effectiveness of the ITC from glucoraphenin against M. laxa was assayed in two further trials to test the effect of ITC concentration on different concentrations of inoculum and to determine the duration of the curative effect of this ITC. ITC concentration directly affected fungus control capacity. The highest ITC concentration (3.6 mg mL -1 ) afforded pathogen control at the highest level of pathogen concentration (10 6 conidia mL -1 ) after 6 days at 20°C. Its curative effect was evident up to 40 h after inoculation.
TL;DR: Fitness of 31 isolates of B. cinerea was determined in vivo by measuring their sporulation and lesion growth rate on leaf disks and no fitness costs were associated with resistance to iprodione (dicarboximide) and benomyl (benzimidazole).
Abstract: Forty-nine greenhouses of vegetable crops were surveyed in southeast Spain at the beginning of the disease season in December 1992 to estimate frequencies of resistance to benzimidazoles, dicarboximides and N-phenylcarbamates (NPC) in B. cinerea. Out of 261 isolates collected, 28% were sensitive to both benzimidazoles and dicarboximides, 15% were benzimidazole-resistant and dicarboximide-sensitive, 8% were benzimidazole-sensitive and dicarboximide-resistant and 46% were benzimidazole- and dicarboximide-resistant. Resistance to benzimidazole, dicarboximide and N-phenylcarbamate was determined by measuring the ability of each isolate to grow in the presence of carbendazim, procymidone and diethofencarb fungicides respectively. Carbendazim- or procymidone-resistant isolates were found in all surveyed greenhouses. Three isolates were found with resistance to carbendazim, procymidone and diethofencarb collected in two adjacent greenhouses that were sprayed with the carbendazim and diethofencarb mixture. All other isolates were sensitive to the mixture because they were either sensitive to carbendazim and resistant to diethofencarb or vice versa. Fitness of 31 isolates of B. cinerea was determined in vivo by measuring their sporulation and lesion growth rate on leaf disks. No fitness costs were associated with resistance to iprodione (dicarboximide) and benomyl (benzimidazole). Isolates with EC 50 values higher than 101 mg/L for benomyl and 1.6 mg/L for iprodione were considered to be field resistant (they caused visible lesions on cucumber leaf disks treated with each fungicide).
TL;DR: Two short sequences, situated in the bacterial 23S rDNA gene, were used as primers for the PCR detection of Erwinia amylovora bacteria, and reliable PCR detection in woody host plant tissue was only obtained with PVP/PVPP-treated sample extracts.
Abstract: Two short sequences, situated in the bacterial 23S rDNA gene, were used as primers for the PCR detection of Erwinia amylovora bacteria. All 34 E. amylovora strains tested, coming from different geographical and host plant origins and of different virulence, produced a 565-bp PCR fragment. The E. amylovora bacteria could be discriminated from all other phytobacteria with which no PCR product was observed. Only Escherichia coli bacteria were cross-recognized by the production of a weaker PCR band of similar size to E. amylovora. In a fast PCR protocol, where two temperatures were cycled, E. amylovora in pure culture could be detected on gel at concentrations as low as 3 x 10 2 cfu mL -1 . This corresponds to a detection limit of 1.5 bacteria per PCR. However, reliable PCR detection in woody host plant tissue was only obtained with PVP/PVPP-treated sample extracts. Using E. amylovora-spiked plant extracts and extracts of fruit tree shoots artificially infected with E. amylovora, the PCR detection sensitivity was determined to be 6.6 x 10 2 cfu mL -1 of extract. Starting from the plant samples, the PCR detection results were visualized on gels within 5 h.
TL;DR: The mode of action of hot water dips on decay of pepper appears to be by direct interaction with the fungi, inversely related to the duration of exposure or to the range of temperature used.
Abstract: The effectiveness of hot water dipping on the control of grey mould, caused by Botrytis cinerea, and black mould, caused by Alternaria alternata on sweet red pepper quality was investigated. Dipping naturally infected or artificially inoculated fruit at 508C for 3 min completely inhibited, or significantly reduced, decay development caused by B. cinerea and A. alternata, respectively. Heat damage was observed on fruit dipped for 5 min at 50°C, or at 55°C for 1 min or longer. Damage appeared as cracks and pitting on the fruit surface. Spore germination and germ tube elongation in vitro was inversely related to the duration of exposure or to the range of temperature used. The ET50 for spore germination for B. cinerea was 3.2, 1.5 and 0.8 min, and for A. alternata 8.8, 4.2 and 1.4 min, at 45, 50 and 55°C, respectively. The ET50 for germ tube elongation for Botrytis was 2.6, 0.9 and 0.5 min, and for Alternaria, 7.2, 2.5 and 1.6 min, at 45, 50 and 55°C. The mode of action of hot water dips on decay of pepper appears to be by direct interaction with the fungi.
