Investigation into components of partial disease resistance,determined <Emphasis Type="Italic">in vitro</Emphasis>, and the concept of types of resistance to <Emphasis Type="Italic">Fusarium</Emphasis> head blight (FHB) in wheat

This work presents an analysis of the relationship between components of partial disease resistance (PDR) detected using in vitro detached leaf and seed germination assays, inoculated with Microdochium majus, and Fusarium head blight (FHB) resistance to Fusarium graminearum assessed using point inoculation, termed Type II resistance. Relationships between in vitro-determined PDR components and FHB resistance using techniques which inoculate the wheat spike uniformly, termed Type I resistance (incidence and severity), have been reported previously. In this study shorter incubation periods, longer latent periods and shorter lesion lengths in the detached leaf assay and higher germination rates in the seed germination assay were related to greater FHB resistance measured by single point inoculation (Type II), collectively explaining 54% of the variation. Overall the relationships observed for Type II FHB resistance were similar to previous findings for Type I resistances. However, the relative magnitude of effects of the individual PDR components determined in vitro varied between FHB disease resistance parameters. Resistance in seed germination and latent period in the detached leaf assay were more strongly related to resistance assessed by point inoculation (Type II) and severity-Type I as opposed to incubation period which was most strongly related to disease incidence-Type I. The results provide evidence that individual components of partial disease resistance differentially affect aspects of FHB disease progression in the wheat spike. This work supports the view that the current model of types of resistance is an oversimplification of the interacting mechanisms underlying expression of FHB resistance.     

Development and Evaluation of an in vitro Detached Leaf Assay forc Pre-Screening Resistance to Fusarium Head Blight in Wheat

An in vitro detached leaf assay, involving the inoculation of detached leaves with Microdochium nivale, was further developed and used to compare with whole plant resistance ratings to Fusarium head blight (FHB) of 22 commercial cultivars and published information on 21 wheat genotypes, identified as potential sources for FHB resistance. An incubation temperature of 10 °C and isolates of M. nivale var. majus of intermediate pathogenicity were found to be the most suitable for the differential expression of several components of partial disease resistance (PDR), namely incubation period, latent period and lesion length, in wheat genotypes used in the detached leaf assay. There were highly significant differences (P < 0.001) for each component of PDR within commercial cultivars and CIMMYT genotypes. Positive correlations were found between incubation period and latent period (r = 0.606; P < 0.001 and r = 0.498; P < 0.001, respectively, for commercial cultivars and CIMMYT genotypes), inverse correlations between incubation period and lesion length (r = -0.466; P < 0.01 and r = –0.685; P < 0.001, respectively) and latent period and lesion length (r = –0.825; P < 0.001 and r = –0.848; P < 0.001, respectively). Spearman rank correlations between individual PDR components and UK 2003 recommended list ratings were significant for incubation period (r_s = 0.53; P < 0.05) and latent period (r_s = 0.70; P < 0.01) but not for lesion length (r _s = –0.26). Commercial cultivars identified with high resistances across all three PDR components in the detached leaf assay also had high whole plant FHB resistance ratings, with the exception of cv. Tanker which is more susceptible than the results of the detached leaf assay suggested, indicating an additional susceptibility factor could be present. Agreement between resistances found in the detached leaf assay and resistance to FHB suggests resistances detected in detached leaves are under the same genetic control as much of the resistances expressed in the wheat head of the commercial cultivars evaluated. In contrast, high resistances in each of the PDR components were associated with higher susceptibility across 19 CIMMYT genotypes previously evaluated as potential breeding sources of FHB resistance (incubation period: r = 0.52; P < 0.01, latent period: r = 0.53; P < 0.01, lesion length: r = –0.49; P < 0.01). In particular, the CIMMYT genotypes E2 and E12 together with Summai #3, known to have high levels of whole plant FHB resistance, showed low levels of resistance in each PDR component in the detached leaf assay. Such whole plant resistances, which are highly effective and not detected by the detached leaf assay, do not appear to be present in Irish and UK commercial cultivars. The most resistant Irish and UK commercial cultivars were comparable to the genotype Frontana and the most resistant CIMMYT germplasm evaluated in the leaf assay.     

