May 2016. Could members identify this condition on melon in Cambodia: the outside is black, but the plants are still healthy.
There was one suggestion that it was Botryodiplodia, but would have to check the two celled spores, or isolate t check. It grows well and sporulates in culture.
For a specific recommendation we would need the correct diagnosis. It looks like it is coming in from the blossom end and moving toward the stem. May be helpful to know what crop was in the area beforehand and were there any problems with it that could be carried over in debris into the watermelon. The stems look healthy so unlikely to be gummy stem blight (Didymella bryoniae) which is a common nemesis of cucurbits in the tropics.
Is there any information on weather conditions. If it had been very dry then it is possible to get breakdown and collapse of fruit at the flower end and colonisation by opportunistic fungi which can then spread back into the fruit. So maybe the problem is incidental to another disorder.
By the size of the fruit it looks like that crop may nearly be finished so emphasis should be on the next crop.
In the absence of a specific diagnosis a generic recommendation would be to rotate and maybe put a monocot crop on that spot e.g., sweetcorn or something like that and plant the next watermelon plot somewhere else. Then keep a close eye on it and apply a broad spectrum protectant fungicide e.g., copper or mancozeb on a regular basis to provide general protection from fungal attack, particularly just after flowering when the flowers are dying off. If we know what the pathogen is we can probably make a more specific recommendation on fungicides but for now broad spectrum protection is probably the way to go – and cheaper than many. Also, make sure they don’t go under water stress at flowering and early fruit development.