A novel technique for greenhouse evaluation of plant extracts for management of downy mildew of sorghum

Antipathogenic potential of 16 plants was evaluated in the form of aqueous extracts against Peronosclerospora sorghi, causing downy mildew of sorghum. A novel method was developed in which conidial suspension and plant extracts were mixed individually and allowed to stand for 5?min and then used to inoculate the host by sprout-dip method. The sprouts thus inoculated were grown in pots, and disease incidence was observed. Three extracts (Parthenium hysterophorus, Duranta repens and Oxalis latifolia) suppressed the disease at par with chemical fungicide (Mancozeb 75%). In form of spray, D. repens extract with tween-20 adjuvant (1.9% disease) performed closest behind mancozeb (0% disease), while negative control showed 21% disease incidence. Organic management of airborne inoculum of downy mildew of sorghum is feasible and preferable as compared to the use of chemical fungicides, considering human and environmental health concerns. Use of water extract keeps the technology simple so that it can be directly prepared and used by farmers. Short listing of three most effective water extracts would help in self-reliance of farmers, reducing their dependence on commercial products.     

Seed treatment with aqueous extract of Viscum album induces resistance to pearl millet downy mildew pathogen

Abstract

Downy mildew (Sclerospora graminicola [Sacc.] Schroet.) is a serious agricultural problem for pearl millet (Pennisetum glaucum [L.] R. Br.) grain production under field conditions. Six medicinally important plant species Azadirachta indica, Argemone mexicana, Commiphora caudata, Mentha piperita, Emblica officinalis and Viscum album were evaluated for their efficacy against pearl millet downy mildew. Seeds of pearl millet were treated with different concentrations of aqueous extract of the plants to examine their efficacy in controlling downy mildew. Among the plant extracts tested, V. album treatment was found to be more effective in enhancing seed quality parameters and also in inducing resistance against downy mildew disease. Germination and seedling vigor was improved in seeds treated with V. album extracts over control. Seeds treated with 10% concentration of V. album showed maximum protection against downy mildew disease under greenhouse and field conditions. The downy mildew disease protection varied from 44–70% with different concentrations. Leaf extract of V. album did not inhibit sporulation and zoospore release from sporangia of Sclerospora graminicola, indicating that the disease-controlling effect was attributed to induced resistance. Seed treatment with V. album extract increased pearl millet grain yield considerably. In V. album, treated pearl millet seedlings increased activities of peroxidase, and phenylalanine ammonia-lyase enzyme was detected. FTIR analysis of V. album extracts showed the presence of amides and other aromatic compounds which are antimicrobial compounds involved in plant defense.     

Evaluation of uncomposted botanical-based soil amendments for organic management of downy mildew of Sorghum

Uncomposted dried, powdered biomass of 38 selected botanicals was evaluated as soil amendments in a pot experiment for their potential to suppress downy mildew of sorghum caused by soil-borne oosporic inoculum of Perenosclerospora sorghi. A negative control showed 11.1% disease incidence. Six botanicals proved effective in form of dry biomass powder at 0.5% (w/w) amendment rate to control infection by oospores (Allium cepa 0%, Lantana camara 1.6%, Polyalthia longofolia 0%, Hyptis suaveolens 1.5%, Eucalyptus globulus 0% and Nerium oleander 1.6% of disease incidence). Dry powders are not only effective in disease suppression but confer additional benefits over composts like reduced bulk, increased shelf-life, consistent composition and results.     

Yield and quality of grapes cultivated under plastic coverings with different downy mildew control strategies

