Strawberry powdery mildew, caused by Podosphaera aphanis , is a major disease of strawberry, capable of leading to yield losses of up to 70%. Its management typically relies on frequent chemical fungicide applications due to the extended infection window of the pathogen. Podosphaera aphanis overwinters as mycelium and chasmothecia on infected plant material, which may act as primary inoculum in spring. This two-year study investigated the effectiveness of post-winter removal of infected leaf and plant debris in disease control and quantified the role of chasmothecia in early season infection under high-tunnel conditions. Complete removal of infected tissues delayed disease onset and reduced severity by up to 88%. In the two years, chasmothecium formation increased in late summer following 8 and 16 cumulative hours below 13 °C, respectively. In spring, ascosporic infection events occurred after the accumulation of 21 and 18 cumulative hours with temperatures above 10 °C combined with at least two consecutive hours of leaf wetness, consistently with previous findings. The efficacy of the sanitation practice was further supported by excluding the possibility that chasmothecia and/or chasmothecia on debris fragments potentially fallen into the substrate could trigger infection. In addition, we investigated the timing of ascospore-derived infections by monitoring disease progression on individual leaves. Infection rate was quantified across different leaf ages to assess their role in epidemic development and their potential contribution to secondary inoculum. The highest infection rate was observed on 15-day-old class age, with a crucial role in supporting early pathogen development. These results indicate the importance of sanitation practices, such as overwintering infected leaf removal in reducing early-season infections and informing more targeted control strategies.

ABSTRACTPowdery mildew, caused by Podosphaera aphanis, poses a significant threat to strawberry production, while current chemical controls raise environmental and food safety concerns. In this study, we have identified the key regulatory module, FaMYB63/FvWRKY75‐PR10.14, that confers enhanced powdery mildew resistance in transgenic strawberry (Fragaria vesca). FaMYB63, an R2R3‐MYB transcription factor, was induced by P. aphanis infection and responsive to the application of defence signalling molecules, including salicylic acid (SA), methyl jasmonate (JA), abscisic acid (ABA), and 1‐aminocyclopropane‐1‐carboxylic acid (ACC). Silencing of FaMYB63 led to reduced SA levels and increased powdery mildew susceptibility, accompanied by suppressed reactive oxygen species (ROS) bursts and down‐regulation of PR10.14 expression. Conversely, overexpressing PR10.14 in transgenic lines inhibited P. aphanis spore germination and enhanced ROS accumulation, indicating a dual role in direct pathogen inhibition and hypersensitive response‐triggered defence. Yeast one‐hybrid, electrophoretic mobility shift assay, β‐glucuronidase, and luciferase assays confirmed that FaMYB63 and FvWRKY75 were directly bound to the MYB‐binding sites and W‐box of the PR10.14 promoter, respectively, and activated its transcription, while WRKY75 negatively regulated the expression of MYB63. PR10.14 exhibited tissue‐specific expression, with the highest levels in red‐ripening fruits, suggesting a role in developmental‐stage‐dependent defence. These findings suggest FaMYB63 as an SA‐dependent regulator of PR10.14‐mediated resistance, bridging hormone signalling and pathogen response. This study provides a molecular target for breeding powdery mildew‐resistant strawberry cultivars through genetic engineering approaches, offering an alternative to fungicides for sustainable and environmental disease management in horticultural crops and advances our understanding of MYB‐ or WRKY‐PR10P networks in plant immunity.

Powdery mildew of strawberry, caused by Podosphaera aphanis, is an economically important disease that infects flowers, fruit, leaves, and petioles. This report evaluates the host resistance to powdery mildew of 21 cultivars and elite breeding lines (genotypes) under greenhouse conditions in San Luis Obispo, California in winter of 2024. Disease incidence and severity were evaluated for all cultivars. Results of this study will inform strawberry growers in selecting resistant cultivars for optimized production.