Studies on the Interaction Between Septoria tritici and Stagonospora nodorum in Wheat

Interactions between Stagonospora nodorum and Septoria tritici were studied. Results from a detached glume experiment indicated that the interaction may be isolate-dependent, as it was shown that the interaction between the two pathogens may be beneficial or antagonistic depending on the isolate of each pathogen present. The number of spores produced by both pathogens was significantly greater when an aggressive isolate of S. tritici was mixed with a non-aggressive isolate of S. nodorum, whereas the number of spores produced by both pathogens was significantly less when two non-aggressive isolates were mixed. There was a significant reduction in disease level when S. tritici was applied prior to S. nodorum, compared to vice versa in the growth chamber. Results from growth chamber and field studies showed that S. nodorum produced significantly more spores when both pathogens were present together. It is concluded that S. tritici has a stimulatory effect on spore production by S. nodorum. However, there was a reduction of S. tritici spores observed in the dual inoculation treatments, suggesting that S. nodorum inhibits S. tritici.     

Towards the Development of a Novel In Vitro Strategy for Early Screening of Fusarium Ear Blight Resistance in Adult Winter Wheat Plants

A novel in vitro bioassay is described for screening Fusarium ear blight (FEB) resistance in adult winter wheat plants. Seven winter wheat cultivars were assessed for components of partial disease resistance as 28 day-old detached leaf segments in the laboratory using isolates of Microdochium nivale var. nivale and M. nivale var. majus. Results were compared with disease data obtained at anthesis using the same cultivars as whole plants and the same isolates under glasshouse conditions. Significant cultivar differences were observed using detached leaves, with cv. Avalon (a Fusarium culmorum ear susceptible cultivar) having the shortest leaf incubation period, greatest leaf lesion development and shortest leaf latent period compared to cv. Spark (a Fusarium culmorum ear resistant cultivar), which had the longest leaf incubation period, least leaf lesion development and longest leaf latent period. Using whole plants, cv. Avalon had the shortest ear incubation period and greatest ear disease severity, whilst cv. Spark had the longest incubation period and least ear disease severity. Overall, cultivars of intermediate F. culmorum ear resistance expressed intermediate responses to M. nivale isolates, using both detached leaves and whole plants. Significant correlations were found with ear disease severity and ear incubation period in whole plants and components of partial disease resistance in detached leaves, with significant correlations obtained between leaf incubation period and ear disease parameters using the M. nivale var. nivale isolate. In addition, leaf latent period and leaf lesion size showed significant correlations with whole plant reactions using M. nivale var. nivale and var. majus isolates. The in vitro screening of cultivars as detached leaves using M. nivale isolates may offer a real possibility of a rapid bioassay for the early screening of FEB resistance in wheat and other cereals.     

Identification of genes expressed during spore germination of Mycosphaerella pinodes

Mycosphaerella blight, caused by Mycosphaerella pinodes, is one of the major diseases of cultivated pea (Pisum sativum L.). To isolate the genes that are up- and down-regulated during spore germination, suppression subtraction hybridization (SSH) was performed between ungerminated and germinated spores. The 232 and 128 clones from forward and reverse libraries, respectively, were collected, sequenced, and analyzed with a BLASTX homology search. About 95% of the 32 selected clones were expressed during spore germination on a paper sheet and during infection of pea leaves. We discuss the applicability of the SSH libraries for analyzing M. pinodes genes involved in the early stage of infection.     

