Development Of Powdery Mildew Research Articles

Powdery mildew-induced changes in phyllosphere microbial community dynamics of cucumber.

As an important habitat for microorganisms, the phyllosphere has a great impact on plant growth and health, and changes in phyllosphere microorganisms are closely related to the occurrence of leaf diseases. However, there remains a limited understanding regarding alterations to the microbial community in the phyllosphere resulting from pathogen infections. Here, we analyzed and compared the differences in phyllosphere microorganisms of powdery mildew cucumber from three disease severity levels (0% < L1 < 30%, 30% ≤ L2 < 50%, L3 ≥ 50%, the number represents the lesion coverage rate of powdery mildew on leaves). There were significant differences in α diversity and community structure of phyllosphere communities under different disease levels. Disease severity altered the community structure of phyllosphere microorganisms, Rosenbergiella, Rickettsia, and Cladosporium accounted for the largest proportion in the L1 disease grade, while Bacillus, Pantoea, Kocuria, and Podosphaera had the highest relative abundance in the L3 disease grade. The co-occurrence network analysis of the phyllosphere microbial community indicated that the phyllosphere bacterial community was most affected by the severity of disease. Our results suggested that with the development of cucumber powdery mildew, the symbiotic relationship between species was broken, and the entire bacterial community tended to compete.

Efficiency of the application of biological systems of the protection of apple trees against powdy dew and scamb in the conditions of the Western Forest-Steppe of Ukraine

Goal. To study the effectiveness of different biological systems of apple tree protection against scab and powdery mildew in the Western Forest-Steppe of Ukraine.&#x0D; Methods. Phytosanitary monitoring was carried out visually and with the help of pheromone traps. Accounting of the spread and development of diseases was carried out according to generally accepted methods in the phases of apple tree development: «bud swelling», «green cone», «bud extension», «bud separation», «pink bud», «flowering», «end of flowering», «fruit formation», «fruit growth» and «fruit ripening». The technical effectiveness of fungicides was determined after 7 days. Statistical processing of the research results was carried out according to generally accepted methods.&#x0D; Results. The results of monitoring the phytosanitary state of apple plantations established that the most common diseases in 2021—2022 in the conditions of the Western Forest Steppe of Ukraine were scab, which affected 15—29% of leaves and 8—13% of apple fruits, powdery mildew, which affected 15—18% of leaves of apple trees, moniliosis, the prevalence of which was 9—13%, and fruit rot, which affected 13—14% of apple fruits. The research results showed that the use of the biological protection system No. 1 (Fluorescin BT, Bactophyt BT, BioHybervit BT, Trihopsin BT, Fluorescin BT) made it possible to reduce the development of powdery mildew by 59.0%, and to reduce the development of scab on leaves and by 59.1%, 62.2% — scab on apple fruits. The use of biological protection system No. 2 (Bactophyt BT + Trihopsin BT, BioHybervit BT, Fluorescin BT, Trihopsin BT, Fluorescin BT) made it possible to reduce the development of powdery mildew by 43.6%, by 47.7% to reduce the development of leaf scab and by 48.6% — scab on apple fruits. The use of biological protection system No. 3 (Ampelomycin BT + Bactophyt BT, Phytosporin + Fluorescin BT, BioGibervit BT, Biospectr BT, Trihopsin BT + Fluorescin BT) made it possible to reduce the development of powdery mildew by 53.8%, and to reduce the development of scab by 68.2% leaves and 67.6% — scab on apple fruits. Using the studied biological systems to protect apple trees from scab and powdery mildew in the conditions of the Western Forest-Steppe of Ukraine yield indicators of 13.2—13.8 t/ha were obtained. The use of the studied biological protection systems made it possible to obtain from 2946 to 8526 UAH/ha of conditional net income and profitability indicators from 72.7 to 229.4%.&#x0D; Conclusions. The results of the research established that all investigated systems of biological protection of apple trees against scab and powdery mildew in the conditions of the Western Forest Steppe of Ukraine made it possible to protect apple plantations at a reliable level and obtain high yields. The technical efficiency of the applied protection systems against powdery mildew was 43.6—59.0%, against scab on apple leaves — 47.7—68.2%, and against scab on apple fruits — 48.6—67.6%. The yield for the application of the investigated biological systems of apple tree protection against scab and powdery mildew was 13.2—13.8 t/ha. Biological protection system No. 1 was the most effective in terms of profitability — 229.4%, and the highest indicator of conditional net income was obtained from the use of biological system No. 3 — UAH 8526/ha.