ABSTRACTPowdery mildew, caused by Podosphaera aphanis, is a severe disease affecting strawberry production in Japan. The biofungicide BotaniGard ES, whose active ingredient is Beauveria bassiana strain GHA, was reported to be effective against P. aphanis. In the present study, the efficacy of BotaniGard was clarified under a range of conditions to evaluate its practicality in the greenhouse. In greenhouse efficacy trials, the control efficacy of BotaniGard was relatively high (70.4%–100%) against the disease under low disease pressure, but low (28.1%–53.4%) under high disease pressure. In addition, the application 6 h before inoculation was the most effective compared with application 7 days before or 7 days after inoculation. The application of BotaniGard at 1‐week intervals (control efficacy of 66.5%) was more effective against powdery mildew on leaves than at 2‐ or 3‐week intervals. In contrast, the application of BotaniGard was not effective against powdery mildew on fruits, regardless of application interval, although most of the chemical fungicides tested for comparison were effective. RNA‐seq analysis indicated that the application of BotaniGard upregulated the defence response, systemic acquired resistance and chitinase activity in leaves. These results indicate that BotaniGard is effective in controlling strawberry powdery mildew on leaves under low disease pressure, but not on fruits, and the mode of action might be the activation of induced plant defences. This study encourages a detailed evaluation of the efficacy of biofungicides to promote their appropriate use in greenhouse production.

Key messageMultiple QTLs for powdery mildew resistance were identified in a pre-breeding population derived from the octoploid progenitor species of garden strawberry, including a stable major novel factor on chromosome 3B.Powdery mildew (PM), caused by the biotrophic fungal pathogen Podosphaera aphanis, poses an increasing threat to garden strawberry (Fragaria × ananassa) production worldwide. While a few commercial cultivars exhibit partial resistance, fungicide application remains essential for managing PM outbreaks. However, breeding offers a more sustainable approach for controlling PM. A better understanding of the genetics of resistance is required for informed breeding strategies, e.g. through identifying novel resistance factors derived from the progenitor species of garden strawberry, F. chiloensis and F. virginiana. We conducted genome-wide association (GWA) and multivariate analyses in a reconstructed (ReC) strawberry population to investigate PM resistance under natural infection. Leveraging multi-year field trial data and 20,779 single-nucleotide polymorphism markers, we identified a novel major quantitative trait locus (QTL) on chromosome 3B, designated as q.LPM.Rec-3B.2, that was consistently associated with high PM resistance in both leaves and fruits. Greenhouse validation with a subset of the ReC population confirmed that this QTL region was stable across field and greenhouse environments. Promising candidate genes for resistance, including two for MLO and one for EXO70, were identified within this major QTL. In addition, multi-locus GWA models and non-additive GWA revealed additional resistance QTLs on multiple chromosomes. Despite previous challenges in breeding for robust PM resistance due to its quantitative nature and complex genetic control, our results provide valuable insights into resistance-contributing QTL regions already existing in strawberry, novel wild-derived resistance QTLs not previously known, candidate genes, and pre-breeding germplasm carrying resistance traits as resources for future genome-informed breeding efforts.

Podosphaera aphanis, causal agent of strawberry powdery mildew, is a foliar pathogen that infects flowers, fruit, leaves, and petioles. The disease can result in declining plant health, deformed fruit, and yield loss. This report evaluates the efficacy of fungicides for the control of powdery mildew under greenhouse conditions in San Luis Obispo, CA, in the summer of 2024. Disease incidence and severity were evaluated for all treatments and controls. Results of this study will inform strawberry growers in developing effective fungicide programs for managing powdery mildew.

