First report of the identification of Moroccan watermelon mosaic virus in papaya in Democratic Republic of Congo (DRC)

Papaya (Carica papaya) is the major cash crop in North Kivu, DRC where it is grown for latex and exported to Europe for papaine production. Since 1997, a disease has gradually spread throughout North Kivu, seriously affecting latex yields. As field production has declined, this has been offset by expansion into new areas of forest; in turn this is having a serious environmental impact on the fragile ecosystems of the equatorial highlands. Plants are affected at all ages and all local papaya varieties are susceptible to the disease. Leaf symptoms ranged from yellow mosaic patterns to deformation and shoestrings as the disease progressed. Oily streaks in petioles and variations in shape and size of the ring spots on the fruits were also seen. Within the trunk, pink discoloration and tumor-like growths appeared, these eventually became brown and necrotic. In the final disease stage, plants resembled a pole with a tuft of small leaves at the apex and the internal trunk became fibrous.

During September 2006, 47 leaf samples from papaya plants showing symptoms and additional healthy plants were collected from surveys carried out in North Kivu. Twenty symptomatic samples were tested by ELISA using a universal potyvirus monoclonal antibody (No. 943710; Agdia, USA) and thirteen of these also by electron microscopy (EM). 17/20 samples tested positive for potyvirus by ELISA, while potyvirus-like particles were observed in 12/13 samples. All samples tested negative for Papaya ringspot virus by ELISA (No. 07060; Loewe, Germany). Healthy controls all tested negative in both ELISA and EM. Total RNA was extracted from all 47 symptomatic samples and used as template in RT-PCR employing a generic potyvirus coat protein primer (Uni 3, 5'-ATG GTR TGG TGC ATT GAG AAT GG-3') and oligo dT (No. O4387, SIGMA, USA); 43/47 samples tested positive for potyvirus. RT-PCR products were obtained from a representative virus isolate using further primers (MWMV F1 (5' GTC GTT GAT ACA CTC TAA TGG TG 3'), MWMV R6 (5' GCG TGC TCA ATC ATT GGC TTC AAC 3'), covering part of the NIb gene, the entire coat protein and the 3’ untranslated region (total 1639 bp). The amplicons were sequenced and analyzed using BLAST searches. The nucleotide sequence obtained (EF211959) shared 84% identity with that of Moroccan watermelon mosaic virus (MWMV) (AF305545). A coat protein amino acid alignment found 93% identity with MWMV.

First reported in 1972 in Morocco, MWMV is a distinct member of the Potyvirus genus (Potyviridae family) (McKern et al., 1993). This is the first record of MWMV both in papaya and in the DRC. Further studies are required to fully characterize the virus and understand its association with this papaya disease.