Pre‐infection efficacy of resistance inducers against grapevine powdery mildew

AbstractPlant resistance inducers (PRIs) are of increasing interest for disease control in sustainable viticulture, but their efficacy in vineyards is still under investigation. We studied the effects of four commercial PRIs, of natural and microbial origin, on the development of powdery mildew (PM) on grapevine leaves over a 3‐year period. Disease and sporulation severity were evaluated following the artificial inoculation of PRI‐treated leaves with Erysiphe necator conidia at 1, 3, 7 and 12 days after treatment (DAT). Laminarin (LAM) and cos‐oga (COS) were the most effective in reducing both disease (57.1% and 49.5%, respectively) and sporulation (85.3% and 90.2%, respectively) severity; the reduction in PM severity provided by cerevisane (CER) was lower (27%), and that of Pythium oligandrum (PYT) was intermediate (44.9%). LAM was effective from 1 DAT, while COS and PYT were more effective when E. necator was inoculated at 7 to 12 DAT. These results open new perspectives on how PRIs are used as single, preventative interventions performed 1 to 3 days before E. necator infection, depending on the product used. The effect of PRIs on the sporulation of E. necator colonies also suggests a broader and longer effect on PM epidemics.

Effect of Environmental Factors in the Development of Powdery Mildew in Okra [Abelmoschus esculentus (L.) Moench

In order to study the effect of environmental factors in the development of powdery mildew in okra [Abelmoschus esculentus (L.) Moench] an experiment was carried out at the College of Agriculture, IGKV, Raipur, Chhattisgarh during kharif (2019-20). The correlation analysis revealed that the age of the crop was significantly, positively correlated and T max, T min and morning RH were significantly, negatively correlated with the disease severity. In addition to this by linear regression analysis, it was observed that plant age (69%), maximum temperature (62%), minimum temperature (60%) and morning relative humidity (46%) contributed in percent disease severity. Based on the proportional measures of extent of infection at different times the apparent rate of infection was found maximum during 70-84 DAS. At this time when the disease progress the average maximum temperature was 29.3℃, minimum temperature 14.9℃, morning relative humidity 87.6% and evening relative humidity 38.33%.

Epidemiology of Powdery Mildew of Apple caused by Podosphaera leucotricha (Ell.and Ev.) Salm. in Kashmir, India

Apple is an economically important horticultural crop that is affected by a number of diseases, including powdery mildew. The disease spread is affected by the environmental conditions during the growth period. The germination and appressoria formation of conidia of powdery of apple initiated after 2 and 4 h, respectively at 20 ±2oC in moist saturated atmosphere. Sporulation started 96 h after incubation on susceptible apple leaves. Cardinal temperatures for powdery mildew development were 10, 20 and 30 oC. Relative humidity levels of 32 percent and above favoured development of powdery mildew. Temperature had negative significant correlation, whereas relative humidity had positive non-significant correlation with incidence of disease.

Beta regression model for predicting development of powdery mildew in black gram

Black gram is a widely grown pulse crop in Asia, prized for its nutritional value and compatibility with various cropping systems. However, the occurrence of powdery mildew, Erysiphe polygoni DC disease poses a significant challenge to black gram production, resulting in potential yield losses in Tamil Nadu. Over a six-year period, spanning from 2017-2018 to 2022-2023, field experiments were conducted during the rabi season at the black soil farm of the Agricultural Research Station in Kovilpatti. The primary objective was to evaluate the incidence of powdery mildew in black gram and establish a statistical model by correlating it with weather variables. Notably, observations of disease index were most frequent during the flowering and pod development stages of the crop. Among the eleven weather parameters considered in the study, maximum temperature, afternoon relative humidity, and sunshine hours emerged as the key contributors to explaining the variation in the Disease Index. Further, a betareg model was developed using these selected variables to predict powdery mildew incidence in black gram.