Studies on red light-induced resistance of broad bean to Botrytis cinerea: I. Possible production of suppressor and elicitor by germinating spores of pathogen

When detached broad bean leaves were preinoculated with virulent strain B304 of Botrytis cinerea 24 h before a challenge inoculation with strain B304, lesion formation by B304 was significantly inhibited in red light but not in the dark. In leaves that were preinoculated with avirulent strain 021 and then challenged by B304, however, lesion formation was not inhibited even under red light. Such differences in lesion formation after the challenge inoculation with an avirulent strain were also observed with lesions caused by Alternaria alternata, a nonpathogen of broad bean and by avirulent strain 021R in the presence of germination fluid from spores of strains B304 and 021R. These results suggest the possibility that virulent B. cinerea produced a suppressor involved in induced susceptibility and an elicitor involved in resistance induced by red light during spore germination.     

Influence of plant stage and organ age on the receptivity of <Emphasis Type="BoldItalic">Pisum sativum</Emphasis> to <Emphasis Type="BoldItalic">Mycosphaerella pinodes</Emphasis>

On spring pea, ascochyta blight (Mycosphaerella pinodes) frequently appears at the plant base on yellowing stipules and disease scores are higher on basal parts of the plants than on the uppermost parts. In order to investigate the relationship between pea plant growth stage and/or organ age, and ascochyta blight on whole plants and detached stipules and pods, two experiments were conducted in 2009 and 2010 under controlled conditions. This study showed that plant stage does not influence receptivity to ascochyta blight before the appearance of the first visual signs of senescence (beginning of yellowing). When stipules were green, regardless of the plant stage, no differences of receptivity was observed (except for the youngest stipule(s) at the top of the plant). Plant stage only had an effect on receptivity by the way of the importance of the visual senescence of the organs and visual senescence has a more predominant effect on plant receptivity than the plant stage considered alone. An effect of the nodal position, linked with the stipule age, was observed on whole plant inoculation, with a decreasing disease severity gradient from the base to the top of the plant which was explained by a greater receptivity for the basal/older stipules which had begun yellowing. On detached stipules, a higher receptivity was observed as soon as visual senescence had been observed. On pods, during their filling, few symptoms appeared and severity was important as soon as they began to yellow. The effect of ascochyta blight on induced senescence of the infected stipules was also observed.     

Components of quantitative resistance to leaf rust in wheat cultivars: diversity,variability and specificity

The aim of this study was to investigate the potential diversity and pathogen‐specificity of sources of quantitative resistance to leaf rust caused by Puccinia triticina in French wheat germplasm. From a set of 86 genotypes displaying a range of quantitative resistance levels during field epidemics, eight wheat genotypes were selected and challenged in a greenhouse with three isolates of the pathogen, belonging to different pathotypes. Five components of resistance were assessed: infection efficiency, for which an original methodology was developed, latent period, lesion size, spore production per lesion, and spore production per unit of sporulating tissue. High diversity and variability for all these components were expressed in the host × pathotype combinations investigated; isolate‐specificity was found for all the components. The host genotypes displayed various resistance profiles, based on both the components affected and the isolate‐specificity of the interaction. Their usefulness as sources of quantitative resistance was assessed: line LD7 probably combines diversified mechanisms of resistance, being highly resistant for all the components, but displaying isolate‐specificity for all the components; cv. Apache did not show isolate specificity for any of the components, which could be related to the durability of its quantitative resistance in the field over more than 11 years.     

Pi34-AVRPi34: a new gene-for-gene interaction for partial resistance in rice to blast caused by Magnaporthe grisea

