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First Report of Powdery Mildew Caused by Golovinomyces orontii on Abelmoschus manihot in Italy

Garibaldi, A., Bertetti, D., Matić, S., Gullino, M. L.
Plant disease 2019 v.103 no.1 pp. 154
Abelmoschus manihot, Arabidopsis thaliana, Erysiphe, Galium aparine, Golovinomyces orontii, Hibiscus syriacus, Oidium, Podosphaera xanthii, conidia, conidiophores, container-grown plants, flowers, fungi, gardens, greenhouses, herbaceous plants, internal transcribed spacers, leaves, mycelium, necrosis, pathogens, polymerase chain reaction, powdery mildew, pressing, ribosomal DNA, temperature, tissues, winter, China, Italy, Japan, New Guinea
Abelmoschus manihot L. (syn.: Hibiscus manihot L.) is an herbaceous plant belonging to the Malvaceae family, appreciated for its large and showy yellow flowers and cultivated in gardens in mixed borders. During the winter 2018, 3- to 5-month-old potted plants of A. manihot started from seed growing in a greenhouse of the Centre of Competence Agroinnova, University of Torino, located in Grugliasco (Torino province, northern Italy), showed symptoms and signs of a powdery mildew. The white mycelium caused by the fungal pathogen colonized in particular the adaxial surface of leaves, causing slightly brown necrosis on the affected tissues. As the disease progressed, the affected leaves yellowed and then dried and fell prematurely. Conidiophores were erect with cylindrical, sometimes curved at the base, foot cells that measured 40 to 76 × 7 to 11 µm (average: 55 × 10 µm). Foot cells were followed by two to three shorter cells that measured 13 to 30 × 8 to 13 µm (average: 20 × 10 µm). Conidia were elliptical, formed short chains (up to three conidia per chain), and measured 27 to 39 × 14 to 23 µm (average: 31 × 19 µm) (n = 50), with length/width ratio between 1.2 and 2.3 (average: 1.7). They germinated apically and were lacking fibrosin bodies. The perfect stage of the pathogen was not observed. Mycelium, conidiophores, and conidia were collected from affected leaves, and the DNA of the microorganism was extracted using the E.Z.N.A. Fungal DNA Mini Kit (Omega Bio-Tek, Darmstadt, Germany). Either primers ITS1/ITS4 (White et al. 1990) or ITS1/PM6 (Takamatsu and Kano 2001) were used for a polymerase chain reaction to amplify the internal transcribed spacer (ITS) region of rDNA. The NCBI BLAST analysis of 446- and 409-bp sequences (GenBank accession nos. MH414543 and MH414544, respectively) showed 99 and 100% identity with Golovinomyces orontii (KX396590 from Galium aparine) and with G. orontii (AB769465 from Arabidopsis thaliana). Therefore, the causal agent of the powdery mildew on A. manihot was identified as G. orontii Heluta (syn.: Erysiphe orontii Cast.), accordingly with the features described for this pathogen by Braun (1987). Three 2-month-old healthy plants of A. manihot were inoculated by gently pressing their leaves onto leaves of the same host affected by G. orontii. Inoculated plants were maintained at daily average temperatures ranging from 12.4 to 21.7°C. Three healthy noninoculated plants were grown separately as controls. Fifteen days later, the first symptoms and signs of powdery mildew developed, on inoculated plants only. Controls remained healthy. On A. manihot was reported Erysiphe abelmoschicola in Japan (Braun and Cook 2012), Sphaerotheca fuliginea in China and Japan, and Oidium sp. in New Guinea. To our knowledge, this is the first report of G. orontii on A. manihot in Italy, as well as worldwide. The economic importance of this disease is at present limited in Italy, and further research is needed to understand the susceptibility of Hibiscus spp. to G. orontii, in particular for H. syriacus, widely used in hedges.