Influence of abiotic factors on the development of powdery mildew on winter barley cultivars in the Western Forest-Steppe of Ukraine

Influence of abiotic factors on the development of powdery mildew on winter barley cultivars in the Western Forest-Steppe of Ukraine

Effectiveness of the use of biopreparations and fungicides in the cultivation of spring wheat in the forest-steppe of the Priobye

The data on the effectiveness of using chemical, fungal and bacterial fungicides to treat seeds and crops of spring soft wheat in order to limit harmfulness of major diseases are presented. The research was conducted in 2020–2022 on chernozem leached forest-steppe of the Novosibirsk region Priobye. Chemical seed protectant Scarlet and its mixture with Vitaplan were the most effective dressers in the fight against root rots. It has been demonstrated that although biopreparations are inferior in efficiency to chemical ones, they act for a longer period of time and protect the plant from the disease for a longer period of time. Seed dressing with chemical preparation reduced the development of leaf diseases by 53–58%, and with biopreparations – by 34–41%. Titul 390 was the most effective of the vegetation treatments – 60–98%. The efficiency of the biopreparations was 42–64% in the years of moderate disease development. In the years of strong development of septoriosis or powdery mildew, the efficiency of biopreparations did not exceed 22–30%. The biological preparations had a growth-stimulating effect on the plants: they increased the flag leaf area by 22–39%. The seed treatment showed an increasing trend in the number of spikelets, grains per ear and grain weight per ear. Application of the preparations during the vegetation period increased reliably the ear length by 16–28%, the number of spikelets – by 15–20, grains – by 24–31, grain weight per ear – by 33–51%. At seed dressing the greatest increase in the yield (0.15 t/ha) was provided by Scarlet, when using a mixture of biological and chemical preparations it amounted to 0.11 t/ha. When treated during the growing season, chemical fungicide increased the yield by 0.33 t/ha, while biopreparations increased the yield by 0.11–0.14 t/ha. Complex use of chemical preparations (Scarlet + Titul 390) provided the maximum yield increase – 0.62 t/ha, from the joint use of chemical and biological preparation the yield increased by 0.15–0.33 t/ha. The biological preparations increased this indicator by 0.13–0.23 t/ha. It has been concluded that it is possible to select combinations of chemical and biological preparations that can control phytosanitary situation and provide acceptable yield.

Мучнистая роса дуба в защитных лесных насаждениях Волгоградской области

In the conditions of the Volgograd region, the source of infection of oak with powdery mildew (the causative agent is Erysiphe alphitoides (Griffon et Maubl.) U. Braun et S. Takam.) is the conidial and marsupial stages of the fungus – ascospores, which overwinter in fruit bodies (cleistothecium) almost within the entire range of the host. The development of the disease on the leaves of the shoots of the first growth depends on the phenological features of the oak and the weather conditions of the first growing season. When studying the regularities of oak phenology and the manifestations of the disease, systematic observations were carried out during the growing season in stands of pedunculate oak (Quercus robur L.) with a periodicity of 10 days. The phenology of oak was studied on 100 specimens of plants in protective forest stands on the territory adjacent to the city of Volgograd. At the same time, the development of powdery mildew of leaves was recorded, and conidium was captured using slides, with the help of agar-strips applied to them. The intensity of spore formation during the growth of the fungus was determined using the Goryaev counting chamber by the number of spores in a certain volume of suspension. Weather conditions for phenological observations were recorded using meteorological data for the period of studying the dynamics of the summer of powdery mildew-conidia and taken from the site “Pogoda i klimat”. It has been established that the seasonal development of conidia Erysiphe alphitoides is determined by a combination of the thermal regime and the conditions of humidification of the growing season. With the help of regression analysis for oak plantations in the arid zone of the Volgograd region, a prognostic model of a short-term meteorological and biological forecast of powdery mildew was developed, which made it possible to establish the dates of the disease and the terms of plant treatment through the dynamics of the spread of conidia.

