|December 2011. Pictures of watermelon growing in fields in the Cameroon Central Province nr. Yaounde (600 masl) showing erect, chlorotic apical shoots. The fields were visited in the wet season, in August.There was one suggestion of thrips, but that was thought unlikely. According to a University of Florida website, there are three viruses, all transmitted by aphids.
This is what is stated at the website:
Mosaic (Papaya ringspot virus type W, Watermelon mosaic virus 2, Zucchini yellow mosaic virus)
Symptoms: The most common symptom is mottling of the leaf (alternate light and dark green areas). However, one of more of the following symptoms may also be associated with mosaic: stunted growth, abnormal leaf shapes, shortened internodes, “bushy” and erect growth habit of the runner tips, and mottled or bumpy appearance of the fruit.
This disease is usually spread by winged aphids during feeding. The aphids pick the virus up from weed hosts such as the creeping cucumber or melonette, Melothria pendula L., in south Florida or alyce clover (Alysicarpus sp.) farther north in the State. Virus transmission requires 9 seconds or less of aphid feeding on watermelon. See Plant Pathology Circular 1184.
Cultural Controls: No effective commercial control but isolation of cucurbit plantings by use of surrounding plantings of solanaceous crops (tomato, potato, eggplant, pepper) might be helpful in delaying initial infection. Elimination of wild hosts in the vicinity of commercial plantings of watermelons and other cucurbits is critical to the control of these viruses
A note from the virologist at AVRDC said the following:
“Concerning Thomas’ watermelon problem, I agree it does look like virus from the pictures, though I am unable to say which one from the symptoms – there is a whole raft of viruses of cucurbits that all cause very similar symptoms. I would also be more confident about diagnosing virus if I could see what the distribution across the fields looked like; if all the plants have the same symptoms at the same stage, then it suggests it is not a virus and rather some environmental/climatic event that has caused the symptoms. If there are patches of symptomful plants in the field with plants showing symptoms at different stages of advancement, then it is more likely to be virus (or viroid or phytoplasma). The chlorotic, curling, crinkling is suggestive of a whitefly-transmitted begomovirus to me, but I would have to receive samples to run through some diagnostics to be sure.
Later: An email from the sender said that samples were taken and analysed for several viruses (CMV, WMV, ZYMV, PRSV, AWMV, MWMV, ZYFV, CABYV, MNSV, SqMV, MRMV, CGMMV, CVYV, CYSDV) by Virologist Herv?? Lecoq from INRA Avignon (France) and WMV was found. It seems that plants at the edge of the field may be the source of infection.