In Vitro Effects of Film-Forming Polymers on the Growth and Morphology of Pyrenophora Avenae and Pyricularia Oryzae

Ethokem (Midkem Agrochemicals, Northampton, UK) and Bond (Newman Agrochemicals, Cambridge, UK) completely inhibited linear growth of Pyricularia oryzae at 1 and 2% concentrations of the compounds and strongly inhibited growth of Pyrenophora avenae in vitro . Vapor Gard (Miller Chemical & Fertilizer Corporation, Hannover, PA, USA) was less effective with relatively little inhibition of P. oryzae , but was a more effective inhibitor of P. avenae . All three compounds decreased cell lengths of both pathogens and increased cell diameters of P. avenae . Vapor Gard and Ethokem did not significantly alter cell diameters of P. oryzae , but a highly significant increase was observed with Bond. Gross changes in hyphal morphology were observed including swollen shortened cells, granulation of the cytoplasm, increased branching and collapsed empty cells. Ethokem and Bond decreased growth rate and inhibited biomass production of P. avenae .     

Antifungal activity of three spermidine conjugates

Three tri-substituted spermidines, di-p-coumaroyl-caffeoylspermidine, tri-caffeoylspermidine and tri-p-coumaroylspermidine, isolated from pollen of Quercus alba, were examined for antifungal activity. Both di-p-coumaroyl-caffeoylspermidine and tri-p-coumaroylspermidine reduced mycelial growth of the oat leaf stripe pathogen, Pyrenophora avenae and reduced powdery mildew (Blumeria graminis f. sp. hordei) infection of barley seedlings when applied as a post-inoculation treatment. When used as a pre-inoculation treatment, only di-p-coumaroyl-caffeoylspermidine reduced powdery mildew infection significantly. Growth of P. avenae in the presence of 100 microM di-p-coumaroyl-caffeoylspermidine reduced activity of S-adenosylmethionine decarboxylase (AdoMetDC), and led to a reduction in the incorporation of labelled ornithine into spermidine. The other two spermidine conjugates increased AdoMetDC activity and the flux label from ornithine into spermine in P. avenae significantly.     

Effects of Sunflower Oil on Tomato Powdery Mildew Caused by Oidium neolycopersici

When tomato leaves were sprayed with 0.1% emulsified canola oil, corn oil, grape seed oil, peanut oil, safflower oil, soya bean oil or sunflower oil, the severity of powdery mildew caused by Oidium neolycopersici was greatly reduced. Among these edible oils tested, sunflower oil was the most effective in the control of powdery mildew. When sprayed with 0.5% sunflower oil, powdery mildew on tomato leaves was reduced to a negligible level. Sunflower oil applied to halves of upper leaf surface did not induce resistance against the pathogen in the non‐treated halves. When applied to halves of lower leaf surface, it also failed to reduce the severity of powdery mildew on the upper leaf surface right above the treated area indicating that control of the powdery mildew by sunflower oil did not result from activation of host defence mechanisms. Scanning electron microscopy showed that control of powdery mildew with sunflower oil resulted mainly from the inhibition of conidial germination and suppression of mycelial growth of the pathogen.     

In vitro effects of the polyamine biosynthesis inhibitor, α-difluoromethylornithine, on ornithine decarboxylase activities from three plant pathogenic fungi

The effects of α-difluoromethylornithine (DFMO) on in vitro ornithine decarboxylase (ODC) activities from three plant pathogenic fungi, Pyrenophora avenae, Pyricularia oryzae and Uromyces viciae-fabae , were studied. DFMO concentrations from 0·01 to 1·0 mmol/l produced no significant effects on ODC activities from the three fungi. However, increasing the DFMO concentration to 5 mmol/l produced a substantial reduction in in vitro ODC activity from Pyre, avenae. The ODC inhibitor, α-monofluoromethylornithine (2 mmol/l), significantly reduced in vitro ODC activity from Pyre. avenae , whereas RR-methyl acetylenic putrescine, an ODC inhibitor based on putrescine, produced no significant effect on the fungal enzyme.     