The japonica rice (Oryza sativa) cultivar Chubu 32 has a high level of partial resistance to blast, which is mainly controlled by a dominant resistance gene located on chromosome 11. The partial resistance to the rice blast fungus (Magnaporthe grisea) in Chubu 32 has isolate specificity; isolate IBOS8-1-1 is more aggressive on Chubu 32 than are other isolates. We hypothesized that the gene-for-gene relationship fits this case of a partial resistance gene in Chubu 32 against the avirulence gene in the pathogen. The partial resistance gene in Chubu 32 was mapped between DNA markers C1172 (and three other co-segregated markers) and E2021 and was designated Pi34. In the 32 F₃ lines from the cross between a chromosome segment substitution line (Pi34⁻) from Koshihikari/Kasalath and Chubu 32, the lines with high levels of partial resistance to the M. grisea isolate Y93-245c-2 corresponded to the presence of Pi34 estimated by graphic genotyping. This indicated that Pi34 has partial resistance to isolate Y93-245c-2 in compatible interactions. The 69 blast isolates from the F₁ progeny produced by the cross between Y93-245c-2 and IBOS8-1-1 were tested for aggressiveness on Chubu 32 and rice cultivar Koshihikari (Pi34⁻). The progeny segregated at a 1 : 1 ratio for strong to weak aggressiveness on Chubu 32. The results suggested that Y93-245c-2 has one gene encoding avirulence to Pi34 (AVRPi34), and IBOS8-1-1 is extremely aggressive on Chubu 32 because of the absence of AVRPi34. This is the first report of a gene-for-gene relationship between a fungal disease resistance gene associated with severity of disease and pathogen aggressiveness.     

A comparative assessment of potential components of partial disease resistance to <Emphasis Type="Italic">Fusarium</Emphasis> head blight using a detached leaf assay of wheat,barley and oats

The relative resistance of 15 winter barley, three winter wheat and three winter oat cultivars on the UK recommended list 2003 and two spring wheat cultivars on the Irish 2003 recommended list were evaluated using Microdochium nivale in detached leaf assays to further understand components of partial disease resistance (PDR) and Fusarium head blight (FHB) resistance across cereal species. Barley cultivars showed incubation periods comparable to, and latent periods longer than the most FHB resistant Irish and UK wheat cultivars evaluated. In addition, lesions on barley differed from those on wheat as they were not visibly chlorotic when placed over a light box until sporulation occurred, in contrast to wheat cultivars where chlorosis of the infected area occurred when lesions first developed. The pattern of delayed chlorosis of the infected leaf tissue and longer latent periods indicate that resistances are expressed in barley after the incubation period is observed, and that these temporarily arrest the development of mycelium and sporulation. Incubation periods were longer for oats compared to barley or wheat cultivars. However, oat cultivars differed from both wheat and barley in that mycelial growth was observed before obvious tissue damage was detected under macroscopic examination, indicating tolerance of infection rather than inhibition of pathogen development, and morphology of sporodochia differed, appearing less well developed and being much less abundant. Longer latent periods have previously been related to greater FHB resistance in wheat. The present results suggest the longer latent periods of barley and oat cultivars, than wheat, are likely to play a role in overall FHB resistance if under the same genetic control as PDR components expressed in the head. However the limited range of incubation and latent periods observed within barley and oat cultivars evaluated was in contrast with wheat where incubation and latent periods were shorter and more variable among genotypes. The significance of the various combinations of PDR components detected in the detached leaf assay as components of FHB resistance in each crop requires further investigation, particularly with regard to the apparent tolerance of infection in oats and necrosis in barley, after the incubation period is observed, associated with retardation of mycelial growth and sporulation.     

Host status and reaction of <Emphasis Type="Italic">Medicago truncatula</Emphasis> accessions to infection by three major pathogens of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) and alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis>)

Ditylenchus dipsaci, the stem nematode of alfalfa (Medicago sativa), Mycosphaerella pinodes, cause of Ascochyta blight in pea (Pisum sativum) and Aphanomyces euteiches, cause of pea root rot, result in major yield losses in French alfalfa and pea crops. These diseases are difficult to control and the partial resistances currently available are not effective enough. Medicago truncatula, the barrel medic, is the legume model for genetic studies, which should lead to the identification and characterization of new resistance genes for pathogens. We evaluated a collection of 34 accessions of M. truncatula and nine accessions from three other species (two from M. italica, six from M. littoralis and one from M. polymorpha) for resistance to these three major diseases. We developed screening tests, including standard host references, for each pathogen. Most of the accessions tested were resistant to D. dipsaci, with only three accessions classified as susceptible. A very high level of resistance to M. pinodes was observed among the accessions, none of which was susceptible to this pathogen. Conversely, a high level of variation, from resistant to susceptible accessions, was identified in response to infection by A. euteiches.     