Strawberry powdery mildew, caused by the obligate biotroph Podosphaera aphanis, is a major threat to commercial strawberry production, reducing both yield parameters and fruit quality. While chemical fungicides remain a standard control method, their non-target effects on phyllosphere microbial communities have raised important ecological and environmental concerns. Nano-pesticides are increasingly applied in plant disease management, however, their influence on the composition and functional potential of phyllosphere microbial communities remains poorly understood. The nano-berberine formulation (BBR-M) used in this study was provided by a collaborative group, with synthesis and physicochemical characteristics consistent with those previously reported for this material. In this study, we compared the field-level effects of a nano-berberine formulation (BBR-M) and conventional chemical fungicides (e.g., bupirimate) on the strawberry phyllosphere microbiota using high-throughput sequencing, bioinformatics analysis, and microbial isolation techniques. The results showed that nano-fungicide application significantly reduced the disease index of powdery mildew and markedly decreased its incidence in field-grown strawberries, ultimately lowering leaf disease incidence to 5.06% with a control efficacy of 96.81%. Furthermore, nano-fungicides and conventional chemical fungicides treatments were associated with distinct impacts on the phyllosphere microenvironment of strawberry. Application of BBR-M was associated with a more structured and potentially stable microbial community, characterized by increased fungal diversity and higher modularity in co-occurrence networks. In contrast, bupirimate treatment increased microbial complexity but coincided with reduced network stability. A strain of Bacillus siamensis—a genus identified as a core taxon within the BBR-M phyllosphere network—was subsequently isolated from nano-berberine–treated leaves and exhibited strong antagonistic activity against Colletotrichum nymphaeae. Field assays showed that this strain effectively suppressed strawberry powdery mildew with 98.18% control efficacy. Collectively, these findings provide important insights into the ecological safety and functional implications of novel pesticide technologies, underscoring the potential of nano-fungicides and native biocontrol agents for sustainable strawberry disease management.

Finding safe and reliable alternatives to fungicides is currently one of the biggest challenges in agriculture. In this regard, this experiment investigated the effectiveness of different elicitors, botanical extracts and essential oils to control grey mold (Botrytis cinerea) and powdery mildew (Podosphaera aphanis) on strawberry plants. This trial was conducted in field conditions under a plastic tunnel with strawberry plants ‘Elsanta’. A first group of strawberry plants was treated before flowering with elicitors [acibenzolar-S-Methyl–(BTH), chitosan], botanical extracts (seaweed extract, alfalfa hydrolysate) and essential oils (thyme and juniper), and grey mold incidence on flowers was evaluated (Experiment 1). Furthermore, a second group of plants was treated before (Experiment 2) and after (Experiment 3) controlled inoculation with P. aphanis. The results indicated that the incidence of flower infected by B. cinerea was reduced by approximately 50% with thyme and juniper essential oils’ applications compared to the untreated control, with no significant difference observed compared to the commercial fungicide penconazole (positive control). As a consequence, the final yield of essential-oil-treated plants was +27% higher than that of non-treated plants. No significant differences emerged for other tested products against grey mold. However, gene expression analysis showed an up-regulation (>2 ÷ 5 folds as compared to control 4 days after application) of FaEDS1, FaLOX and PR gene expression (FaPR1, FaPR5, FaPR10) in leaves treated with BTH. The other natural substances tested also induced defense-related genes, albeit at a lower level than BTH. In Experiment 2, all treatments applied prior to inoculation significantly reduced the incidence and severity of powdery mildew as compared to control. At 28 days after inoculation, chitosan and thyme essential oil applications performed similarly to their positive controls (BTH and penconazole, respectively), showing a significant reduction in disease incidence (by −84 and −92%) as compared to control. Post-inoculum application of essential oils (Experiment 3) showed an efficacy similar to that of penconazole against powdery mildew. These results indicated that the tested substances could be used as alternatives to fungicides for the control of grey mold and powdery mildew in strawberry, therefore representing a valuable tool for the control of these fungal diseases under the framework of sustainable agriculture.

Salmonberry (Rubus spectabilis) and thimbleberry (Rubus nutkanus) are important native berry plants for Indigenous communities and restoration sites in the coastal region of British Columbia (BC). Compared to cultivated berry plants, information on the diseases impacting native berry plant species is limited. In late August 2022, powdery mildew symptoms and signs were observed on field plots of salmonberry and thimbleberry plants at the Agassiz Research and Development Centre, BC. The disease symptoms progressed until the end of September. The morphology of conidiophores and conidia when examined under a compound and scanning electron microscope showed they matched with those of Podosphaera aphanis. DNA from diseased foliage was subjected to PCR amplification with primers representing the internal transcribed spacer (ITS) region of rDNA followed by sequencing. The corresponding sequences from both hosts showed >99% similarity to P. aphanis in GenBank and grouped with several accessions of P. aphanis following phylogenetic analysis. In artificially inoculated trials conducted in the greenhouse, powdery mildew symptoms developed within 3−4 weeks post-inoculation and P. aphanis isolates from salmonberry and thimbleberry were cross-pathogenic to each host. To our knowledge, this is the first study characterizing the powdery mildew pathogen infecting salmonberry and thimbleberry and demonstrating cross-pathogenicity to both hosts. The outcomes from this study provide insight into the biology of the powdery mildew pathogen infecting native Rubus spp.