Synthesis and antifungal activity of two novel spermidine analogues

Two spermidine analogues were synthesised and examined for antifungal activity. Both compounds used as 1 mM post-inoculation sprays reduced infection of barley seedlings by the powdery mildew fungus, Erysiphe graminis f.sp. hordei, infection of broad bean seedlings by the rust fungus, Uromyces viciae-fabae, and infection of apple seedlings by the powdery mildew fungus, Podosphaera leucotricha. Since these fungal pathogens cannot be cultured axenically, the effects of the two spermidine analogues on mycelial growth in vitro, as well as preliminary investigations on polyamine biosynthesis, were undertaken using the oat stripe pathogen, Pyrenophora avenae. Although neither compound affected radial growth of the fungus on plates, both analogues reduced fungal biomass in liquid culture substantially. The two spermidine analogues, used at a concentration of 1 mM, had no significant effect on the conversion of labelled ornithine into polyamines in P. avenae.     

Leaf growth characteristics of Vitis vinifera and prediction of final lamina length for studies of powdery mildew

Total functional leaf area is a key factor in determining crop yield. A nonlinear mixed‐effects model was employed to estimate growth responses for individual leaves using repeated measures of lamina length ≥30 mm, in the absence of disease. Resulting growth curves make allowances for, and allow assessment of, leaf to leaf variability. The major source of variability in leaf growth was identified as differences in thermal time required to reach half final lamina length. Juvenile leaves of Vitis vinifera are susceptible to infection by the powdery mildew fungus (Erysiphe necator) which impairs leaf function. The model was used to predict unobserved final lamina length for a subset of leaves inoculated with E. necator immediately after observations ceased. The severity of infection by E. necator varies among infected leaves. A previous study identified which of the inoculated leaves developed symptoms of severe powdery mildew. Maximum severity of infection was found to occur when individual leaves were at 85.3–97.9% of predicted final lamina length.     

Antifungal activity of essential oil and its constituents from Calocedrus macrolepis var. formosana Florin leaf against plant pathogenic fungi

Resistance to conventional fungicides causes the poor disease control of agriculture. Natural products from plants have great potential as novel fungicide sources for controlling pathogenic fungi. In this study antipathogenic activity of the leaf essential oil and its constituents from Calocedrus macrolepis var. formosana Florin were evaluated in vitro against six plant pathogenic fungi. Chemical analysis of leaf oil by GC/MS allowed identification of alpha-pinene (44.2%), limonene (21.6%), beta-myrcene (8.9%), beta-caryophyllene (8.2%), caryophyllene oxide (2.4%), alpha-cadinol (1.6%), beta-pinene (1.2%), and T-muurolol (1.1%) as main components. Sesquiterpenoid components of the oil were more effective than monoterpenoid components of the oil. In particular, T-muurolol and alpha-cadinol strongly inhibited the growth of Rhizoctonia solani and Fusarium oxysporum, with the IC(50) values < 50 microg ml(-1). These compounds also efficiently inhibited the mycelial growths of Colletotrichum gloeosporioides, P. funerea, Ganoderma australe and F. solani. These results showed that T-muurolol and alpha-cadinol possess antifungal activities against a broad spectrum of tested plant pathogenic fungi and could be used as potential antifungal agents for the control of fungal diseases in plants.     

Antifungal activities of Bacillus thuringiensis isolates on barley and cucumber powdery mildews

Fourteen Bacillus thuringiensis isolates having both insecticidal activity and in vitro antifungal activity were selected and tested for in vivo antifungal activity against tomato late blight, wheat leaf rust, tomato gray mold, and barley powdery mildew in growth chambers. All the isolates represented more than 70% disease control efficacy against at least one of four plant diseases. Specifically, 12 isolates exhibited strong control activity against barley powdery mildew. Under glasshouse conditions, four (50-02, 52-08, 52-16, and 52- 18) of the isolates also displayed potent control efficacy against cucumber powdery mildew. To our knowledge, this is the first report of B. thuringiensis isolates that have disease control efficacy against powdery mildew of barley and cucumber as well as insecticidal activity.     