Effective management of powdery mildew in cantaloupe plants using nighttime applications of UV light.

Ultraviolet light at wavelengths from 254 to 283 nm has been reported to effectively suppress powdery mildews in several crops, including some cucurbits. Its use to suppress powdery mildew (Podosphaera xanthii) specifically in cantaloupe has not been previously reported. We evaluated the foregoing technology in cantaloupe fields for suppression of powdery mildew and possible effects on plant growth, and yield. In a controlled laboratory study, greenhouse-grown cantaloupe plants were exposed to a gradient of UV-C (254 nm) doses during darkness, and the effects upon powdery mildew development and the plant were evaluated. We also evaluated the efficacy of nighttime applications of UV-C at 100 and 200 J/m2 against powdery mildew on adaxial leaf surfaces in greenhouse, high-tunnel and open-field plantings. UV-C at the foregoing doses reduced sporulation and germination of conidia P. xanthii without damaging plants. On cantaloupe seedlings in the greenhouse, disease severity was equivalently suppressed at all doses and frequencies of applications of the light. In high-tunnel and open-field experiments, the most effective control of powdery mildew was provided by UV-C applied at 200 J/m2 twice every week, where suppression provided by UV-C was generally equal to and sometimes better than the fungicide treatment. The foregoing UV-C dose and frequency of application also provided the highest yield under field conditions, indication that UV-C treatment is a promising technology for commercially-relevant suppression of powdery mildew on cantaloupe in a variety of growing systems.

Refining Management of Apple Powdery Mildew in New York State with Weather-Based Fungicide Application Timing Programs.

In the absence of durable host resistance among commercial cultivars, chemical management continues to be an essential component of disease control in apple production. Apple powdery mildew, caused by the ascomycete Podosphaera leucotricha, is largely managed with regular fungicide applications from the host phenological stages of tight cluster to terminal bud growth set, with applications typically being made in a prophylactic manner irrespective of existing disease pressure. Here we evaluated two management programs that aligned fungicide applications to specific weather thresholds conducive to powdery mildew development using a rotation of single-site fungicides and sulfur. In three separate orchards among four cultivars, we compared powdery mildew disease progression over the growing season for each of the weather factor-based programs and a typical calendar-based application program. In each year of the trial, we found that management programs with weather-based fungicide applications provided levels of disease control similar to the calendar program but required 50 to 83.3% fewer mildew-specific fungicide applications throughout the growing season. Our results provide a framework with which to evaluate future weather-based management programs for apple powdery mildew management. This knowledge could be implemented in the creation of a powdery mildew disease management decision support system to better inform and aid fungicide application programs for continued sustainable apple production in the northeast United States.

Germination Development of Powdery Mildew on Natural and Artificial Wheat Leaf Surfaces: A Study to Investigate Plant Wax Signals

Germination Development of Powdery Mildew on Natural and Artificial Wheat Leaf Surfaces: A Study to Investigate Plant Wax Signals

Assessment of Fungicide Resistance via Molecular Assay in Populations of Podosphaera leucotricha, Causal Agent of Apple Powdery Mildew, in New York.

Podosphaera leucotricha, causal agent of apple powdery mildew, is a pathogen endemic worldwide where apples are produced. In the absence of durable host resistance, the disease is most effectively managed in conventional orchards with single-site fungicides. In New York State, increasingly erratic precipitation patterns and warmer temperatures due to climate change may create a regional environment more conducive to apple powdery mildew development and spread. In this scenario, outbreaks of apple powdery mildew may supplant the apple diseases of current management concern: apple scab and fire blight. Presently, there have been no reports from producers of fungicide control failures for apple powdery mildew, though increased disease incidence has been reported to and observed by the authors. As such, action was needed to assess the fungicide resistance status of populations of P. leucotricha to ensure key classes of single-site fungicides (FRAC 3, demethylation inhibitors, DMI; FRAC 11, quinone outside inhibitors, QoI; and FRAC 7, succinate dehydrogenase inhibitors, SDHI) remain effective. In a 2-year survey (2021 to 2022), we collected 160 samples of P. leucotricha from 43 orchards, representing conventional, organic, low-input, and unmanaged orchards from New York's primary production regions. Samples were screened for mutations in the target genes (CYP51, cytb, and sdhB) historically known to confer fungicide resistance in other fungal pathogens to the DMI, QoI, and SDHI fungicide classes, respectively. Across all samples, no nucleotide sequence mutations that translated into problematic amino acid substitutions were found in the target genes, suggesting that New York populations of P. leucotricha remain sensitive to the DMI, QoI, and SDHI fungicide classes, provided no other fungicide resistance mechanism is at play in the population.