We evaluated the effects of fungicides on strawberry powdery mildew (PM) fungus, Podosphaera aphanis, using an electrostatic technique. Thirty-six fungicides were sprayed on single colonies of P. aphanis on leaves of strawberry seedlings (Fragaria × ananassa Duchesne ex Rozier). Colony development varied depending on the tested fungicides. Particularly, pyraziflumid, triflumizole, triforine, polyoxin, sodium hydrogen carbonate + copper wettable powder, and flutianil + mepanipyrim were highly effective for reducing colony development. P. aphanis colonies were histochemically stained to observe the morphological characteristics of fungal cells forming normal and abnormal conidiophores. Abnormal conidiophores were classified into seven types based on their morphological and cytological characteristics. Finally, asexual conidia were collected from single P. aphanis colonies on the leaves spray-treated with fungicides using a dielectrically polarized insulator plate (electrostatic spore collector); conidia attracted to the insulator plates were counted using a high-fidelity digital microscope. Most tested fungicides highly inhibited the production and/or germination of asexual conidia. The germination of asexual conidia was observed only in thiophanate-methyl (methyl benzimidazole carbamates fungicides; MBC fungicides) and azoxystrobin (quinone outside inhibitors; QoI fungicides). Assessing with the electrostatic technique, we clarified that P. aphanis has developed resistance to both thiophanate-methyl and azoxystrobin. Thus, the methodological assessment analyzing the colony development and the number of conidia released from single colonies will be helpful information for screening effective fungicides.

Powdery mildew (Sphaerotheca macularis Mag. (syn. Podosphaera aphanis Wallr.)) is a dangerous disease of strawberry (Fragaria L.). The resistance of strawberry to powdery mildew is controlled polygenically. Several genetic loci with a large contribution to disease resistance have been identified in various strawberry varieties. Diagnostic DNA markers have been developed for QTL 08 To-f. They showed a high level of reliable gene detection in mapping populations. The purpose of this study was assessment of a strawberry genetic collection for resistance to powdery mildew and identification of promising strawberry forms for breeding for resistance to S. macularis. The objects of the study were wild species of the genus Fragaria L., varieties and selected seedlings of strawberry (Fragaria × ananassa Duch.) created in the I.V. Michurin Federal Scientific Center, and strawberry varieties introduced from various ecological and geographical regions. To identify QTL 08 To-f, DNA markers IB535110 and IB533828 were used. Locus 08 To-f was detected in 23.2 % of the analyzed strawberry genotypes, including wild species F. moschata and F. orientalis, strawberry varieties of Russian breeding (Bylinnaya and Sudarushka) and foreign breeding (Florence, Korona, Malwina, Ostara, Polka and Red Gauntlet). The correlation between the presence of markers IB535110 and IB533828 and phenotypic resistance (powdery mildew effect on strawberry plants is absent) was 0.649. The determination coefficient (R2 ) showing the contribution of the studied locus to the manifestation of the trait was 0.421, that is, in 42.1 % of cases resistance was explained by the presence of QTL 08 To-f, and in 57.9 % of cases, by other genetic factors. All strawberry genotypes with locus 08 To-f were characterized by high field resistance to S. macularis in the conditions of Michurinsk, Tambov region. Thus, locus 08 To-f is promising for conferring resistance on local powdery mildew races, and markers IB535110 and IB533828 can be used in marker-assisted breeding programs to create powdery mildewresistant strawberry genotypes.