Viticulture has been expanding in tropical regions. However, the climate in these areas is generally favourable to the incidence of plant diseases, especially downy mildew. Plastic covers and warning systems have shown very positive results in disease control, but they are tools that have never been used simultaneously in a tropical area. The Vitis vinifera cv. BRS Morena table grape was evaluated as regards yield and quality under different downy mildew control strategies as carried out on vineyards trained on an overhead trellis system, covered by a black shading screen (BSS) or a braided polypropylene film (BPF), over a 3‐year period. Different grapevine downy mildew management approaches defined the treatments: Co) Control (no spraying); Ca) Conventional control (calendar); Ba) “Rule 3–10” (Atti Istituto Botanico, 8, 1947, 45); Ma25) Low‐infection efficiency—i₀>25%; and Ma75) High‐infection efficiency—i₀>75% (Plant Disease, 84, 2000, 549). The occurrence of downy mildew and the amount of damage inflicted on vine yield and grape quality are directly related to the period of the crop cycle when there is rainfall. The use of the Ma75 warning system (Plant Disease, 84, 2000, 549) under braided polypropylene film resulted, for the most part, in similar vineyard productivity compared to Ca, but did not influence the number of branches and its fertility. The other warning systems decreased productivity by 31.9% compared to Ca. It was not possible to establish a relationship between the occurrence of downy mildew and its influence on grape sweetness and acidity. The use of warning systems led to a substantial reduction in fungicide sprays, approximately 66.7 to 71.3%, compared to the calendar system commonly used by the vine growers, with the Ba (Atti Istituto Botanico, 8, 1947, 45) and Ma75 controls (Plant Disease, 84, 2000, 549) leading to the highest fungicide saving.     

Exploiting medicinal plants and phylloplane microflora for the management of grapevine downy mildew

Abstract

Grapevine downy mildew is the most devastating disease throughout the world causing huge monetary losses. Twenty medicinal plant extracts and six phylloplane microfloras were evaluated for their efficacy against sporangial germination of grapevine downy mildew pathogen Plasmopara viticola in vitro. The results revealed that the Neem Seed Kernel Extract (NSKE) at 5% significantly inhibited the sporangial germination (75.36%) of P. viticola. Among the phylloplane microflora Pseudomonas fluorescens was highly effective in reducing the sporangial germination (64.26%). Post inoculation spraying of NSKE (5%) and P. fluorescens (0.2%) effectively inhibited the disease development in the greenhouse. Three sprays with NSKE (5%) and phylloplane P. fluorescens (0.2%): first spray after initial appearance of disease and the second and third at 10 day intervals were found to be promising in reducing disease incidence in the field.     

The influence of foliar applications of sugars on the susceptibility of sugar beet to downy mildew

Spraying sugar-beet seedlings in the glasshouse, with 1% or 10% solutions of sucrose, 24 h before inoculation with Peronospora farinosa, significantly reduced their susceptibility to downy mildew. The proportion of inoculated plants that became infected was reduced by spraying with sucrose but the main effect was the inhibition of sporulation. Applications of glucose or fructose also increased the resistance of beet seedlings to P. farinosa. Spraying with sucrose 1 or 2 days before inoculation was much more effective than was spraying shortly before inoculation, or 24 h afterwards, or adding sucrose to the inoculum. Washing sucrose-sprayed seedlings with distilled water 1–2 h before inoculation removed only part of the effect of sucrose on sporulation. Although the mechanism by which applications of sugars affected susceptibility to downy mildew is not understood, the results suggest that the main effects occurred inside the host plant rather than externally. The possible significance of these results, in breeding for resistance to downy mildew and in the control of this disease in the field, is discussed.     

Downy mildew incidence of pearl millet hybrids with different male-sterility inducing cytoplasms

The use of different sources of cytoplasmic male sterility (CMS) in hybrid seed production of pearl millet [Pennisetum glaucum (L.) R. Br.] is advocated to avoid possible disease epidemics occurring due to cytoplasmic uniformity. The effects of commercially unexploited, but potentially exploitable, sources of CMS, like A₂, A₃ and A₄, on downy mildew [Sclerospora graminicola (Sacc.) Schroet] incidence were studied by using the disease incidence of isonuclear hybrids with male-sterile and fertile cytoplasm. The mean downy mildew incidence of hybrids carrying different male-sterile cytoplasm was similar to that of hybrids retaining the fertile cytoplasm. The cytoplasm accounted for only 0.6% of the total variation and its effect was non-significant; pollinators could explain most of the variation in determining the disease incidence of hybrids. This suggested that these male-sterile cytoplasms are not linked to downy mildew susceptibility and thus can be exploited commercially to broaden the cytoplasmic base of the male-sterile lines and, ultimately, of hybrids.     