Fungitoxic action of dimethirimol and ethirimol

The systemic fungicides dimethirimol and ethirimol were shown to be toxic to spores of Erysiphe graminis and Sphaerotheca fuliginea, in germination tests in vitro. Toxicity of dimethirimol to spores of Botrytis fabae, Phytophthora infestans, Puccinia recondita, Uncinula necator and Venturia inaequalis was relatively feeble or absent. When applied in foliage sprays or in root treatments, both compounds inhibited the emergence of germ tubes from powdery mildew spores on cucumber and barley plants. The specific disease control shown by these fungicides can be explained by their direct fungitoxic action. The effects of dimethirimol and ethirimol on powdery mildew infections and on spore germination were greatly decreased by the presence of riboflavin in the light. A rapid photo-chemical interaction between riboflavin and dimethirimol was demonstrated. Marked reversals both of disease control and of fungitoxicity were also given by folic acid, and it is suggested that folic acid overcomes a metabolic block induced in powdery mildew fungi by the fungicides.     

Temporal Dynamics of Powdery Mildew (Oidium neolycopersici) and its Effects on the Host Growth Dynamics of Tomato

Controlled glasshouse experiments were conducted to investigate the temporal progress of powdery mildew and its effects on host dynamics of tomato, without and with one fungicide application. Healthy tomato transplants (5‐ to 6‐week old) were artificially inoculated with powdery mildew, and disease progress as well as host growth were monitored in both fungicide sprayed and unsprayed treatments and compared with non‐inoculated plants. Actual disease severity on a plant basis increased in unsprayed plants reaching maximum severity in the proportionate range of 0.53–0.83. One fungicide spray significantly reduced the maximum disease severity by two‐ to fourfolds. Despite adjustments for defoliation, declines in the proportion of disease severity between successive assessments were evident. Whereas the estimated growth rates of diseased plants were significantly lower than that of healthy plants, no significant differences were observed in the maximum leaf area formed of inoculated and non‐inoculated plants. A considerable effect of the powdery mildew epidemics was manifested through hastened shrivelling and defoliation of diseased leaves within the tomato canopy. An average of 18–29% and 40–52% of leaves had abscised from the plant canopy at the last date of assessment in sprayed and non‐sprayed plants, respectively. Accordingly, defoliation accounted for 14–33.3% and 58.3–63.1% losses in leaf area of sprayed and non‐sprayed plants, respectively. Duration of healthy leaf area and yield of inoculated plants were also significantly reduced by powdery mildew epidemics.     

Prospects for exploitation of disease resistance from Hordeum chilense in cultivated cereals

Hordeum chilense is a South American wild barley with high potential for cereal breeding given its high crossability with other members of the Triticeae. In the present paper we consider the resistance of H. chilense to several fungal diseases and the prospects for its transference to cultivated cereals. All H. chilense accessions studied are resistant to the barley, wheat and rye brown rusts, the powdery mildews of wheat, barley, rye and oat, to Septoria leaf blotch, common bunt and to loose smuts, which suggests that H. chilense is a non-host of these diseases. There are also lines resistant to wheat and barley yellow rust, stem rust and to Agropyron leaf rust, as well as lines giving moderate levels of resistance to Septoria glume blotch, tan spot and Fusarium head blight. Some H. chilense lines display pre-appressorial avoidance to brown rust. Lines differ in the degree of haustorium formation by rust and mildew fungi they permit, and in the degree to which a hypersensitive response occurs after haustoria are formed. Unfortunately, resistance of H. chilense to rust fungi is not expressed in tritordeum hybrids, nor in chromosome addition lines in wheat. In tritordeum, H. chilense contributes quantitative resistance to wheat powdery mildew, tan spot and loose smut. The resistance to mildew, expressed as a reduced disease severity, is not associated with macroscopically visible necrosis. Hexaploid tritordeums are immune to Septoria leaf blotch and to common bunt although resistance to both is slightly diluted in octoploid tritordeums. Studies with addition lines in wheat indicate that the resistance of H. chilense to powdery mildew, Septoria leaf blotch and common bunt is of broad genetic basis, conferred by genes present on various chromosomes.     

