First Pakistani report of Erysiphe betae on the invasive weed Chenopodium ambrosioides

During September–October 2017, powdery mildew symptoms were observed on both surfaces of leaves of Chenopodium ambrosioides in Abbottabad, Malakand, and Upper Dir districts of Pakistan. The causal agent was identified as Erysiphe betae, based on its asexual morphology and chasmothecia, and its identity was confirmed by molecular data. This is the first Pakistani report of Erysiphe betae on this host.

QoI Resistance in Sugar Beet Powdery Mildew (Erysiphe betae) in Scandinavia

Powdery mildew caused by Erysiphe betae is one of the major fungal diseases in sugar beet in Denmark and Sweden. Frequent applications of fungicides mitigate the risk of powdery mildew epidemics and, consequently, reduce yield losses conferred by the disease. So far, mixtures of quinone outside inhibitors (QoIs) and triazoles have provided good efficacy against E. betae in field trials and common farming practice. However, development of fungicide resistance is a real risk, because only a limited number of active ingredients are available for the control of powdery mildew in sugar beet, and several other active ingredients are expected to be banned following reevaluation when the most recent European Union legislation is implemented. The G143A mutation associated with QoI resistance has been previously found in the United States. In this brief, its presence in Europe is reported for the first time. The current finding strongly encourages the adoption of anti-resistance strategies that minimize the spread of QoI resistance in sugar beet powdery mildew. Those strategies should be based on integrated pest management measures, including disease monitoring, the use of resistant cultivars, and the use of biological products. A sole reliance on QoI fungicides for sugar beet powdery mildew control should be avoided.

First Report of Powdery Mildew Caused by Erysiphe betae on Swiss Chard in China

Swiss chard (Beta vulgaris L. subsp. cicla) is a widely planted vegetable in China. From May to June 2013, an outbreak of powdery mildew on Swiss chard cultivar Fangzheng was observed in the commercial fields in Zhoukou city of Henan Province, located in central China. More than 80% of the plants exhibited symptoms of the disease. At the beginning of infection, circular, white, dust-like colonies of powdery mildew occurred mainly on adaxial surfaces of leaves. As the disease progressed, white mycelia covered the foliar parts of plant. No cleistothecia were found on or in collected samples. Upon microscopic evaluation, conidiophores were unbranched with the length of 63 to 126 and width of 7 to 10 μm (n = 50), produced conidia singly, and composed of a cylindrical foot cell followed by one to three short cells. Conidia were colorless, hyaline, ovoid, measured 29 to 40 × 12 to 18 μm (n = 100), lacked fibrosin bodies, and produced germ tubes on the ends of the conidia. The fungus was identified as Erysiphe betae according to the morphological features (1). To verify the identity, the internal transcribed spacer (ITS) region was amplified with the universal primers ITS1 and ITS4 (2) and sequenced. The ITS sequence obtained was assigned as Accession No. KF268348 in GenBank, which showed 100% homogeneity with two ITS sequences of E. betae isolates from UK (DQ164432 and DQ164436). Koch's postulates were conducted by inoculating 15 healthy 5-week-old plants (cv. Fangzheng) with detached infected leaves, which grew in a growth chamber under 22/16°C (day/night), 50% relative humidity, 120 μmol/m2/s light and a 16-h photoperiod. Fifteen non-inoculated plants grew in another growth chamber with the same conditions as control. Symptoms consistent with the infected field plants were observed on the inoculated plants, while no symptoms were found on the control plants. Microscopic observation revealed that the pathogen growing on the inoculated plants was consistent with the morphology of the original fungus. To our knowledge, this is the first report of E. betae infection on Swiss chard in China (3). References: (1) S. Francis. Mol. Plant Pathol. 3:119, 2002. (2) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, CA, 1990. (3) R. Y. Zheng et al. Page 63 in: Flora Fungorum Sinicorum, Vol. 1, Erysiphales. Science Press, Beijing, 1987.