Effectiveness of fungicidal protection of spring wheat against powdery mildew and tan spot

The results of the study of the effectiveness of plant protection products against powdery mildew (Blumeria graminis) and tan spot (Pyrenophora tritici-repentis) for spring wheat are presented. It has been noted that the probability of mass destruction of crops by phytopathogens is determined by the stock of the infectious beginning of pathogens, the susceptibility of the host plant and the favorable weather conditions for their development and spread. The experiment was carried out using the methods generally accepted in the Russian Federation. During the observation period, the degree of plant damage of more than 20% in the heading phase was noted in 46% of the years of observation (in 2009, 2013, 2014, 2016, 2017). Epiphytotic development of powdery mildew together with brown leaf rust (Puccinia triticina Eriks.) was registered in 2013–2014 and 2016–2017. For 2009–2019 the degree of plant damage of more than 20% in the heading phase was noted in 46% of the years of observation (in 2009, 2013, 2014, 2016, 2017). Effective protection of wheat against powdery mildew (75–87%) was provided by fungicides based on 2–3 active ingredients. The biological effectiveness of the biofungicide (Bacillus subtilis) was manifested only in the years with moderate wheat damage and did not exceed the average level of 58%. It is more expedient to carry out operational control of yellow spotting of wheat leaves with preparations based on such active substances as "azoxystrobin + epoxiconazole", "tebuconazole + propiconazole". The level of statistically significant yield preserved due to fungicidal protection during the years of depression was 5–6%, with a moderate development of leaf diseases due to chemical fungicides - 18%, and the biological product provided an increase in productivity of 9%. During the years of epiphytoties, the chemical protection of crops retained an average of 24% of the wheat yield, polycomponent preparations had the best performance and stability of action, and the biological protection of plants under these conditions was ineffective.

The optimization of grain crops’ phytosanitary condition with the associative rhizobacteria application

The effectiveness of the associative rhizobacteria influence on the intensity of the soft wheat and triticale diseases’ development was studied. According to the experimental scheme, seeds were soaked and plants were sprayed twice with a working fluid, containing rhizobacteria strains of Bacillus subtilis 124-11, Sphingomonas sp. K1B and Pseudomonas fluorescens SPB2137 (titer 108-109 cl/ml), and other variants with combined treatment with a liquid organomineral composition Batr Gum were used. In the control plants were treated with water and Batr Gum (by 10 ml/l of water). The biopreparations influence on the diseases development was investigated using generally accepted accounting scales and additional phytopathological indicators. The Bacillus subtilis strain 124-11, had the maximal effectiveness against yellow and brown rust of wheat, and Sphingomonas sp. K1B - against triticale rusts. In particular, the treatment of the local cultivar Leningradskaya 6 with Bacillus subtilis 124-11 led to a decrease in the development of yellow rust by 12 %, the pustules number by 47 %, the strip length by 41 %, the pustule area by 39 %; the brown rust development by 9 %, the pustules number by 50 %, the pustule area the by 40 %. After the Dua, k-828 cultivar treatment with the Sphingomonas sp. K1B strain, a decrease in the plants affection by brown rust was observed by 13 %, the pustules number decreased by 59 %, the pustule area by 52 %. A significant decrease in the powdery mildew development on soft wheat and a decrease in the number of spots with plaque were recorded when using the Sphingomonas sp. K1B strain on cultivars: Leningradskaya 6 - by 12 % (79.4 %), Ajeeba - 19 % (72.5 %), Trizo - 13 % (87.2 %), Sudarynya - 3 % (60 %). On the soft wheat cultivar Sudarynya and triticale cultivars Aist Kharkovskiy and Dua, a decrease in helminthosporiotic root rot was revealed when using associative rhizobacteria; the maximal decrease in the disease development (by 32 %) was noted when using the strain Sphingomonas sp. K1B. When bacterial strains were used combined with the organomineral fertilizer Batr Gum, the development of powdery mildew and yellow rust was much slower than when they were used separately, while the greatest effectiveness against these diseases (by 31 % and 91 %) was revealed in the «Batr Gum + Pseudomonas fluorescens SPB2137» experimental variant.