Assessment of Partial Resistance to Powdery Mildew (Podosphaera pannosa) in a Tetraploid Rose Population Using a Spore-suspension Inoculation Method

Powdery mildew (Podosphaera pannosa) is the most important fungal disease in greenhouse roses and is in practice controlled by fungicides. The creation of novel cultivars with durable resistance to powdery mildew is highly desirable. To understand the inheritance of mildew resistance, a tetraploid rose population with a size of 181 seedlings was obtained by crossing two tetraploid genotypes each having partial resistance. The population and its parents were tested under greenhouse conditions with two well-defined monospore isolates (2 and F1) using artificial inoculation with spore suspensions. Disease score at 11 days post-inoculation, latent period and rate of symptom development were used to describe seedling resistance. The tests for both isolates exhibited a wide and significant variation among genotypes for resistance. The distribution of the genotypic means of the disease scores was continuous and showed a considerable transgression. Statistical analysis, scatter plot of disease scores for the isolates, and correlation analyses indicated that the two isolates differed in pathogenicity. The outcome of the tests showed that the inoculation assay with spore suspensions was a reliable and effective way to screen large numbers of genotypes under greenhouse conditions for genetic and breeding studies. This is the first report on spore-suspension inoculation to be used successfully in rose.     

蚕豆和豌豆锈病病原菌的分子鉴定

为明确我国热带和亚热带地区蚕豆Vicia faba和豌豆Pisum sativum锈病的病原菌种类,通过致病性测定和ITS序列系统发育分析对来自我国云南省玉溪市的4份豌豆锈菌分离物及云南、广西、重庆和四川省(区、市)的5份蚕豆锈菌分离物进行系统鉴定。结果显示,分离自豌豆的锈菌WX1分离物对蚕豆和豌豆均具有高致病性,在侵染叶片上产生大量锈子器;分离自蚕豆的锈菌CX3分离物仅对蚕豆具有高致病性,能在叶片上产生大量夏孢子,而对豌豆的致病性相对较低,仅产生少量的夏孢子堆;分离物WX1和CX3对小扁豆和鹰嘴豆不具有致病性。基于ITS序列系统发育分析表明,所有不同寄主来源的蚕豆单胞锈菌分离物均聚类于一个系统发育组,但分离自蚕豆和豌豆的分离物分别聚类在不同的亚组。表明分离自云南省玉溪市豌豆上的蚕豆单胞锈菌Uromyces viciae-fabae应为豌豆专化型,定名为U. viciae-fabae ex P. sativaum,而来源于云南、广西、重庆和四川省(区、市)的蚕豆锈病病原菌为蚕豆专化型U. viciae-fabae ex V. faba。     

Durability of natural and transgenic resistances in rice to <Emphasis Type="Italic">Rice yellow mottle virus</Emphasis>

A monogenic recessive resistance to Rice yellow mottle virus (RYMV) found in the Oryza sativa indica cultivar Gigante and in a few Oryza glaberrima cultivars provided a higher level of resistance than either a polygenic partial resistance found in some japonica cultivars which delayed symptom expression or transgenic resistances which were partial and temporary. This high resistance was overcome by several isolates, but the percentage of such virulent isolates in the fields was low. There was no relationship between the virulence of an isolate towards the high resistance and its aggressiveness in other cultivars. Isolates with either of the two components of pathogenicity – virulence and aggressiveness – were found in each strain and in all regions of Africa, in both wild and cultivated grass species. There was no loss of fitness of resistance-breaking (RB) isolates as they were not counter-selected, impaired or outperformed after serial passages in susceptible cultivars, even in mixture with avirulent quasi-isogenic wild type isolates. Resistance breaking was highly dependent on the amount of virus inoculated and on the mode of transmission. Implications of these results for the durability of the resistances to RYMV and for the development of integrated disease management strategies are discussed.     