TL;DR: There is a strong relationship between the presence of the fas-1 gene and virulence of the organism.
Abstract: Thirty-six isolates of the fasciation-inducing bacterium Rhodococcus fascians were evaluated for the presence and location of the fas-1 gene, which codes for an isopentenyl transferase, the committed step in cytokinin biosynthesis. The presence of fas-1 was determined by PCR using a set of primers to the most conserved regions of the gene and by Southern hybridization to genomic digests using the PCR fragment as a probe. Both methods revealed the presence of the gene in 18 virulent isolates and the absence of the gene in 18 avirulent isolates. Thus, there is a strong relationship between the presence of the gene and virulence of the organism. The location of fas-1 was determined by probing blots of linear and circular DNA. For most of the virulent isolates, the gene was localized to a 200 ± 10 kb linear plasmid. Three virulent isolates lacked a plasmid of this size, but contained fas-1 either on a linear plasmid of 130 kb or on a large circular plasmid.
TL;DR: Spore trapping showed that both pycnospore dispersal and ascospore discharge were initiated by rainfall or dew, suggesting that ascospores also play an important role in secondary infections.
Abstract: The pattern of development of pycnidia and perithecia of Mycosphaerella pinodes was studied in the glasshouse on pea plants (cv. Solara) sprayed with a pycnospore suspension and in field plots inoculated with barley grains colonized by the fungus. The numbers of pycnidia and perithecia were estimated on each stipule and internode of infected plants, and were related to ratings of disease severity (0–5 scale). Pycnidia were produced on both green and senescent organs, whereas perithecia only appeared on senescent organs. The development and quantity of pycnidia were related to initial inoculum concentration and the physiological stage of the plants. The formation of fruiting bodies progressed from the bottoms to the tops of plants during crop development. Spore trapping showed that both pycnospore dispersal and ascospore discharge were initiated by rainfall or dew. Pycnospores were principally trapped in the first 20 cm above the soil surface while ascospores were also trapped above the crop canopy. Pycnospores and ascospores were dispersed throughout the growing season, suggesting that ascospores also play an important role in secondary infections.
TL;DR: Soilborne inoculum of R. solani produced economically significant levels of stem canker and its incidence and severity varied with rotation, with most disease in 2-year and less in 4- and 6-year rotations.
Abstract: Bait plants, comprising micropropagated and commercial seed tubers, were used to estimate the effects of rotation on the density and spatial pattern of inoculum of Rhizoctonia solani in large field plots of potatoes. Soilborne inoculum of R. solani produced economically significant levels of stem canker and its incidence and severity varied with rotation, with most disease in 2-year and less in 4- and 6-year rotations. The rates of loss of inoculum during intercrop periods differed amongst rotations with a rapid fall to low levels occurring after 1 year in a 6-year rotation and after 2 years in a 4-year relation. Replenishment of inoculum to soil was rapid following the growth of a susceptible crop, with comparatively high levels of infection and disease, even in long rotations. Disease occurred in patches and the size of patches and the density of R. solani within patches differed with cropping frequency. The degree of spatial autocorrelation also differed amongst rotations but there was no evidence for any significant differences in the rate of change of spatial autocorrelation during intercrop periods in the three rotations.
TL;DR: Erysiphe pisi, the powdery mildew pathogen of Pisum sativum, followed a developmental sequence that allowed the identification of 10 distinct growth stages (GS) over 30 h following inoculation, which showed considerable variation in structure.
Abstract: Erysiphe pisi, the powdery mildew pathogen of Pisum sativum, followed a developmental sequence that allowed the identification of 10 distinct growth stages (GS) over 30 h following inoculation. The growth stages were ungerminated conidia (GS1), germinated conidia, having produced a germ tube (GS2), germlings where the germ tube had forked (GS3), germlings with a multi-lobed germ tube (GS4), germlings with a single hypha (GS5), germlings with two (GS6 and 7) or three (GS8 and 9) hyphae, one of which may have formed from the appressorium (GS7 and 9), and germlings with abnormally long germ tubes (GS10), which did not develop hyphae. Conidia germinated rapidly, with a quarter of conidia producing germ tubes by 2 h after inoculation (hai). Most germlings produced multi-lobed appressoria, which showed considerable variation in structure. Haustoria, although often difficult to visualize, were first seen 4 hai, and the first hyphae 14 hai, growing from the body of the conidium. Subsequent hyphae developed from both the body of the conidium and from the appressorium.
