Lemon balm is a well-known perennial, medicinal and culinary herb, and also a melliferous plant that is grown commercially in many parts of the world including Hungary. In October 2009, symptoms of powdery mildew infection were observed on lemon balm plants grown in several gardens in Budapest, Maklár, and Eger, Hungary, as well as in Ghenci, Romania. Abundant mycelium and conidial sporulation was observed on both leaf surfaces and stems. Conidia were produced in chains and were ellipsoid-ovoid to subcylindrical, measured 29 to 44 × 15 to 18 μm, and germinated with germ tubes produced apically or subapically on conidia. The basal septa of the conidiophores were sometimes displaced from the point of branching. The width of their foot cells increased from base to top and sometimes enlarged considerably at a particular point. Hyphal appressoria were nipple shaped. On the basis of these characteristics, the pathogen was identified as an Oidium sp. belonging to the subgenus Reticuloidium. The teleomorph stage was not found. To precisely identify the pathogen, DNA was extracted from mycelia collected from single leaves collected in Budapest and Ghenci with a Qiagen (Valencia, CA) DNeasy Plant Kit. The internal transcribed spacer (ITS) sequences of the nrDNA were amplified and sequenced as described in Jankovics et al. (3). The two ITS sequences, deposited in GenBank under Accession Nos. HM156493 and HM156494, were identical to several ITS sequences of Golovinomyces biocellatus, such as AB307675, AF011291, and EU035602. Thus, the pathogen was identified as G. biocellatus based on the host plant species, anamorph morphology, and ITS sequence. It was clearly distinguished from Neoerysiphe galeopsidis, another powdery mildew species known to infect lemon balm in Europe (1). Specimens were deposited under Accession Nos. HAL 2369F and HAL 2370F at the Herbarium of Martin Luther University, Halle, Germany. Pathogenicity tests were carried out in cabinets within a controlled environment as described for other powdery mildews in Jankovics et al. (3) using five potted healthy lemon balm plants bought from a nursery. The first powdery mildew colonies appeared 7 to 10 days after inoculation, and 2 to 3 weeks later, nearly all the leaves and stems of the three inoculated plants became covered with powdery mildew mycelium. Light microscopy confirmed that the pathogen was the anamorph of G. biocellatus. The two noninoculated plants remained healthy. This confirmed the pathogenicity of the fungus collected from the field to lemon balm. G. biocellatus has long been known to infect lemon balm in some European countries and elsewhere (1), but was not listed as a pathogen of this plant species in Hungary (4). However, it was reported from Romania, a neighboring country, more than 30 years ago (2). This suggests that G. biocellatus might have occurred in Hungary on lemon balm during the past decades without being reported in the literature.
References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) O. Constantinescu and G. Negrean. Sydowia 29:75, 1976-77. (3) T. Jankovics et al. Phytopathology 98:529, 2008. (4) Sz. Nagy and L. Kiss. Acta Phytopathol. Entomol. Hung. 41:79, 2006.
