Cryptocoryne rot is a documented phenomenon that effects Cryptocorynes of many different species. Although they don’t appear to be, Cryptocorynes are sensitive plants. Environmental changes in their habitat may cause a completely healthy plant to partially or totally disintegrate.
Things such as water changes or fertilization can initiate this phenomenon. Relocation or changes in lighting can also cause Cryptocoryne rot. The rhizomes remain alive underground and will grow new leaves eventually if one is patient.
The best method of prevention is to maintain a stable environment. Perform small water changes on a more regular basis and fertilize gently and more regularly.
Most commercially produced Cryptocorynes are grown emmersed (above water) at this point and are very strong in this growth form. Only after the submersed from is produced in the fish tank will you observe this phenomenon.