Strawberry powdery mildew, caused by Podosphaera aphanis, is an economically important disease for strawberry production. Typical symptoms are white mycelium on all aerial parts of affected plants, with young host tissues being the most susceptible. The pathogen overwinters on infected leaves, either as mycelium or chasmothecia, although the quantitative role of chasmothecia in epidemics are not fully understood. In spring, under favourable conditions, the fungus sporulates, disseminating conidia and causing polycyclic infections. The disease is mainly controlled using synthetic fungicides, but there is increasing interest in sustainable alternatives, including microbial biocontrol agents (e.g., Ampelomyces quisqualis, Bacillus spp., Trichoderma spp.) and substances of plant or animal origin (e.g., Equisetum arvense, orange oil, chitosan, whey). Physical methods, (e.g. UV-C, ozone) are also promising alternatives to fungicides. All of these strategies should be combined with appropriate agronomic practices (e.g., overhead irrigation, canopy management) to create unfavourable environments for the pathogen. However, agronomic practices have never been assessed for P. aphanis. Disease forecasting models and DSSs, though available, are underutilized due to their complexity and lack of validation across locations. This review presents the current state of knowledge on P. aphanis the available methods for control of strawberry powdery mildew, and highlights knowledge gaps relating to this host/pathogen relationship.

Goal. To identify fungi associated with the tissues of strawberry plants and evaluate the frequency of their isolation at different stages of plant growth.
 Methods. The research was conducted in the Right Bank Forest Steppe of Ukraine (Cherkasy Region) in 2021—2023, cv. ‘Clery’. Plant samples were taken in the phases of rosette formation (April), budding-flowering (May), fruit ripening (June) and in the post-harvest period (July-August). The analysis was carried out in laboratory conditions using macroscopic and biological methods.
 Results. During the research period, fungi belonging to 27 genera were isolated from strawberry tissues. In general, they can be divided into three groups: pathogens of leaves, which is the most numerous, berries, roots and crown. Representatives of the genera Alternaria, which were isolated from 57—100% of samples, and Fusarium (40—88%) occurred most often. Less often, but also during the entire growing season, Penicillium spp. (36—69%), Podosphaera aphanis (11—80%), Botrytis cinerea (27—67%), Paraphomopsis obscurans (24—77%) were isolated. Among the leaf diseases powdery mildew was recorded during all growing season, with a maximum in the fruit ripening phase. White leaf spot was observed starting from the budding-flowering phase, and leaf scorch was found from the fruit ripening phase. Gray rot was the most common fruit disease. The root system was more often affected by Fusarium spp.
 Conclusions. The obtained results demonstrate that during the entire growing season fungi of the genera Alternaria, Fusarium, Penicillium, as well as Podosphaera aphanis, Botrytis cinerea and Paraphomopsis obscurans were most often isolated from strawberry tissues. R.grevilleana was isolated starting from the budding-flowering phase, and D. earlianum from the fruit ripening phase. Species from the genera Podosphaera, Ramularia, Diplocarpon, Colletotrichum, Pestalotiopsis, Verticillium, Sclerotinia, Gnomoniopsis, Rhizoctonia, Coniella, Rhizopus, Septoria, Cylindrocarpon require constant control of their spread. Under favorable conditions for their development, they can cause damage to plants and a significant lack of harvest. The seasonal monitoring of fungi on strawberry provides a means for establishing the optimal periods for their control and developing effective disease protection system.

Abstract Powdery mildew, caused by the ascomycete Podosphaera aphanis, is an important disease of strawberry. A slightly modified version of a method using steam thermotherapy to rid diseases and pests from strawberry transplants was tested against strawberry powdery mildew. Experiments took place in Norway and Florida, with potted strawberry plants heavily infected with the fungus. Aerated steam treatments of the plants were carried out as follows: a pre-treatment with steaming at 37 °C for 1 h was followed by 1 h at ambient temperature before plants were exposed to steaming at 40, 42, or 44 °C for 2 or 4 h in Norway and 44 °C for 4 h in Florida. Following steaming, plants from the different treatments and the untreated control were kept apart and protected from outside contamination of powdery mildew by growing them in closed containers with over-pressure. On steamed plants, hyphae of P. aphanis were dead and without any new spore formation after treatments, independent of temperature or exposure time; however, up to 99% of the area infected with powdery mildew prior to treatments contained actively sporulating lesions on non-steamed plants. None of the new leaves formed after steaming had powdery mildew, whereas more than half of the new leaves on non-treated plants were infected by P. aphanis. This investigation clearly indicates that steam thermotherapy can eradicate powdery mildew from strawberry transplants, and this can be achieved at lower temperatures and exposure times than previously reported for other pathogens.