Temporal isolation explains host-related genetic differentiation in a group of widespread mycoparasitic fungi

Understanding the mechanisms responsible for divergence and specialization of pathogens on different hosts is of fundamental importance, especially in the context of the emergence of new diseases via host shifts. Temporal isolation has been reported in a few plants and parasites, but is probably one of the least studied speciation processes. We studied whether temporal isolation could be responsible for the maintenance of genetic differentiation among sympatric populations of Ampelomyces, widespread intracellular mycoparasites of powdery mildew fungi, themselves plant pathogens. The timing of transmission of Ampelomyces depends on the life cycles of the powdery mildew species they parasitize. Internal transcribed spacer sequences and microsatellite markers showed that Ampelomyces populations found in apple powdery mildew (Podosphaera leucotricha) were genetically highly differentiated from other Ampelomyces populations sampled from several other powdery mildew species across Europe, infecting plant hosts other than apple. While P. leucotricha starts its life cycle early in spring, and the main apple powdery mildew epidemics occur before summer, the fungal hosts of the other Ampelomyces cause epidemics mainly in summer and autumn. When two powdery mildew species were experimentally exposed to Ampelomyces strains naturally occurring in P. leucotricha in spring, and to strains naturally present in other mycohost species in autumn, cross‐infections always occurred. Thus, the host‐related genetic differentiation in Ampelomyces cannot be explained by narrow physiological specialization, because Ampelomyces were able to infect powdery mildew species they were unlikely to have encountered in nature, but instead appears to result from temporal isolation.     

Host and non-host pathogens elicit different jasmonate/ethylene responses in Arabidopsis

Arabidopsis does not support the growth and asexual reproduction of the barley pathogen, Blumeria graminis f. sp. hordei Bgh). A majority of germlings fail to penetrate the epidermal cell wall and papillae. To gain additional insight into this interaction, we determined whether the salicylic acid (SA) or jasmonate (JA)/ethylene (ET) defence pathways played a role in blocking barley powdery mildew infections. Only the eds1 mutant and NahG transgenics supported a modest increase in penetration success by the barley powdery mildew. We also compared the global gene expression patterns of Arabidopsis inoculated with the non-host barley powdery mildew to those inoculated with a virulent, host powdery mildew, Erysiphe cichoracearum. Genes repressed by inoculations with non-host and host powdery mildews relative to non-inoculated control plants accounted for two-thirds of the differentially expressed genes. A majority of these genes encoded components of photosynthesis and general metabolism. Consistent with this observation, Arabidopsis growth was inhibited following inoculation with Bgh, suggesting a shift in resource allocation from growth to defence. A number of defence-associated genes were induced during both interactions. These genes likely are components of basal defence responses, which do not effectively block host powdery mildew infections. In addition, genes encoding defensins, anti-microbial peptides whose expression is under the control of the JA/ET signalling pathway, were induced exclusively by non-host pathogens. Ectopic activation of JA/ET signalling protected Arabidopsis against two biotrophic host pathogens. Taken together, these data suggest that biotrophic host pathogens must either suppress or fail to elicit the JA/ET signal transduction pathway.     

