First Report of <i>Erysiphe necator</i> Causing Powdery Mildew Disease of Cashew in Ghana

Cashew (Anacardium occidentale) is an important cash crop in Ghana and the export of raw cashew nuts generated $378 million in 2018 (Ghana Business News 2020). In October 2024, symptoms of suspected powdery mildew disease were detected on cashew in the Tain, Wenchi (Bono Region) and Techiman (Bono East Region) districts in Ghana. Infected leaves were covered with white powdery spores on the adaxial surface (Figure 1a,b) and brown to yellow necrotic lesions on the abaxial surface (Figure 1c). The disease was mostly found on mature cashew trees of at least 10 years’ old. Disease incidences of 67, 87 and 79% were observed on infected farms in the Tain, Techiman and Wenchi, districts, respectively. Disease severity, estimated according to the proportion of leaves on each tree damaged by the disease, in the surveyed farms ranged from 9.8% to 64.0%. For morphological identification, conidia from the infected leaves were removed using sticky tape and mounted in sterile distilled water on a microscope slide. Microscopic examination (Leica, Germany) showed that the conidia were produced in pseudo chains and were an ellipsoid to ovoid shape. The conidia measured 25.0–42.5 µm in length and 10.0–17.5 µm in width (Figure 2). Molecular identification was done by washing conidia from infected leaves into microfuge tubes. Genomic DNA was extracted using a DNeasy Extraction Kit (Qiagen, Germany). Primers PM3 (5′-GKGCTYTMCGCGTAGT-3′) (Takamatsu and Kano 2001), ITS1 (5′-TCCGTAGGTGAACCTGCGG-3′) (White et al. 1990), P3R (5′-G CCGCTTCACTCGCCGTTAC-3′) (Kusaba and Tsuge 1995) and TW14 (5′-GCTATCCTGAGGGAAACTTC-3′) (Mori et al. 2000) were used for PCR amplification. A BLASTn search showed that the sequence from all three samples had a query cover of 95%–99% and 97.6%–99.5% identity to Erysiphe necator with GenBank Accession Nos. LC777882.1 and KY312008.1. Sequences of the three isolates were deposited in GenBank (PV778036, PV778048 and PV778061). A pathogenicity test was done by making a conidia print of the fungus from infected mature leaves using transparent sticky tape. Six-month-old healthy cashew seedlings of mixed genotypes with three to six leaves were washed with mild soap and sodium hypochlorite (1%) under running water and rinsed with sterile distilled water. The seedlings were inoculated with the conidia print and non-inoculated seedlings served as controls. The seedlings were kept in a gauze house with a 12-h day-night cycle at 25°C ± 2°C. Powdery mildew symptoms, similar to those on field-infected cashew trees, were observed on inoculated seedlings after 30 days. This indicates that the disease can infect younger plants, although this has not been observed in the field in Ghana. No symptoms were recorded on the uninoculated cashew seedlings. Conidia from leaves of infected seedlings were obtained as previously described and identified under a microscope to fulfil Koch's postulates. To the best of our knowledge, this is the first report of Erysiphe necator causing powdery mildew disease of cashew in Ghana. The disease, but not the causal pathogen, has been reported in neighbouring countries. Confirmation of the causal pathogen highlights the threat of a new disease to the flourishing cashew industry in the country. This disease requires further study and the development of control systems to ensure the sustainable growth of the cashew sector in Ghana. We acknowledge the technical support of staff of the Pathology Division of the Cocoa Research Institute of Ghana. This paper is published with the kind permission of the Executive Director of the Cocoa Research Institute of Ghana (CRIG/04/2025/066/001).