First Report of Moroccan watermelon mosaic virus in Cucurbits in Brandenburg, Germany

In September 2024 viral disease symptoms were observed in field-grown cucurbits at two sites in the state of Brandenburg, Germany. Symptoms including dark green blisters and chlorosis of the leaves were first observed on pumpkin (Cucurbita pepo) and later on zucchini squash (C. pepo) at a second farm 80 km from the first location (Fig. 1a), of which ca. 10% or 30% of the plants were symptomatic, respectively. Zucchini fruits were malformed and displayed a weak mosaic (Fig. 1b), pumpkin fruits displayed ringspots. To identify the suspected virus, total RNA was extracted from the zucchini fruit using the InnusPREP Virus RNA Kit (ITS Innuscreen). The sample tested positive by RT-PCR with generic potyvirus primers targeting the CI coding region (Ha et al. 2008). Sanger sequencing of the amplified product confidently identified Moroccan watermelon mosaic virus (MWMV; Potyvirus citrullimoroccense). Mechanical inoculation of symptomatic zucchini fruit tissue to seedlings of C. pepo cv. Rigas resulted in chlorotic mottle and ringspots on systemic leaves. An RNA library was constructed from an RNA extract (Spectrum™ Plant Total RNA Kit, Sigma) of the inoculated plant using the NEBNext Ultra II RNA Library Prep Kit and sequenced on an Illumina NextSeq2000 platform (2x150 bp reads). Assembly of the 7,839,654 reads (Geneious Prime) enabled the identification of one viral contig (9760 nt in length) to which 476,863 reads mapped (mean coverage 6785,7), showing in blast n 89.9% identity to MWMV (GenBank accession no. OM718859). The MWMV isolate propagated on C. pepo cv. Rigas is available at DSMZ under accession no. PV-1479 (GenBank accession no. PX681984). To check for the possible occurrence of MWMV in pumpkin, five additional symptomatic leaf samples were tested using the generic potyvirus RT-PCR. Direct sequencing of the amplificates obtained from all five samples showed the highest identities to isolates of MWMV for four samples and to ZYMV for one sample in blastn. However, the sequence obtained for ZYMV showed numerous wobble positions, which is a possible indication of a potyvirus mixed infection. The same five pumpkin samples were also tested using DAS-ELISA for MWMV (AS-0747, DSMZ) and ZYMV (AS-0234, DSMZ). One sample showed a clear mixed infection with both viruses, while all others were infected with MWMV only. The mixed-infected sample was the same one that showed numerous wobble positions in sequencing. The following year 2025, MWMV was again identified using DAS-ELISA in five symptomatic pumpkin samples collected from the two original locations and three additional sites in the region. This virus was first described in Morocco in 1972 (Fischer and Lockhart 1974) and has since been found in countries in sub-Saharan Africa and around the Mediterranean basin, including Spain, France, Italy and Greece (Lecoq and Desbiez 2012; Malandraki et al. 2014). Curcurbits are an important crop in Germany, with a general trend of increasing production over the past few years (FAOSTAT 2023). Although the presence of other viruses in the original samples cannot be ruled out, this is to our knowledge the first report of MWMV in cucurbits in Germany. Cucurbit crops should be monitored for disease symptoms during the following seasons to assess the significance of the virus for growers in the region. This should also include the screening of young plants before planting and the testing of potential overwintering hosts in order to verify the possibility of permanent establishment in Brandenburg.