The article presents the results of mycological studies conducted in the Altyn-Emel National Park and in adjacent territories. The collection of samples was carried out in the summer 2021-2022 in various gorges of the Altyn-Emel ridge, the Chulak Mountains and in the Konyrolen intermountain plain. A taxonomic list is given, which includes 46 species of fungi from 33 genera, 24 families, 16 orders, 9 classes, 2 kingdoms. The largest is the class Dothideomycetes with 18 species. 43 species of micromycetes are new for the study area, for 3 species a new host is given: for Leveillula taurica (Lev.) G. Arnaud - Astragalus sp., Hedysarum sp., Peganum harmala L.; for Neoerysiphe galeopsidis (DC.) U. Braun -Dracocephalum integrifolium Bunge, Lamium sp.; for Podosphaera aphanis (Wallr.) U. Braun et S. Takam. - Agrimonia asiatica Juz., Geum urbanum L. Of the first recorded species, 31 species (72.1 %) are plant parasites, 10 species (23.2 %) are saprotrophs on plant remains, two species (4.7 %) live on stones and basic rocks and belong to lichenized fungi. Leveillula taurica was found most frequently on various hosts (especially on Peganum harmala).

Evaluation of host resistance and susceptibility to Podosphaera aphanis NWAU1 infection in 19 strawberry varieties

Powdery mildew is one of the most economically destructive diseases in protected strawberry production. Here we present the first genome assembly for Podosphaera aphanis, the causal agent of powdery mildew on strawberry. This obligate-biotrophic fungal pathogen was sampled from a naturally occurring outbreak on Fragaria × ananassa 'Malling Centenary' plants grown under cover in the United Kingdom. Assembled reads resolved a 55.6 Mb genome, composed of 12,357 contigs whose annotation led to prediction of 17,239 genes encoding 17,328 proteins. The genome is highly-complete, with 97.5% of conserved single-copy Ascomycete genes shown to be present. This annotated P. aphanis genome provides a molecular resource for further investigation into host-pathogen interactions in the strawberry powdery mildew pathosystem.

Powdery mildew fungi produce progeny conidia on conidiophores, and promote the spread of powdery mildew diseases by dispersal of the conidia from conidiophores in the natural environment. To gain insights and devise strategies for preventing the spread of powdery mildew infection, it is important to clarify the ecological mechanism of conidial dispersal from conidiophores. In this study, all of the progeny conidia released from single colonies of strawberry powdery mildew fungus (Podosphaera aphanis (Wallroth) U. Braun and S. Takamatsu var. aphanis KSP-7N) on true leaves of living strawberry plants (Fragaria × ananassa Duchesne ex Rozier cv. Sagahonoka) were consecutively collected over the lifetime of the colony with an electrostatic rotational spore collector (insulator drum) under greenhouse conditions, and counted under a high-fidelity digital microscope. The insulator drum consisted of a round plastic container, copper film, thin and transparent collector film, electrostatic voltage generator, and timer mechanism. When negative charge was supplied from the voltage generator to the copper film, the collector film created an attractive force to trap conidia. The electrostatically activated collector film successfully attracted progeny conidia released from the colony. Experiment was carried out at just one colony on one leaf for each month (in February, May, July, October, November, and December in 2021), respectively. Each collector film was exchanged for a new collector film at 24 h intervals until KSP-7N ceased to release progeny conidia from single colonies. Collection experiments were carried out to estimate the total number of conidia released from a single KSP-7N colony over a 35-45-day period after inoculation. During the fungal lifetime, KSP-7N released an average of 6.7 × 104 conidia from each of the single colonies at approximately 816 h. In addition, conidial release from KSP-7N colonies was largely affected by the light intensity and day length throughout a year; the number of conidia released from single KSP-7N colonies in night-time was clearly smaller than that in daytime, and the time of conidial release from single KSP-7N colonies was shorter by approximately 2 to 4 h in autumn and winter than in spring and summer. The ecological characteristics related to conidial releases from KSP-7N colonies will be helpful information for us to successfully suppress the spread of strawberry powdery mildews onto host plants under greenhouse conditions.