Loop-mediated isothermal amplification assay for the detection of Plasmopara viticola infecting grapes

Grapes downy mildew caused by obligate oomycete plant pathogen Plasmopara viticola is a devastating disease worldwide, resulting in significant yield and quality losses. A field survey was conducted in two major grapes cultivated areas of Tamil Nadu for the incidence of grapevine downy mildew. The disease incidence was 43.42%–76.69%, and the highest disease incidence of 76.69% was observed in the Theni district. Totally eight P. viticola isolates were collected from different places in Coimbatore and Theni districts. These isolates were confirmed through microscopic observation and sequencing of COX 2 gene, and the phylogenetic tree was developed to study their phylogenetic relationship among the isolates which shows 97–100% sequence similarity with other P. viticola isolates and less sequence similarity with Plasmopara species. The loop-mediated isothermal amplification (LAMP) assay was developed based on the CesA4 gene sequence of P. viticola. The assay developed was more sensitive as it detected P. viticola genomic DNA up to 20 fmg. LAMP assay specificity was proved by carrying out the assay with genomic DNA extracted from other Oomycetes and fungal plant pathogens. Finally, LAMP assay was validated by testing seventy-eight grapevine leaf samples collected from seven different locations. LAMP assay showed a positive reaction in sixty-two samples tested out of seventy-eight samples tested. Therefore, the LAMP assay described should helpful for early and specific detection of downy mildew pathogen and help in mitigating disease incidence.     

Identification and characterization of potential NBS-encoding resistance genes and induction kinetics of a putative candidate gene associated with downy mildew resistance in <Emphasis Type="Italic">Cucumis</Emphasis>

Background  

Due to the variation and mutation of the races of Pseudoperonospora cubensis, downy mildew has in recent years become the most devastating leaf disease of cucumber worldwide. Novel resistance to downy mildew has been identified in the wild Cucumis species, C. hystrix Chakr. After the successful hybridization between C. hystrix and cultivated cucumber (C. sativus L.), an introgression line (IL5211S) was identified as highly resistant to downy mildew. Nucleotide-binding site and leucine-rich repeat (NBS-LRR) genes are the largest class of disease resistance genes cloned from plant with highly conserved domains, which can be used to facilitate the isolation of candidate genes associated with downy mildew resistance in IL5211S.     

Reclassification of an enigmatic downy mildew species on lovegrass (Eragrostis) to the new genus Eraphthora, with a key to the genera of the Peronosporaceae

Graminicolous downy mildews affect economical important crops like sorghum, maize and pearl millet, and also a variety of wild grasses, which pathogens have hitherto been much less studied. This also applies to an enigmatic downy mildew from Eragrostis cilianensis, which exhibits a chimeric appearance, with similarities to Basidiophora, Sclerophthora, and Sclerospora. In this study, the phylogenetic relationship of lovegrass downy mildew with respect to other genera of the Peronosporaceae was investigated and the characteristics of genera potentially associated with lovegrass downy mildew summarised from historic studies. A key based on the morphology of asexual stages is presented for all genera of downy mildews hitherto described. Based on morphological characteristics and sequence divergence, Sclerospora butleri is transferred to a new genus, Eraphthora. Lovegrass downy mildew seems to be widely distributed, as in addition to previous records from Malawi, India and Australia, we could now sample one specimen from Europe (Spilimbergo, Italy). Currently, it is unclear if Eraphthora butleri is able to infect the crop species Eragrostis tef, which might be a potential host of economic importance.     

