First report of powdery mildew on cranberry (<i>Vaccinium macrocarpon</i>) caused by <i>Erysiphe elevata</i>

Vaccinium macrocarpon is an important domesticated fruit crop, commonly known as the large cranberry (Hancock et al., 2003). Cranberry is native to North America, with the USA and Canada being two of the largest producers of the fruit crop. In the UK cranberries aregrown in gardens and allotments. In February 2022, powdery mildew symptoms were found initially on the upper and lower surface of several young leaves of a two-year-old cranberry plant (cv. ‘Pilgrim’) in Kent, UK (Fig. 1), which then continued to spread from young leaves at the tips of the stems towards the crown, along stems and on mature leaves. Approximately 10% of the plant material was diseased. Disease symptoms appeared as epiphyllous, irregular, white patches of mycelia on the surface of the plant tissue, with conidiophores and conidia causing a white covering of the leaf that resulted in infected leaves becoming discoloured and dropping prematurely. Conidiophores were erect, unbranched, and 60–96 × 8–10 μm in size, with flexuous or straight foot cells at the base 20–35 μm in length, followed by 2 or 3 cells (Fig. 2a,b). Conidia were singular and ellipsoid to oblong elliptical in shape, 29–38 × 13–20 μm in size (n = 40), without fibrosin bodies (Fig. 2c,d). Hyphae were septate, branched at right or acute angles, hyaline, with simple or multi-lobed appressoria formed. Some spores germinated to produce a germ tube from the end of the spore with a lobed appressorium (Fig. 2d). Chasmothecia were not observed on the sample plant material. These morphological observations were consistent with the characteristics described for the conidial stage of the genus Erysiphe (Braun & Cook, 2012). To confirm the fungal identification, genomic DNA was extracted from the mycelium and conidia, collected from single juvenile leaves. The internal transcribed spacer (ITS) region of the rDNA gene was then amplified using ITS1 and ITS4 primers (White et al., 1990) and sequenced. The 577 bp consensus sequence was deposited in GenBank (Accession No. ON540088). A BLAST analysis revealed 100% identity to Erysiphe elevata isolates from China (MH985631, MT802112, MZ379159) and the UK (KY660910). Phylogenetic analysis in Geneious (v2021.2) revealed the similarities of the ITS region between other Erisyphe species with one single nucleotide polymorphism found between some isolates of E. elevata and Erisyphe vaccinii (Fig. 3). To fulfil Koch's postulates, four healthy pot-grown cranberry plants cv. ‘Pilgrim’ were inoculated by pressing infected leaves onto 20 healthy juvenile leaves. Plants were maintained in a growth room at 22–23°C, with a 12-h photoperiod. After 21 days, inoculated leaves exhibited powdery mildew symptoms (Fig. 4a,b) and the associated fungal structures observed were morphologically identical to those on the original infected leaves. E. elevata has been recorded to cause powdery mildew disease in Catalpa species across North America, Canada, Hungary, Germany, Slovakia and was also reported in the UK in 2003 (Braun, 1987; Ale-Agha et al., 2004; Vajna et al., 2004; Pastircakova et al., 2006). However, to our knowledge, this is the first report of E. elevata on cranberry in the UK and globally. Due to the economic importance of cranberry, this disease is potentially a serious threat to production.

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