First Report of Powdery Mildew Caused by Erysiphe betae on the Invasive Weed Dysphania ambrosioides in Korea

Dysphania ambrosioides (L.) Mosyakin & Clemants (formerly Chenopodium ambrosioides L.), commonly known as epazote, is an herb that is native to Central America, South America, and southern Mexico. As well as in its native areas, it is used as an herb, tea, and food commodity in warm temperate to subtropical areas of Europe, the United States, and Asia. In Korea, however, this plant was accidentally introduced around the 1970s and has become widely naturalized by replacing indigenous plants and disrupting native ecosystems (3). Since 2006, powdery mildew infections of epazote have been consistently found in the southern part of Korea, including Jeju Island. Specimens (n = 8) have been deposited in the Korea University Herbarium (KUS). White mycelial and conidial growth was present mostly on leaf surfaces with sparse growth on young stems and inflorescences. Severely infected leaves were malformed. Slight purplish discoloration was present on the leaves contiguous with colony growth. Mycelial colonies were conspicuous, amphigenous, and epiphytic. Appressoria on the mycelia were lobed. Conidiophores were 110 to 200 μm long and produced conidia singly. Conidia were hyaline, oblong-elliptical, measured 30 to 48 × 13 to 18 μm, lacked fibrosin bodies, and produced germ tubes on the subterminal position. Chasmothecia were amphigenous, scattered or partly clustered, dark brown, spherical, 110 to 130 μm in diameter, and contained four to seven asci. Appendages were mycelioid, numbered 50 to 80 per chasmothecium, 0.5 to 1.5 times as long as the chasmothecial diameter, one- to three-septate, and brown at the base while becoming paler toward the tip. Asci were short stalked, 60 to 75 × 30 to 38 μm, and contained three to five spores. Ascospores were ellipsoid-ovoid with dimensions of 20 to 28 × 14 to 18 μm. On the basis of these morphological features, this fungus was identified as Erysiphe betae (Vanha) Weltzien (1). To confirm the identification, the complete internal transcribed spacer (ITS) region of rDNA from KUS-F23213 was amplified with primers ITS5 and P3 and sequenced (4). The resulting sequence of 560 bp was deposited in GenBank (Accession No. JQ041419). A GenBank BLAST search with the current data showed >99% (558 of 560 bp) similarity with the results for E. betae ex Beta vulgaris (sugar beet). Therefore, the sequence analysis verified the pathogen to be E. betae. Previous epazote infections by E. betae have been recorded in Argentina, Mexico, Romania, India, and Japan (1,2). In Taiwan, an epazote powdery mildew associated with Oidium erysiphoides f. sp. chenopodii J.M. Yen, an anamorph of E. betae, was recorded (1,2). To our knowledge, this is the first record of E. betae on epazote in Korea, and the first confirmation of epazote powdery mildew being identified as E. betae on the basis of holomorphic characteristics and ITS rDNA sequences. Our field observation suggests that the powdery mildew is acting as one of several limiting factors to suppress the expansion of this invasive weed in Korea. References: (1) U. Braun. Beih. Nova Hedw. 89:1, 1987. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ , November 22, 2011. (3) C. G. Song and Y. H. Yang. The Naturalized Plants in Jeju Island. Nam-Jeju County, Jeju, Korea, 2005. (4) S. Takamatsu et al. Mycol. Res. 113:117, 2009.

Characterization of Resistance Mechanisms to Powdery Mildew (Erysiphe betae) in Beet (Beta vulgaris)

Beet powdery mildew incited by Erysiphe betae is a serious foliar fungal disease of worldwide distribution causing losses of up to 30%. In the present work, we searched for resistance in a germplasm collection of 184 genotypes of Beta vulgaris including fodder (51 genotypes), garden (60 genotypes), leaf (51 genotypes), and sugar (22 genotypes) beet types. Resistant genotypes were identified in the four beet types under study. In addition, mechanisms underlying resistance were dissected through histological studies. These revealed different resistance mechanisms acting at different fungal developmental stages, i.e., penetration resistance, early and late cell death, or posthaustorial resistance. Most genotypes were able to hamper fungal development at several stages. The later are interesting for breeding aiming to resistance durability. Furthermore, characterization of defense mechanisms will be useful for further cellular and molecular studies to unravel the bases of resistance in this species.