Pearl millet downy mildew as affected by integrating seed dressing,sowing date and cultivar

The effects of seed dressing, sowing date and cultivar on incidence and severity of downy mildew of pearl millet induced by Sclerospora graminicola and yield were studied in a two-year field trial conducted at the Research farm of University of Maiduguri. The millet cultivars, Ex-Borno, SOSAT-C88, GB 8735 and Gwagwa were each dressed with metalaxyl at 0.75 and 1.50 g a.i./kg seed; and a batch of undressed seeds of each cultivar served as control. Both dressed and undressed seeds were used for dry-planting and wet-planting in early and late seasons. The results showed that seed dressing with the fungicides significantly (p ≤ 0.05) reduced the incidence and severity of downy mildew and increased grain yield. Dry-planting also significantly (p ≤ 0.01) increased grain yield irrespective of disease incidence. Delay in sowing led to a significant reduction in incidence and severity of downy mildew. Differences between the cultivars in relation to incidence and severity of downy mildew and grain yield were significant. SOSAT-C88 developed low or no downy mildew in both seasons. Sowing of dressed SOSAT-C88 as soon as rainfall established appeared most beneficial in the control of downy mildew. Dry- or wet-planting Ex-Borno dressed with any of the metalaxyl formulations proved to be effective for downy mildew management and for high yield.     

Enhancement of downy mildew of sorghum by some organic soil amendments: a caution to organic farmers

Matured botanical compost of Croton sparsiflorus and Azadirachta indica were evaluated for promotion of sorghum downy mildew at the rate of 0.5% (w/w). Exceptionally high promotional rates of 26.9% (C. sparsiflorus) and 30.1% (A. indica) were observed as compared to negative control (8.8%). The enhancement of disease was higher than that obtained by the supply of recommended dose of urea (13.3%). As the pathogen (Peronosclerospora sorghi) is a biotroph and cannot multiply by using organics as nutrient source in the absence of a host, the mechanism of disease enhancement is inferred to be through stimulation of oospore germination by organic compounds.     

Breakdown of resistance to grapevine downy mildew upon limited deployment of a resistant variety

Background  

Natural disease resistance is a cost-effective and environmentally friendly way of controlling plant disease. Breeding programmes need to make sure that the resistance deployed is effective and durable. Grapevine downy mildew, caused by the Oomycete Plasmopara viticola, affects viticulture and it is controlled with pesticides. Downy mildew resistant grapevine varieties are a promising strategy to control the disease, but their use is currently restricted to very limited acreages. The arising of resistance-breaking isolates under such restricted deployment of resistant varieties would provide valuable information to design breeding strategies for the deployment of resistance genes over large acreages whilst reducing the risks of the resistance being defeated. The observation of heavy downy mildew symptoms on a plant of the resistant variety Bianca, whose resistance is conferred by a major gene, provided us with a putative example of emergence of a resistance-breaking isolate in the interaction between grapevine and P. viticola.     

Comparative evaluation of Pseudomonas fluorescens and their lipopolysaccharides as implicated in induction of resistance against pearl millet downy mildew

Two plant growth promoting Pseudomonas fluorescens isolates namely UOM SAR 14 and UOM SAR 80 most effectively induced resistance against downy mildew disease of pearl millet both under greenhouse and field conditions. Relative assessment of live cultures of P. fluorescens UOM SAR 14 and UOM SAR 80 and their lipopolysaccharides (LPS) extracted from their cell walls were evaluated for their ability to induce resistance against pearl millet downy mildew. Treatment with P. fluorescens and their LPS enhanced the seed germination and seedling vigour considerably. Although both live cultures and their LPS treatment induced resistance in pearl millet against downy mildew disease both under greenhouse and field conditions as evidenced by the significant reduction of the disease, live cultures were more effective than the LPS in level of resistance induced. Live cultures of UOM SAR 14 and UOM SAR 80 induced 66% and 57% protection while their respective LPS extracts offered 59 and 53% protection against downy mildew disease under greenhouse conditions. Similarly, under field conditions with very heavy inoculum pressure live cultures offered 75% and 70%, and their LPS offered 71% and 67% protection, respectively. In either case, the time gap required for the building up of resistance was found to be 3 days and nature of the resistance induced was systemic and durable with both live cultures and their lipopolysaccharides. It was also noticed that the live bacteria significantly varied in the degree of protection offered and so also their respective LPS.     

