Causal agent, Inconclusive. A variety of bacterial organisms have been previously described as associated with watermelon rind necrosis, but there is no conclusive evidence of the casual organism/s yet. 

Symptoms: Internal vascular browning of watermelon rind. Brown, dry, and hard necrosis of the rind, that rarely extends to the flesh of the most watermelon cultivars. The fruits looks normal from outside, but is unmarketable.

Management: No management options are currently available. Varietal resistances have been previously reported, however was not consistent in different locations.

References: 

http://aggie-horticulture.tamu.edu/extension/newsletters/vpmnews/jul01/art2jul.html