Localisation of genes for resistance against Blumeria graminis f.sp. hordei and Puccinia graminis in a cross between a barley cultivar and a wild barley (Hordeum vulgare ssp. spontaneum) line

The aims of this investigation have been to map new (quantitative) resistance genes against powdery mildew, caused by Blumeria graminis f.sp. hordei L., and leaf rust, caused by Puccinia hordei L., in a cross between the barley ( Hordeum vulgare ssp. vulgare) cultivar "Vada" and the wild barley ( Hordeum vulgare ssp. spontaneum) line "1B-87" originating from Israel. The population consisted of 121 recombinant inbred lines. Resistance against leaf rust and powdery mildew was tested on detached leaves. The leaf rust isolate "I-80" and the powdery mildew isolate "Va-4", respectively, were used for the infection in this experiment. Moreover, powdery mildew disease severity was observed in the field at two different epidemic stages. In addition to other DNA markers, the map included 13 RGA (resistance gene analog) loci. The structure of the data demanded a non-parametric QTL-analysis. For each of the four observations, two QTLs with very high significance were localised. QTLs for resistance against powdery mildew were detected on chromosome 1H, 2H, 3H, 4H and 7H. QTLs for resistance against leaf rust were localised on 2H and 6H. Only one QTL was common for two of the powdery mildew related traits. Three of the seven QTLs were localised at the positions of the RGA-loci. Three of the five powdery mildew related QTLs are sharing their chromosomal position with known qualitative resistance genes. All detected QTLs behaved additively. Possible sources of the distorted segregation observed, the differences between the results for the different powdery mildew related traits and the relation between qualitative and quantitative resistance are discussed.     

荒漠固沙植物河西菊的白粉病菌

2009–2011年间,在对新疆塔里木盆地进行白粉菌物种多样性调查时,发现荒漠固沙植物河西菊Hexinia polydichotoma白粉病大发生,导致植株大面积死亡。通过对病原菌形态学特征观察和rDNA-ITS序列分析,证明引发新疆塔里木盆地河西菊白粉病大发生的病原菌为野莴苣内丝白粉菌Leveillula lactucae-serriolae,为中国新记录种,河西菊为新寄主。     

氨基寡糖素对黄瓜白粉病菌侵染的抑制作用*

用透射电子显微镜,研究了寡聚糖类药剂中科6号(2%氨基寡糖素)处理黄瓜Cucumis sativus 植株叶片后对白粉病菌Sphaerotheca fuliginea侵染的抑制作用。超微结构观察表明,病菌发育受到明显抑制,表现为白粉菌菌丝细胞质凝聚,细胞器解体和细胞组织崩解;吸器内的原生质电子致密度加深,吸器畸形,吸器壁增厚,细胞器解体,最终吸器坏死。     

Molecular phylogenetic analyses reveal a close relationship between powdery mildew fungi on some tropical trees and Erysiphe alphitoides, an oak powdery mildew

To investigate the phylogenetic relationships among the powdery mildew fungi of some economically important tropical trees belonging to Oidium subgenus Pseudoidium, we conducted molecular phylogenetic analyses using 30 DNA sequences of the rDNA internal transcribed spacer (ITS) regions and 26 sequences of the domains D1 and D2 of the 28S rDNA obtained from the powdery mildews on Hevea brasiliensis (para rubber tree), Anacardium occidentale (cashew), Bixa orellana, Citrus spp., Mangifera indica (mango), and Acacia spp. The results indicate that the powdery mildew fungi isolated from these tropical trees are closely related to one another. These powdery mildews are also closely related to E. alphitoides (including Erysiphe sp. on Quercus phillyraeoides). Because of the obligate biotrophic nature of the powdery mildew fungi, the relationship between powdery mildews and their host plants is conservative. However, the present study suggests that a particular powdery mildew species has expanded its host ranges on a wide range of the tropical trees. This article also suggests that a powdery mildew fungus distributed in temperate regions of the Northern Hemisphere expanded its host ranges onto tropical plants and may be a good example of how geographical and host range expansion has occurred in the Erysiphales.     

