Effective Management of Cucumber Powdery Mildew with Essential Oils

This research evaluated the efficacy of essential oils in the management of cucumber powdery mildew. Essential oils of lemongrass, lemon, thyme, peppermint, abundance blend, purification blend, and thieves blend were tested in vitro and under greenhouse conditions in two separate experiments. The effects of essential oils were tested against powdery mildew disease at concentrations of 1.0–2.5 mL/L, and the consequent impact of the oils on plant growth was evaluated. Powdery mildew fungus, Podosphaera xanthii, was identified using sequencing of the ITS region. The essential oils significantly reduced disease incidence up to 77.3% compared with the positive control (p < 0.5). Moreover, the essential oils increased the plant length (up to 187 cm), leaf area (up to 27.5 cm2), fresh weight (up to 123 g), dry weight (up to 22.5 g), number of flowers (16.3), and metabolite content compared with the positive control (p < 0.5). Cell membrane injury decreased significantly in the oil-treated pants (p < 0.5), indicating the protective effect of essential oils. This study recommends the application of essential oils in an appropriate dose (2.5 mL/L) to protect cucumber plants against powdery mildew. Overdose of the oils (more than 2.5 mL/L) should be avoided due to adverse effects.

Genus Capsicum L. and Diseases of Sweet and Hot Peppers (Review)

This paper reviews taxonomy, importance and diseases of pepper plants belonging to Capsicum genus. Five species of the genus are domesticated, Capsicum annuum being the most cultivated species followed by C. chinense and C. frutescens while C. baccatum and C. pubescens are grown in limited areas of some regions. Review of the available literature data has shown that more than 122 species of microorganisms can cause pepper diseases, including >58 species of fungi, 11 oomycetes, 15 bacteria, 32 viruses, > 6 species of nematodes, and some higher parasitic plants. From these 18 species of fungi, 2 oomycetes, one bacterium, two viruses, one root-knot nematode, two species of each of dodder and broomrape have been recorded in Uzbekistan. However, all of these organisms but one powdery mildew fungus has been registered on other than pepper plants. Previously the authors of the current paper have found that deadly crown rot of both sweet and hot peppers caused by Fusarium oxysporum (supposedly f. sp. radici-capsici) occurred widely in six districts of Uzbekistan.

Gene Mining for Conserved, Non-Annotated Proteins of Podosphaera xanthii Identifies Novel Target Candidates for Controlling Powdery Mildews by Spray-Induced Gene Silencing

The powdery mildew fungus Podosphaera xanthii is one of the most important limiting factors for cucurbit production worldwide. Despite the significant efforts made by breeding and chemical companies, effective control of this pathogen remains elusive to growers. In this work, we examined the suitability of RNAi technology called spray-induced gene silencing (SIGS) for controlling cucurbit powdery mildew. Using leaf disc and cotyledon infiltration assays, we tested the efficacy of dsRNA applications to induce gene silencing in P. xanthii. Furthermore, to identify new target candidate genes, we analyzed sixty conserved and non-annotated proteins (CNAPs) deduced from the P. xanthii transcriptome in silico. Six proteins presumably involved in essential functions, specifically respiration (CNAP8878, CNAP9066, CNAP10905 and CNAP30520), glycosylation (CNAP1048) and efflux transport (CNAP948), were identified. Functional analysis of these CNAP coding genes by dsRNA-induced gene silencing resulted in strong silencing phenotypes with large reductions in fungal growth and disease symptoms. Due to their important contributions to fungal development, the CNAP1048, CNAP10905 and CNAP30520 genes were selected as targets to conduct SIGS assays under plant growth chamber conditions. The spray application of these dsRNAs induced high levels of disease control, supporting that SIGS could be a sustainable approach to combat powdery mildew diseases.

