First report of <i>Exobasidium gracile</i> causing galls on <i>Camellia sasanqua</i> in Switzerland and in Europe

In May 2024, symptoms consistent with those caused by an Exobasidium species were observed on Camellia sasanqua in a private garden in the Maggia Valley, southern Switzerland (46.25°N, 8.7°E). Affected leaves became swollen, deformed, and spongy, with a light green hue and red tones, in contrast to the original dark green appearance and the leathery, coriaceous texture (Figure. 1). The galls formed due to the intercellular growth of the fungus within the leaf epidermis, causing the leaf thickness to more than double. These symptoms and the host plant C. sasanqua are in agreement with previous reports of Exobasidium gracile in Asia (Shirai, 1896; Akai, 1939; Mims et al., 2007; Lee et al., 2015). The total rainfall in May was 305.9 mm, which is higher than the mean of 189.6 mm calculated between 1991 and 2020; the temperature ranged from 10.6 – 18.7°C in May, where a temperature deviation of -1,3°C from the 1991–2020 mean was observed (Locarno Monti; Federal Office of Meteorology and Climatology, 2022). A month later, the leaves showed signs of rot at fungal proliferation sites (Figure. 2). As the season progressed, the cool, humid conditions favourable to Exobasidium declined (Chen et al., 2023). Since no spores were found for morphological identification, cultivation or infection tests, DNA was directly extracted from a 4 mm2 piece of infected leaf (isolation PHP24_0812) for molecular analysis. The ITS region of rDNA was amplified and sequenced using the standard ITSF1/ITS4 primers for fungi. A BLAST search with the obtained ITS sequence (GenBank Accession No. PQ238300) showed more than 99% identity with sequences of E. gracile deposited in GenBank (EU692760, EU692761), only 98% identity with the sequence of E. camelliae-oleiferae and 97% identity with those of E. camelliae. In a phylogenetic analysis, the Swiss sample clustered with all other E. gracile sequences in a single clade (Figure. 3). A voucher specimen is deposited at the Museo cantonale di storia naturale, Lugano (LUG 20000). The very close match of the barcode sequence, in combination with the typical symptoms on C. sasanqua, lead to the conclusion that the fungus belongs to E. gracile (even if the microscopic features of the fungus could not be checked). This is the first report of this fungus in Switzerland and Europe. There is a potential risk to ornamental plantations of the host plant. We thank Mauro Bordoni and the mycological society “Gruppo Micologico Bellinzonese e Valli” for the organisation of the excursion which led to the discovery of Exobasidium gracile.