Since aquatic plants in a planted tank live in an enclosed and clean environment, a smaller number of diseases affect them than land plants. I have seen a few rare diseases, such as the blackish, nearly transparent wrinkles that spread rapidly through Microsorium. The most common seems to be Cryptocoryne disease, where the plant's leaves and stems crumple up instantly. In fact, this is not a disease at all, but rather a shock reaction to sudden changes in the water chemistry such as a drastic pH value drop from adding too much CO2, or a sudden increase in light intensity from a bulb change. However, if measures are taken to rectify the cause, the recovery of the plant is almost as swift as the onset.
If the aquarium's upkeep is good, aquatic plants almost never get sick. Many plants are sensitive to fish medicines, even those whose labels say they are safe. Generally, an unhealthy enviroment is caused by one of the following:
Regular water changes, especially in warm weather, a large power filter, a small number of fishes, and cleaning up after feedings add up to healthy and beautiful plants. If, by a stroke of bad luck, some disease does afflict the plants, quickly transplant the unaffected plants to another tank and start over. It is just not worth the trouble and cost to cure ailing plants, introduction of chemical treatments will trigger off a chain of reaction that affects other flora and fauna.