Germination Development of Powdery Mildew on Natural and Artificial Wheat Leaf Surfaces: A Study to Investigate Plant Wax Signals

Wax chemistry, especially long‐chain aldehydes, and surface wettability are discussed to have a stimulating effect on the development of Blumeria graminis, the pathogen of powdery mildew on wheat. Here, these specific surface properties are investigated on leaves of different wheat varieties and on developed test systems coated with plant wax and wax components. So far, the wettability of leaves has not been achieved by artificial substrates used in in vitro studies. With the test systems developed here, a wettability comparable to that on leaf surfaces and the signal character of wax chemistry could be investigated individually and in combination. The results show that wax morphology and chemistry (analyzed by scanning electron microscopy and gas chromatography) as well as wettability of leaves have no differences that would suggest a relationship with variety susceptibility. Furthermore, the influence of wax chemistry and wettability on the process of prepenetration of B. graminis on leaves and test systems is investigated. Germination and differentiation are not stimulated on any of the test systems compared to surfaces without the signals offered. Wettability, wax chemistry, and components such as long‐chain aldehydes could not be identified as decisive signals for the germination development of B. graminis.

Estimating yield in commercial wheat cultivars using the best predictors of powdery mildew and rust diseases.

This four-year research determined the best predictors of black, brown and yellow rusts and powdery mildew development in different wheat cultivars and planting dates across 282 experimental field plots. Parameters estimated by exponential (for black rust and powdery mildew) and Gaussian (for brown and yellow rusts) models, area under disease progress curve (AUDPC), and maximum disease severity were considered as disease progress curve elements. Factor analysis determined the most predictive variables among 19 indicators in order to describe wheat yield. According to principal component analysis (PCA), 11 selected wheat diseases and yield predicators accounted for 60% of total variance in datasets. This PCA test described four principal components involving these selected predictors. Next, multivariate regression model, which developed according to four independent principal components, justified a noticeable part of yield variability over and within growing seasons. Present findings may improve accuracy of future studies to examine seasonal patterns of powdery mildew and rusts, predict wheat yield and develop integrative disease management programs.

Sources of spring common wheat resistance to powdery mildew

Effective breeding work to create disease-resistant wheat varieties provides for regional, retrospective studies to determine effective sources of resistance. One of the most harmful and globally significant wheat diseases is powdery mildew. In the conditions of Tatarstan, located in the Volga economic region of Russia, the mass development of powdery mildew on spring common wheat is noted annually, regardless of the prevailing weather conditions. The prevailing high natural infectious background allows us to objectively differentiate spring wheat samples by disease resistance. The purpose of our research was to search for sources of resistance to powdery mildew among the samples of spring common wheat of the VIR collection, which retain stability for a long time. The studies were conducted in the period from 2011 to 2021. In total, during the entire period, the resistance to powdery mildew of 702 samples was studied in the conditions of the Pre-Kama zone of the Republic of Tatarstan. The reaction of the samples was evaluated to a natural population of the fungus Blumeria graminis f. sp. tritici, caused by powdery mildew of wheat. The screening of spring common wheat samples for the period from 2011 to 2020 allowed us to identify sources of field resistance to powdery mildew that retain their effectiveness for a long time: Tybalt (k-64897, Netherlands), Zebra (k-64707, Sweden), CH Rubli (k-65003, Germany), Visa (k-64390, Belarus). During this period, there was a decrease in the resistance of spring common wheat samples to powdery mildew from 3.5 points in 2011-2015 to 4.6 points in 2016-2020 (on a 9-point scale). At varietals: Toma (k-66193), Sudarynya (k-6647), Laska (k-66421) from Belarus; KWS Torridon (k-66273, UK); Tybalt (k-64897, Netherlands); Griwa (k-66701), Zura (k-66702) from Poland; Florens (k-66391, France); Odeta (66394, Czech Republic); Lavett (k-66095), Boett (k-66353) from Sweden no symptoms of powdery mildew were recorded in 2019-2021. The collection of Russian varieties and varieties from the countries of the former USSR (except Belarus) is poor in resistant varieties, only two varieties of Barakat and Sitara from Tatarstan turned out to be highly resistant. Resistant to B. graminis samples mainly originate from the European part of the world (band: Great Britain ?Belarus), approximately in geographical coordinates - 50...55? north latitude.