First record of Phyllactinia moricola (Erysiphales, Ascomycota) on Morus alba in Europe

Information on the record of Phyllactinia moricola, a new in Ukraine alien powdery mildew fungus, is reported. The teleomorph of the fungus was collected in October–December 2020 on Morus alba in Odesa. The description and illustrations of the studied material are provided. The disease caused by this species can significantly reduce the ornamental properties of mulberry, a popular plant that is often used in greenery in the settlements of Ukraine. It is noted that the fungus can spread to Southern Europe, where it may cause significant damage to horticulture and silk production.

First Report of Powdery Mildew Caused by Podosphaera xanthii on Salvia farinacea in Korea

Salvia farinacea Benth. (Lamiaceae) is an herbaceous perennial plant, native to Mexico and southern parts of the United States. This plant is cultivated worldwide for its ornamental value. In November 2019, hundreds of S. farinacea ‘Blue Bedder” grown in a flower garden in Jeju (33°30'57"N 126°32'50"E), Korea have been found to be infected with a powdery mildew fungus. The disease severity was estimated to be 100%. Likewise in October 2020, a similar situation with this plant was also observed in a flower garden in Seoul (37°35'19"N 127°01'07"E), Korea. Leaves, stems and inflorescence of plants were covered by white, thin mycelial felt, bearing an abundance of conidiophores and conidia. Eventually, infected plants lose their ornamental value. Two voucher specimens have been deposited in the Korea University Herbarium (KUS-F31478 and F32164). Fresh materials were examined. Hyphal appressoria were nipple-shaped, but rarely found. Conidiophores (n = 30) were straight, 95 to 160 × 10 to 12 μm and produced 2 to 7 immature conidia in chains with a crenate outline. Foot-cells were cylindric and 36 to 60 μm long. Conidia (n = 30) were ellipsoid-ovoid to barrel-shaped, 32 to 38 × 18 to 24 μm, and contained conspicuous fibrosin bodies. Dark brown chasmothecia were found partly embedded in the mycelial felt on leaves, mostly hypophyllous, spherical, and 82 to 100 µm diameter, with a single ascus in each. Appendages were few, mycelioid, 1- to 4-septate, brown near the base when mature, but paler above. Asci were broadly ellipsoid to subglobose, 56 to 68 × 50 to 62 μm, sessile and 8-spored. Ascospores were colorless, oval to subglobose, and 14 to 18 × 12 to 15 µm. These characteristics were consistent with those of Podosphaera xanthii (Castagne) U. Braun & Shishkoff (Braun and Cook 2012). For further confirmation, genomic DNA was extracted from chasmothecia from KUS-F31478 and F32164. PCR amplification was performed using the primer pair ITS1F/PM6 for internal transcribed spacer (ITS) and PM3/TW14 for the large subunit (LSU) of the rDNA (Takamatsu and Kano 2001). Obtained sequences were deposited to the GenBank under the accession numbers MZ359847 and MZ359859 for ITS, MZ359858 and MZ359861 for LSU. For ITS regions 99.80-100% similarity was found with sequences MT131256 (Salvia farinacea), MT131254 (Mazus pumilus) and MT131252 (Erigeron bellioides) of P. xanthii, whereas it was 99.90% with sequences of this fungus on Echinacea purpurea (MT826247 and MT826245) for 28S rDNA gene. Pathogenicity tests were carried out by touching an infected leaf onto healthy leaves of disease-free 30 days old potted ‘Blue Bedder’ using replication of five plants, with five non-inoculated plants used as controls. The typical signs of powdery mildew started to develop on the inoculated leaves in 7 to 10 days, and microscopic examination revealed the morphological identity with the fungus observed from the field. All non-inoculated control plants remained symptomless. Hitherto Golovinomyces powdery mildews on Salvia spp. were reported globally (Farr and Rossman 2021). However, Podosphaera elsholtziae on Salvia sp. and P. xanthii on S. farinacea were reported from China and Taiwan (Zheng and Yu 1987, Yeh et al. 2021). To our knowledge, this is the first report of P. xanthii on S. farinacea in Korea. The occurrence of Podosphaera powdery mildew on S. farinacea could pose a serious threat to the beauty of this plant, causing premature senescence of young leaves and gray to purplish discoloration of the leaves.