Epidemiological Characteristics of Angular Leaf Spot of Bean: A Systems Analysis

In this review, available knowledge on angular leaf spot (ALS) of bean, caused by Phaeoisariopsis griseola, is analysed and synthesised. This is done through a systems-analytical approach, and successive flowcharts of the system, in order to identify knowledge gaps and guide further research. Six connected sub-models of the ALS monocycle are used as a framework: lesion establishment, lesion extension, defoliation, sporulation, spore liberation, and spore deposition. Each of the sub-models enables the linking of processes to various effects of environmental (physical and host) factors. Disease-induced defoliation is one feature of the pathosystem, leading to a depletion of infectious tissues from the canopy, which are transferred to the ground. Consequences of defoliation may include: strong reductions of the amount of inoculum and of vacant sites in the canopy, limited maximum disease severity, and progressive accumulation of inoculum below the canopy, which may become important a later stage of disease epidemics. These elements are hypothesised to explain the typical behaviour of ALS epidemics in the field, especially late onset, high apparent rate of disease increase, and low level of terminal disease severity in the standing canopy. Epidemiological consequences of lesion expansion, sporulation, and survival of spores deposited onto the canopy are other knowledge gaps in this pathosystem.     

Prior Inoculation with Non-pathogenic Fungi Induces Systemic Resistance to Powdery Mildew On Cucumber Plants

A spray inoculation of the first leaf of 2-leaf stage cucumber plants with a non-pathogenic isolate of Alternaria cucumarina or Cladosporium fulvum before a challenge inoculation with the pathogen Sphaerotheca fuliginea induced systemic resistance to powdery mildew on leaves 2–5. Systemic resistance was expressed by a significant (p < 0.05) reduction in the number of powdery mildew colonies produced on each leaf of the induced plants, as compared with water-sprayed plants. Systemic resistance was evident when a prior inoculation with each of the inducing fungi was administered 1, 3 or 6 days before the challenge inoculation with S. fuliginea. Increasing the inoculum concentration of A. cucumarina or C. fulvum enhanced the systemic protection and provided up to 71.6% or 80.0% reduction, respectively, in the number of colonies produced on upper leaves, relative to controls. Increasing the inoculum concentration of S. fuliginea used for challenge inoculation, increased the number of powdery mildew colonies produced on both induced and non-induced plants. Pre-treated plants, however, were still better protected than controls, indicating that the level of systemic protection was related to the S. fuliginea inoculum concentration. The induction of systemic resistance against powdery mildew by biotic agents, facilitates the development of a wide range of disease management tools.     

稻梨孢菌与其他寄主梨孢菌在水稻植株上的交互作用

作者研究了1个稻梨孢菌株与4个稻以外寄主梨孢菌株在混合接种和间隔接种条件下,在水稻植株上的交互作用。结果表明:弱致病菌与强致病菌间交互作用较强,非致病菌与强致病菌间较弱。弱致病菌可明显降低强致病菌的致病性,非致病菌对强致病菌的作用,多数组合表现为减轻病害,但有少数组合表现为促进发病。弱致病菌先接种;2天后接种强致病菌,比两者混合接种具有更强烈的互作效应,病斑数减少达35.7%-38.1%。先接种菌的孢子浓度对后接种菌的致病性也有影响。     

Assessment of early blight (<Emphasis Type="Italic">Alternaria solani</Emphasis>) resistance in tomato using a droplet inoculation method