Control of pearl millet downy mildew caused by Sclerospora graminicola with systemic fungicides in an artificially-contaminated plot

Among four fungicides, viz. metalaxyl (two formulations), fosetyl-Al, pro-pamocarb and cyomaxanil tested in vitro against sporangial germination inhibition of Sclerospora graminicola, cyomaxanil was found to be most inhibitory. In an artificially contaminated plot, when used as seed treatment or foliar spray for the control of downy mildew of pearl millet, only metalaxyl was effective. Metalaxyl 25 (Ridomil) and metalaxyl 35 (Apron) seed treatments protected the pearl millet plants from downy mildew up to 30 days. As a foliar spray, metalaxyl 25 used once at 20 days or twice after 20 and 38 days of plant growth gave less disease at harvest time. Seed treatment (metalaxyl 25 or 35) followed by one metalaxyl 25 spray was found to be effective in controlling the downy mildew. These treatments improved the growth of plants and yield significantly.     

Plant Growth‐promoting Fungus Penicillium oxalicum Enhances Plant Growth and Induces Resistance in Pearl Millet Against Downy Mildew Disease

Plant Growth‐promoting Fungus (PGPF) Penicillium oxalicum was isolated from rhizosphere soil of pearl millet and was tested for its ability to promote growth and induce systemic resistance in pearl millet against downy mildew disease. The fungal isolate P. oxalicum UOM PGPF 16 was identified as P. oxalicum using ITS sequencing and morphological analysis and sequence was deposited at NCBI with accession number KF150220. Pearl millet susceptible seeds were treated with three different inducers (CS, CF and LCF) of PGPF P. oxalicum and all the inducers significantly reduced the downy mildew disease and enhanced plant growth. Among the inducers tested, CS treatment recorded highest seed germination of 91% and 1427 seedling vigour followed by LCF and CF treatments. The vegetative growth parameter and NPK uptake studies under greenhouse conditions revealed that the CS treatment of P. oxalicum remarkably enhanced the parameters tested when compared to control plants. A significant disease protection of 62% and 58% against downy mildew disease was observed in plants pretreated with CS of P. oxalicum under greenhouse and field conditions, respectively. The spatio‐temporal studies revealed that inducers P. oxalicum required a minimum of 3 days for developing maximum disease resistance which was maintained thereafter. The maximum Peroxidase (POX) activity (62.7 U) was observed at 24 h in seedlings treated with CS of PGPF P. oxalicum and the activity gradually reduced at later time points after pathogen inoculation. Chitinase (CHT) activity was significantly higher in inducer treated seedlings when compared to control seedlings inoculated with pathogen after 48 h and remained constant at all time points.     

Elevated atmospheric carbon dioxide and ozone alter soybean diseases at SoyFACE

Human driven changes in the Earth's atmospheric composition are likely to alter plant disease in the future. We evaluated the effects of elevated carbon dioxide (CO₂) and ozone (O₃) on three economically important soybean diseases (downy mildew, Septoria brown spot and sudden death syndrome‐SDS) under natural field conditions at the soybean free air concentration enrichment (SoyFACE) facility. Disease incidence and/or severity were quantified from 2005 to 2007 using visual surveys and digital image analysis, and changes were related to microclimatic variability and to structural and chemical changes in soybean host plants. Changes in atmospheric composition altered disease expression, but responses of the three pathosystems varied considerably. Elevated CO₂ alone or in combination with O₃ significantly reduced downy mildew disease severity (measured as area under the disease progress curve‐AUDPC) by 39–66% across the 3 years of the study. In contrast, elevated CO₂ alone or in combination with O₃ significantly increased brown spot severity in all 3 years, but the increase was small in magnitude. When brown spot severity was assessed in relation to differences in canopy height induced by the atmospheric treatments, disease severity increased under combined elevated CO₂ and O₃ treatment in only one of the 3 years. The atmospheric treatments had no effect on the incidence of SDS or brown spot throughout the study. Higher precipitation during the 2006 growing season was associated with increased AUDPC severity across all treatments by 2.7 and 1.4 times for downy mildew and brown spot, respectively, compared with drought conditions in 2005. In the 2 years with similar precipitation, the higher daily temperatures in the late spring of 2007 were associated with increased severity of downy mildew and brown spot. Elevated CO₂ and O₃ induced changes in the soybean canopy density and leaf age likely contributed to the disease expression modifications.     