First Report of Powdery Mildew Caused by Podosphaera macularis on Hemp in Oregon

In Oregon, hemp (Cannabis sativa) production has increased substantially after cultivation was legalized in the 2014 and 2018 Farm Bills. Typically, hemp species are affected by powdery mildew caused by Golovinomyces species. This paper reports on hop powdery mildew caused by Podosphaeria macularis, found colonizing hemp in natural conditions. The occurrence of the hop powdery mildew fungus on hemp may have management implications for both the hemp and hop industry.

Occurrence of Powdery Mildew Caused by Erysiphe buhrii on Dianthus chinensis in Inner Mongolia, China

Dianthus chinensis is widely cultivated for ornamental and medicinal use in China (Guo et al. 2017). The plant has been used in traditional Chinese medicine for the treatment of urinary problems such as strangury and diuresis (Han et al. 2015). In June and July 2020, powdery mildew-like signs and symptoms were seen on leaves of D. chinensis cultivated on the campus of Inner Mongolia Agricultural University, Hohhot city, Inner Mongolia Province, China. White powder-like masses occurred in irregular shaped lesions on both leaf surfaces and covered up to 50% of leaf area. Some infected leaves were deformed on their edges and some leaf senescence occurred. More than 40 % of plants (n = 180) exhibited these signs and symptoms. Conidiophores (n = 50) of the suspect fungus were unbranched and measured 70 to 140 µm long × 6 to 10 µm wide and had foot cells that were 25 to 48 µm long. Conidia (n = 50) were produced singly, elliptical to cylindrical shaped, 30 to 45 µm long × 12 to 19 µm wide, with length/width ratio of 2.0 to 3.2, and lacked fibrosin bodies. No chasmothecia were found. Based on these morphological characteristics, the fungus was tentatively identified as an Erysiphe sp. (Braun and Cook 2012). Fungal structures were isolated from diseased leaves and genomic DNA of the pathogen extracted utilizing the method described by Zhu et al. (2019). The internal transcribed spacer (ITS) region was amplified by PCR employing the primers PMITS1/PMITS2 (Cunnington et al. 2003) and the amplicon sequenced by Invitrogen (Shanghai, China). The sequence for the powdery mildew fungus (deposited into GenBank under Accession No. MW144997) showed 100 % identity (558/558 bp) with E. buhrii (Accession No. LC009898) that was reported on Dianthus sp. in Japan (Takamatsu et al. 2015). Pathogenicity tests were done by collecting fungal conidia from infected D. chinensis leaves and brushing them onto leaves of four healthy plants. Four uninoculated plants served as controls. Inoculated and uninoculated plants were placed in separate growth chambers maintained at 19 ℃, 65 % humidity, with a 16 h/8 h light/dark period. Nine-days post-inoculation, powdery mildew disease signs appeared on inoculated plants, whereas control plants remained asymptomatic. The same results were obtained for two repeated pathogenicity experiments. The powdery mildew fungus was identified and confirmed as E. buhrii based on morphological and molecular analysis. An Oidium sp. causing powdery mildew on D. chinensis previously was reported in Xinjiang Province, China (Zheng and Yu 1987). This, to the best of our knowledge, is the first report of powdery mildew caused by E. buhrii on D. chinensis in China (Farr and Rossman 2020). The sudden occurrence of this destructive powdery mildew disease on D. chinensis may adversely affect the health, ornamental value and medicinal uses of the plant in China. Identifying the cause of the disease will support efforts for its future control and management.