Mechanisms of resistance to powdery mildew in cucumber

Podosphaera xanthii causes powdery mildew of cucumber, and is associated with significant yield and quality losses. Development of resistant or tolerant varieties is the most effective and eco-friendly strategy for powdery mildew management. An important host resistance mechanism is based on the recognition of conserved resistance genes, resulting in durable resistance. To determine powdery mildew resistance mechanisms in cucumber, total RNAs were isolated from the powdery mildew resistant cultivar Meltem, the tolerant line VT18, and the susceptible local variety Camlica. Expression levels of nine genes in these plants were analysed by Reverse Transcription Polymerase Chain Reaction (RT-PCR). The host reactions were assessed using microscope observations of stained specimens. Serine/threonine (STN7), transcription factor (WRKY22), serine/threonine-protein kinase (D6PKL1), and serine/threonine receptor kinase (NFP) genes were induced, as positive regulators in defence mechanisms against powdery mildew. Polygalacturonase Inhibitor (PGIP) did not express after P. xanthii inoculation of Camlica, resulting in susceptibility. After inoculation, callose synthase (CALLOSE) and cinnamyl alcohol dehydrogenase (CAD) gene expression levels were increased in resistant Meltem, but Hypersensitive Reaction (HR) and ROS formation were only linked in the tolerant VT18. Powdery mildew development was less in Meltem than in VT18, indicating that cell wall thickening and HR play separate roles in resistance to this disease.

Temperature Influences on Powdery Mildew Susceptibility and Development in the Hop Cultivar Cascade.

The hop cultivar 'Cascade' possesses partial resistance to powdery mildew (Podosphaera macularis) that can be overcome by recently emerged, virulent isolates of the fungus. Given that hop is a long-lived perennial and that brewers still demand Cascade, there is a need to better understand factors that influence the development of powdery mildew on this cultivar. Growth chamber experiments were conducted to quantify the effect of constant, transient, and fluctuating temperature on Cascade before, concurrent to, and after inoculation as contrasted with another powdery mildew-susceptible cultivar, 'Symphony'. Exposure of plants to supraoptimal temperature (26 and 32°C) before inoculation led to more rapid onset of ontogenic resistance in intermediately aged leaves in Cascade as compared with Symphony. Cascade was overall less susceptible to powdery mildew when exposed to constant temperature ranging from 18 to 32°C directly after inoculation. However, cultivar also interacted with temperature such that proportionately fewer and smaller colonies developed on Cascade than Symphony at supraoptimal yet permissive temperatures for disease. When plants were inoculated and then exposed to high temperature, colonies became progressively more tolerant to temperatures of 26 to 30°C with increasing time from inoculation to exposure, as moderated by cultivar, the specific temperature, and their interaction. Subjecting plants to simulated diurnal temperature regimes at the time of inoculation or 24 h later indicated Cascade and Symphony responded proportionately similarly on days predicted to be marginally unfavorable or marginally favorable for powdery mildew, although Cascade was quantitatively less susceptible than Symphony. In sum, this research indicates that Cascade is overall less susceptible to powdery mildew than Symphony, and supraoptimal temperature before, concurrent to, or after infection may interact differentially to moderate disease risk in Cascade. Therefore, cultivar-specific risk assessments for powdery mildew appear warranted.