Inheritance of downy mildew resistance, β-1,3-glucanases and peroxidases in pearl millet [Pennisetum glaucum (L.) R. Br.] crosses

The inheritance of resistance to downy mildew disease and the defense-related enzymes β-1,3-glucanase and peroxidase was studied in crosses of pearl millet using a generation-mean analysis. The study material comprised six generations (susceptible and resistant parents, F₁, F₂, BC₁ and BC₂) in three crosses. Seedlings from these generations were inoculated with the downy mildew pathogen Sclerospora graminicola and disease incidence was recorded. Analysis of constitutive levels of β-1,3-glucanase and peroxidase in the seedlings of different generations indicated that the resistant populations showed higher enzyme activities, while lower activities of the enzymes were recorded in the susceptible populations. In the generation-mean analysis, the significance of scaling tests revealed the existence of non-allelic interactions in the inheritance of resistance to downy mildew as well as with the enzymes. Among the gene effects, both additive and dominant effects were significant. All the non-allelic interaction effects were significant in the crosses. Studies on the isozyme patterns of the enzymes substantiated the results of the disease-incidence experiments in most of the generations. The results indicated that the inheritance of downy mildew disease resistance and the expression of β-1,3-glucanase and peroxidase in pearl millet is not only under the control of additive and dominant genes but are also governed by complex non-allelic interactions. Received: 30 April 2000 / Accepted: 17 October 2000     

Synergistic effects of Trichoshield on enhancement of growth and resistance to downy mildew in pearl millet

Trichoshield, a talc formulation consisting of spores of Trichoderma harzianum, Trichoderma lignorum, Gliocladium virens and Bacillus subtilis was tested, following different application methods, for its ability to promote growth of pearl millet plants and to induce resistance to downy mildew of pearl millet. Under laboratory conditions, trichoshield seed treatment enhanced seed germination and seedling vigor of pearl millet significantly over the control; under greenhouse conditions vegetative growth parameters like height, fresh and dry weight, leaf area and number of tillers were significantly enhanced over the control: Trichoshield formulation offered greater protection against downy mildew in comparison with individual strains of T. harzianum, T. lignorum, G. virensand B. subtilis. Among the methods of application, foliar spray was found to be a more efficient delivery method than seed treatment or slurry treatment. Combinations of foliar spray with seed treatment and slurry treatment produced the same effect as foliar spray alone. Under field conditions, trichoshield treatment enhanced reproductive parameters like number of earheads, length and girth of earheads, 1000 seed weight and yield significantly over the control. Days required for 50% flowering was reduced by 4 days compared to the control. Yield enhancement of 28% over the control was highly significant. Trichoshield treatment offered protection ranging from 52 to 71% under field conditions, depending on the application method. However, the chemical fungicide metalaxyl Apron provided the highest protection against downy mildew, both under greenhouse and field conditions.     

58种植物提取液对葡萄霜霉病菌的抑菌活性筛选研究

通过 5 8种植物提取液对葡萄霜霉病菌 (Plasmoparaviticola)的游动孢子囊萌发抑制率及接种叶圆盘的发病控制效果的研究 ,表明 :虎耳草 (ChlorisvirgataSwartz) ,黄檀 (DalbergiahuoeanaHance) ,马尾松 (PinusmassonianaLarmb .) ,牡丹 (Paeoniasuffruticosa) ,刺槐 (RobiniapseudoacciaL .) 5种植物提取液对葡萄霜霉病有抑菌效果 ,对游动孢子囊萌发抑制率分别为 :84 70 %、87 5 2 %、88 4 6 %、83 78%、83 15 % ,而叶圆盘生测结果 ,其发病率均为 0级 ,该效果和目前常用药剂波尔多液的效果等同 ,有的还优于对照。该研究为开发植物源控制葡萄霜霉病杀菌剂奠定了基础。