Strawberry powdery mildew, caused by Podosphaera aphanis, can be particularly destructive in glasshouse and plastic tunnel production systems, which generally are constructed of materials that block ultraviolet (UV) solar radiation (about 280 to 400 nm). We compared epidemic progress in replicated plots in open fields and under tunnels constructed of polyethylene, which blocks nearly all solar UV-B, and two formulations of ethylene tetrafluoroethylene (ETFE), one of which contained a UV blocker and another that transmitted nearly 90% of solar UV-B. Disease severity under all plastics was higher than in open-field plots, indicating a generally more favorable environment in containment structures. However, the foliar severity of powdery mildew within the tunnels was inversely related to their UV transmissibility. Among the tunnels tested, incidence of fruit infection was highest under polyethylene and lowest under UV-transmitting ETFE. These effects probably transcend crop, and the blocking of solar UV transmission by glass and certain plastics probably contributes to the widely observed favorability of greenhouse and high-tunnel growing systems for powdery mildew.

Wild blackberry species (Rubus spp. L.; Rosaceae) represents an invaluable source of genes for the generation of new varieties, but also serve as a primary source of disease inoculum. During April of 2020, symptoms of powdery mildew were observed on four populations of wild blackberry species located in the states of Chiapas (16°59'11"N, 92°59'07"W; 16°47'08"N, 92°31'05"W) and Michoacán (19°37'17"N, 100°08'59"W; 19°29'25"N, 101°32'54"W), Mexico. Signs of the pathogen were white powdery masses mainly on the top of new shoots. Symptoms included yellowing, necrosis, and early defoliation of the plants. Hyphae were tin-walled, hyaline, smooth, and 4.0-9.0 mm wide. Appressoria were indistinct -to- nipple-shaped. Conidiophores (n=30, 75-225 × 10.5-13.5 μm) were straight, and unbranched with cylindrical foot cells (n=30, 31.5-158 × 8-13.5 μm), straight, somewhat widening upwards, followed by 1-3 shorter cells. Conidia (n=100; 25.5-38.5 × 9.5-22.5 μm) were catenulate, ellipsoid-ovoid -to- doliiform, containing fibrosin bodies (in 3% KOH). Germ tubes (n=30, 13.5-40.5 × 4.5 μm) emerged laterally, and were unbranched with slightly swollen tips. Chasmothecia were not found. Morphological characters of the fungus in all samples corresponded to the previous descriptions of Podosphaera aphanis by Braun and Cook (2012) and Stevanovi´c et al. (2020). Voucher specimens were deposited in the Department of Agricultural Parasitology Herbarium at the Chapingo Autonomous University under accessions UACH421, UACH423, UACH425, UACH426. To confirm the species identification, the internal transcribed spacer (ITS) of one sample was amplified using the primers ITS5 (White et al. 1990) and P3 (Kusaba and Tsuge, 1995) and sequenced. The sequence was deposited in GenBank (accession number MW988591). A phylogenetic analysis using Bayesian inference and maximum likelihood was performed (Hernández-Restrepo et al. 2018) and included other Podosphaera species (Takamatsu et al. 2010). The sequence from the isolate UACH426 clustered with the strain MUMH1871 of P. aphanis forming a definite clade and remained as a sister taxon of P. pannosa. Pathogenicity was verified through inoculation by gently dusting conidia from one powdery mildew patch onto leaves of five healthy blackberry plants of each specie. The same number of noninoculated plants served as controls. All plants were maintained in a greenhouse at 25-30°C with 75% relative humidity. All inoculated plants developed powdery mildew symptoms after 12 days, whereas no symptoms were observed on noninoculated plants. The fungus recovered from the inoculated plants was morphologically identical to that originally observed on diseased blackberry plants, demonstrating the pathogenicity of the fungus. Based on morphological data and phylogenetic analysis, the fungus was identified as P. aphanis. This fungus has been reported to cause powdery mildew on blackberry plants in Serbia (Stevanovi´c et al. 2020). This is the first report of P. aphanis causing powdery mildew on wild backberry species in Mexico according to Farr and Rossman (2021). The primary source of inoculum of powdery mildew for commercial plantings is wild blackberry plants from noncultivated areas and may warrant control of wild populations.