Because aquatic plants live in a controlled environment, they do not tend to encounter a large number of diseases and infections. However, failure to maintain proper conditions for your aquatic plants as well as a variety of other environmental issues can lead to health problems for your aquarium inhabitants. Closely monitoring your aquatic plant community so that you can take quick action in the event of signs or symptoms of aquatic plant diseases will help you maintain the population of your aquarium in a happy, healthy fashion.
Cryptocoryne disease, also called cryptocoryne rot, starts out with small holes forming in the leaves of the afflicted plant. Left untreated, these holes will grow and spread, eventually breaking down the entire plant. The cause for cryptocoryne is uncertain, but excess nitrate and impure water probably play a role. You should do a "large water" change, which means you change out as much of the water in the aquarium as possible. Remove all affected and dead plant material from the infected aquatic plant. Continue to monitor your plant and remove dead plant material as necessary. If your plant does not recover after three weeks, you might need to remove it from the aquarium completely to prevent the spread of the disease.
While mineral deficiencies are not a contagious disease, they are the source of nearly all aquatic plant afflictions in an aquarium. They also appear in much the same way a disease would, spreading quickly from one plant to another as each succumbs to the impact of the problem. An iron deficiency is characterized by yellow, brittle, glassy leaves, while manganese shortages cause the leaves to yellow while the veins remain green. Too much phosphate causes plants to turn brown or black and die. Too little potassium causes new growth in aquatic plants to yellow and die off. All of these issues can be addressed by filtering the water effectively and fertilization.
When the outside air is too dry, aphids and other small insects may actually colonize aquatic plants instead of dry-land plants. You will easily be able to spot these little insects in your aquarium feasting on your wilting, puny-looking aquatic plants. Rinse all the insects off of the plant using fresh water and remove any parts of the plant that are heavily damaged or heavily infected. Continue to monitor the plant for signs of further infection, repeating the treatment if necessary.