Infection of papaya (Carica papaya) by four powdery mildew fungi

Papaya (Carica papaya L.) is an important fruit crop in many tropical and subtropical countries. Powdery mildew commonly affects this host, causing premature leaf loss, reduced yields and poor fruit quality. At least fifteen different fungi have been identified as the causal agents of papaya powdery mildew. Powdery mildew symptoms were detected on potted papaya plants growing in two locations in Hungary. This study aimed to identify the causal agents. Morphology of powdery mildew samples was examined, and sequences of two loci were used for molecular taxonomic identifications. Only anamophs were detected in all samples, and four morphological types were distinguished. Most samples had Pseudoidium anamorphs, while some were of the Fibroidium anamorph. Based on morphology and molecular taxonomy, the Fibroidium anamorph was identified as Podosphaera xanthii. The Pseudoidium anamorphs corresponded to three different Erysiphe species: E. cruciferarum, E. necator and an unidentified Erysiphe sp., for which molecular phylogenetic analyses showed it belonged to an unresolved species complex of E. malvae, E. heraclei and E. betae. Infectivity of P. xanthii and E. necator on papaya was verified with cross inoculations. A review of previous records of powdery mildew fungi infecting papaya is also provided. Podosphaera xanthii was known to infect, and E. cruciferarum was suspected to infect Carica papaya, while E. necator was recorded on this host only once previously. No powdery mildew fungus belonging to the E. malvae/E. heraclei/E. betae species complex is known to infect papaya or any other plants in the Caricaceae, so the unidentified Erysiphe sp. is a new record on papaya and the Caricaceae. This study indicates host range expansion of this powdery mildew fungus onto papaya.

Ampelomyces strains isolated from diverse powdery mildew hosts in Japan: Their phylogeny and mycoparasitic activity, including timing and quantifying mycoparasitism of Pseudoidium neolycopersici on tomato

A total of 26 Ampelomyces strains were isolated from mycelia of six different powdery mildew species that naturally infected their host plants in Japan. These were characterized based on morphological characteristics and sequences of ribosomal DNA internal transcribed spacer (rDNA-ITS) regions and actin gene (ACT) fragments. Collected strains represented six different genotypes and were accommodated in three different clades of the genus Ampelomyces. Morphology of the strains agreed with that of other Ampelomyces strains, but none of the examined characters were associated with any groups identified in the genetic analysis. Five powdery mildew species were inoculated with eight selected Ampelomyces strains to study their mycoparasitic activity. In the inoculation experiments, all Ampelomyces strains successfully infected all tested powdery mildew species, and showed no significant differences in their mycoparasitic activity as determined by the number of Ampelomyces pycnidia developed in powdery mildew colonies. The mycoparasitic interaction between the eight selected Ampelomyces strains and the tomato powdery mildew fungus (Pseudoidium neolycopersici strain KTP-03) was studied experimentally in the laboratory using digital microscopic technologies. It was documented that the spores of the mycoparasites germinated on tomato leaves and their hyphae penetrated the hyphae of Ps. neolycopersici. Ampelomyces hyphae continued their growth internally, which initiated the atrophy of the powdery mildew conidiophores 5 days post inoculation (dpi); caused atrophy 6 dpi; and complete collapse of the parasitized conidiphores 7 dpi. Ampelomyces strains produced new intracellular pycnidia in Ps. neolycopersici conidiophores ca. 8–10 dpi, when Ps. neolycopersici hyphae were successfully destroyed by the mycoparasitic strain. Mature pycnidia released spores ca. 10–14 dpi, which became the sources of subsequent infections of the intact powdery mildew hyphae. Mature pycnidia contained each ca. 200 to 1,500 spores depending on the mycohost species and Ampelomyces strain. This is the first detailed analysis of Ampelomyces strains isolated in Japan, and the first timing and quantification of mycoparasitism of Ps. neolycopersici on tomato by phylogenetically diverse Ampelomyces strains using digital microscopic technologies. The developed model system is useful for future biocontrol and ecological studies on Ampelomyces mycoparasites.