It is hard to identify the disease unless we see the spores through the microscope. Do you have microscope facility in your area? Also, please provide us more information on weather, agronomic practices carried out etc.
Alternaria Leaf Blight on Watermelon
Gummy stem blight on watermelon
Anthracnose lesions on cucurbits are angular in appearance
Downy mildew disease on watermelon