Not sure if watermelons experience blossom end rot? Anyway here's an article @ UF on watermelon diseases. I always get a little depressed reading plant disease descriptions & looking @ the pictures - seems like so much can go wrong!

Here is a link that might be useful: Watermelon Diseases

Ah yes- gummy stem blight.
Good link carolb!
I had a huge problem with fungus this year on all the veggies.
Who could imagine that we would have fungal problems in such a dry year?
I used fresh seeds and had no old plant debris and NO rain!
Gave up and yanked everything out.
No saving this year's garden.

I've had a number of disease probs as well, I understand insects can be vectors for many of them, so dryness isn't always the reason.

I think it is blossom end rot. Lack of calcium.

FWIW, I understand that BER isn't necessarily a lack of calcium in the soil, but rather an inability of the plant to utilize it, due to factors like excessive heat & lack of water.