A droplet inoculation method was used for evaluation of tomato resistance to early blight, a destructive foliar disease of tomato caused by Alternaria solani (Ellis and Martin) Sorauer. In this test method, leaflets are inoculated with small droplets of a spore suspension in either water or a 0.1% agar solution. Early blight resistance was evaluated based on lesion size. The droplet method better discriminated the level of resistance (P < 0.001) for a range of spore densities in comparison with the more commonly used spray inoculation method. Lesions generated by droplet inoculation at 7 days after inoculation ranged from small flecks to almost complete blight with an exponential-like distribution of lesion sizes. Significant correlations (r = 0.52, 0.58, and 0.63, P < 0.001) were observed across three glasshouse tests of 54 accessions including wild species using the droplet method. The most resistant accessions included wild species: one accession of Solanum arcanum, three accessions of Solanum peruvianum, one accession of Solanum neorickii, and one of Solanum chilense. Solanum pennellii and Solanum pimpinellifolium accessions were susceptible, whereas Solanum habrochaites and Solanum lycopersicum accessions ranged from susceptible to moderately resistant. The droplet test method is simple to apply, offers a fine discrimination of early blight resistance levels, and allows objective evaluation.     

Evaluation of biocontrol preparations and plant extracts for the control of <Emphasis Type="Italic">Phytophthora infestans</Emphasis> on potato leaves

The potential of biocontrol products and plant extracts for control of late blight on potato plants, caused by Phytophthora infestans was evaluated in detached leaf assays and on potted plants. Based on an initial screening of 22 preparations and plant extracts, the 10 most active treatments were selected for further investigation. In the detached leaf assays the commercial preparations Elot-Vis, Serenade and Trichodex, and plant extracts of Rheum rhabarbarum and Solidago canadensis showed a significant effect on the level of infestation by P. infestans. However, none of the treatments was as effective as copper. In the case of Serenade, the metabolites produced by its active micro-organism, Bacillus subtilis, were demonstrated to be the effective component of the formulation, and not the micro-organism itself. In order to take curative and protective modes of action into account, the test substances were applied 24 h before, or 90 min after inoculation with P. infestans. Generally, better effects were obtained when the applications were made 24 h before inoculation. For defining the optimum time of application, potted plants were treated 72 or 24 h before, and 1 and 24 h after inoculation with P. infestans. In these tests, Trichodex showed no activity, while Elot-Vis gave best results when applied 1 day before inoculation. Serenade and the extracts of R. rhabarbarum and S. canadensis (all at 5% concentration) however, were effective when applied up to 3 days before and just after inoculation with P. infestans. The results of the experiments on potted plants indicated direct effects on the pathogen for all agents except the extract of S. canadensis, but other mode of actions, e.g. induced resistance, could not be ruled out. None of the treatments had a curative effect.     

RETRACTED ARTICLE: Effect of calcium chloride on the biocontrol efficacy of two antagonistic yeasts against <Emphasis Type="Italic">Penicillium expansum</Emphasis> on apple fruit

Two antagonistic yeasts, Candida membranaefaciens and Pichia guilliermondii, were evaluated for the control of the blue mold of apple caused by Penicillium expansum. Dual culture, cell-free metabolite and volatile tests were used for in vitro assay. Yeast strains of two genera inhibited growth of P. expansum; inhibition varied from 30.27% to 44.19% in dual culture, from 79.40% to 90.57% in the volatile metabolite test, and from 72.99% to 88.77% in the cell-free metabolite test. Calcium chloride (2% w/v) significantly inhibited the growth of the pathogen P. expansum, but did not affect the colony-forming units (CFU) of the yeasts C. membranaefaciens and P. guilliermondii in potato dextrose broth. The concentration of yeast suspension influenced spore germination and germ tube growth of P. expansum in vitro, as well as disease incidence and lesion development in fruits. There were significant negative relationships between the suspension concentrations of the yeasts and the growth as well as infectivity of the pathogen. The addition of calcium resulted in lower spore germination rates and slower growth of germ tubes in vitro, as well as in lower disease incidences and smaller lesion diameters compared with treatments with yeast antagonists alone. When yeast cell suspensions reached a concentration of 10⁷ CFU ml^-1, growth of the pathogen was completely limited in vitro, and no infection was found in apple fruits treated with or without calcium. This article has been retracted because part of the data shown has already been published before, by different authors. An erratum to this article can be found at