Paecilomyces fumosoroseus (Deuteromycotina: Hyphomycetes) as a potential mycoparasite on Sphaerotheca fuliginea (Ascomycotina: Erysiphales)

Hyphomycete Paecilomyces fumosoroseus that is well known as saprophytic and entomopatogenic fungus was investigated for its mycoparasitism on the cucumber powdery mildew pathogen. Mycoparasitism was documented by using standard bioassay and SEM. Effects of mycoparasitism were evaluated in three types of experiments. Paecilomyces fumosoroseus was applied in the form of graded suspensions into a colony of powdery mildew on a leaf segment. Interaction between both fungi was observed as the percentage of colonized area vs. experimental time. In the second experiment, young cucumber plants were sprayed with a suspension of Paecilomyces fumosoroseus 24 h before inoculation of Sphaerotheca fuliginea. Pre-treatment with P. fumosoroseus reduced development and spreading of powdery mildew infection significantly 15 days post-inoculation in contrast to pre-treatments with sulfur fungicide and distilled water. The development of pure culture powdery mildew under determined experimental conditions was observed and compared with treated variants. In the third experiment, mildewed plants were treated with a suspension of P. fumosoroseus. The control treatments with sulfur fungicide and distilled water were tested. Effects of P. fumosoroseus on the dispersion of powdery mildew during a 21-day period were observed.P. fumosoroseus suppressed the development and spread of cucumber powdery mildew significantly during the time of the experiment. The mechanical and physical damages and disruptions of vegetative and fruiting structures of powdery mildew were recorded under light microscopy and S.E.M.Results were concluded in pursuance to differences between the natural behaviour and development of S. fuliginea on cucumber plants treated with P. fumosoroseus and non-treated plants.     

Characters of compost teas from different sources and their suppressive effect on fungal phytopathogens

Compost teas (CT) are fermented watery extracts of composted materials that are used to control plant diseases and on crop fertilization. In this work, aerated (ACT) and non-aerated compost teas (NCT) were obtained from four different composts: spent mushroom substrate compost, grape marc compost, greenhouse horticultural crop residues compost, and vermicompost. Physico-chemical and microbiological analysis were carried out to determine their properties. In vitro assays were performed to assess their suppressive effect on the mycelial growth of eight fungal phytopathogens. In vivo trials aimed to assess their effect on gummy stem blight (Didymella bryonae) and powdery mildew (Podosphaera fusca) in melon plants. Results showed that ACT and NCT filtrates inhibited the in vitro growth of all tested pathogens while autoclaved CT did not completely lose their inhibitory effect, and CT sterilized by microfiltration had no effect on the pathogen growth. The severity of powdery mildew was highly reduced by ACT and NCT from all sources, though in gummy stem blight assay only a delay in disease development was observed. In general, all compost teas showed a high level of microbial populations and nutrients. Results suggest that the efficacy of ACT and NCT firstly depend on the microbiota present in them. We consider compost teas from the four tested sources as a viable way to manage plant diseases and crop fertilization, throughout its integration in pest management programs and fertirrigation systems under different dilution rates.     

Mutual interplay between phytopathogenic powdery mildew fungi and other microorganisms

Powdery mildew is a common and widespread plant disease of considerable agronomic relevance. It is caused by obligate biotrophic fungal pathogens which, in most cases, epiphytically colonize aboveground plant tissues. The disease has been typically studied as a binary interaction of the fungal pathogen with its plant hosts, neglecting, for the most part, the mutual interplay with the wealth of other microorganisms residing in the phyllo- and/or rhizosphere and roots. However, the establishment of powdery mildew disease can be impacted by the presence/absence of host-associated microbiota (epi- and endophytes) and, conversely, plant colonization by powdery mildew fungi might disturb indigenous microbial community structures. In addition, other (foliar) phytopathogens could interact with powdery mildews, and mycoparasites may affect the outcome of plant–powdery mildew interactions. In this review, we discuss the current knowledge regarding the intricate and multifaceted interplay of powdery mildew fungi, host plants and other microorganisms, and outline current gaps in our knowledge, thereby setting the basis for potential future research directions.