TL;DR: It is concluded that much more information is required about individual breeding systems, gene flow, ecology, phylogeny and distribution before informed decisions about the delimitation of most species can be made.
Abstract: Inadequate species definitions present a serious problem to the pathologist working in plant hygiene or quarantine which demands urgent attention. Species concepts in the downy mildews have not kept pace with developments in evolutionary and molecular biology, or with advances in ecological genetics, because the downy mildews are obligate biotrophs that are not easily cultured in the laboratory. Existing approaches to species concepts in this group (morphometric, Ga¨umann’s ‘biological species’ and Skalicý’s ‘eco–physio–phentic’ concepts) are examined and found to be inadequate and potentially misleading. The systematic treatment of the downy mildews is beginning to benefit from the application of modern methods of systematic analysis. The contribution and potential of ultrastructure, karyotyping, sterol and fatty acid composition, isoenzyme patterns, molecular biology, numerical methods, immunoassay and hypotheses of coevolution to the development of species concepts are reviewed and their wider application is seen as a priority. The application to the downy mildews of two widespread species concepts, the biological and phylogenetic concepts, is examined in the light of the information gained from modern methods of analysis, but neither is found adequate to describe species of downy mildews as they occur in nature. Modern methods can suggest phylogenetic relationships on the basis of statistical probabilities and may also detect microevolutionary change, but it is concluded that much more information is required about individual breeding systems, gene flow, ecology, phylogeny and distribution before informed decisions about the delimitation of most species can be made. Until patterns of genetic diversity can be established, a modified phylogenetic species concept may offer one interim solution to the problem of species definition in the downy mildews.
TL;DR: It is suggested that the combination of race specific and race non-specific resistance could provide an effective strategy for establishing durable resistance.
Abstract: One thousand and forty-eight Phaseolus bean accessions were evaluated for resistance to six races of Pseudomonas syringae pv. phaseolicola. The accessions originated from regions of the Americas and Africa where the disease is important and included wild type accessions and some known resistance sources. Resistance, graded on a five-point scale, was of two types: qualitative, which was shown to be race-specific, and quantitative. Race specific resistance genes (R-genes) were detected in 49.4% of accessions with the following gene frequencies: R1 (10.3%), R2 (0.3%), R3 (25.0%), R4 (35.0%) and R5 (0.2%).
Evidence for quantitative variation in resistance, in the absence of specific R-genes, was shown by the distribution of infection scores, 76% of accessions showing maximum susceptibility (grades 4–5), 23% showing intermediate resistance (grades 2–4), and 1% showing high levels of quantitative resistance (grades 1–2). The last 1% of accessions showed interactions which were not race-specific and it is suggested that they may possess race non-specific resistance. It is possible that several of the accessions in this category carry the recessive gene derived from PI 150414. Other accessions were of unknown parentage and may represent new sources of quantitative, potentially race non-specific, resistance. It is suggested that the combination of race specific and race non-specific resistance could provide an effective strategy for establishing durable resistance.
TL;DR: In 9 years of advisory experience in the British Isles and in virulence tests in the laboratory and field, only one of three distinct taxa of T. harzianum, namely Th2, consistently colonized mushroom compost either in vitro or in mushroom houses.
Abstract: In 9 years of advisory experience in the British Isles and in virulence tests in the laboratory and field, only one of three distinct taxa of T. harzianum, namely Th2, consistently colonized mushroom compost either in vitro or in mushroom houses. A single particle of contaminated spawn initiated colonies up to 30 cm in diameter. Four other Trichoderma species common in the mushroom environment rarely caused problems. Circumstantial evidence suggested airborne dust to be a main source of contamination of compost or its packing machinery, along with transmission on workers' clothing, pallets, load covers, trailers and by vectors such as mites, mushroom flies and mice. The Th2 isolate of T. harzianum was recovered from all of these sources but, in two tests, it did not survive peak heating. The control measures described here, based on strict hygiene, proved effective. Timing of colonization was important, also temperature of spawn run. Type of compost, as judged by analysis, appeared not to be important. In the field, spawning with peat based 'spawn' instead of grain spawn failed to prevent colonization, whereas in vitro, it reduced it. In the absence of any spawn, Th2 only grew strongly in microwaved or autoclaved compost. Reasons for this are discussed.