T. S. de Oliveira and
L. J. Dallagnol, Faculdade de Agronomia Eliseu Maciel, Universidade Federal de Pelotas, Rio Grande do Sul, Brazil;
J. V. de Araujo Filho,
F. R. de Castro Moretti, and
L. E. A. Camargo, Escola Superior de Agricultura “Luiz de Queiroz,” Universidade de São Paulo, São Paulo, Brazil
Platanus × acerifolia (Aiton) Willd. (London planetree) is a tree commonly used as an ornamental and in the furniture industry. In the summer of 2013, powdery mildew was observed on shoots of P. × acerifolia plants in the cities of Pelotas and Canela (State of Rio Grande do Sul, Brazil). Voucher specimens (n = 2) were deposited in the Phytopathological Museum Manoel Alves Oliveira at Federal University of Pelotas. Dense white powdery masses of conidia and mycelium were observed on leaves (abaxial and adaxial surfaces), petioles, and young stems. Leaves with high disease severities (≥70%) were deformed with curved edges to the adaxial side, and they often died. Mycelia were superficial with lobed appressoria. Conidiophores were straight, sometimes curved at the base, unbranched, cylindrical, 98 to 236 μm long (137.3 ± 41.2 μm) and composed of a cylindrical foot cell 49 to 102 μm long (66.9 ± 19.5 μm) and 4.4 to 6.4 μm wide (5.3 ± 0.8 μm) followed by two to four cells. Conidia were produced singly or in short chains (two to three), without distinct fibrosin bodies, ellipsoid to ovoid and measuring 24 to 37 μm long (29.5 ± 3.2 μm) and 12 to 19 μm wide (15.2 ± 1.4 μm), often with a wrinkled appearance. Primary conidia had truncate bases and rounded apex while both base and apex were truncated in secondary conidia. Germ tubes were produced apically (pseudoidium type). Chasmothecia were not observed. Genomic DNA was used to amplify the internal transcribed spacer (ITS) region using the ITS1 and ITS4 primers. The resulting sequence (602 bp) was deposited (Accession No. KF499270) in GenBank. BLASTn searches revealed similarity of 100 and 99% with Erysiphe platani from P. orientalis L. (Accession No. JQ365943.1) and P. occidentalis L. (Accession No. JX997805.1), respectively. Phylogenetic analysis placed our sequence in a clade (99% bootstrap support) which included only other E. plantani sequences. In short, morphological and molecular approaches allowed us to identify the infecting fungus as E. platani. For Koch's postulates, 10 detached leaves were inoculated (10 to 15 conidia cm2) on their adaxial surface using an eyelash brush. Non-inoculated leaves served as control. All leaves were kept inside trays with petiole immersed in humidified cotton and maintained at 25 ± 1°C. Symptoms identical to those of the original leaves were observed 6 to 8 days after inoculation, whereas the control leaves remained symptomless. Although E. platani has been previously reported on P. × acerifolia in the city of Poços de Calda, state of Minas Gerais, Brazil (1) and on P. occidentalis in Korea (2), to our knowledge, this is the first record of E. platani on P. × acerifolia in Rio Grande do Sul, Brazil.
References: (1) E. M. Inokuti et al. New Dis. Rep. 15:38, 2007. (2) Y. J. La and H. D. Shin. Plant Dis. 97:843, 2013.