Answer: Few pests and diseases cause problems to these plants. Waterlily beetle is the most common pest, damaging leaves and flowers, but does not seem to affect the plant?s general health. No approved chemicals are available to deal with this pest, so control measures are cultural. Remove old leaves and flowers in summer to reduce insect eggs and larvae. Surface-feeding fish such as golden orfe also help, eating larvae and adult beetles.
The most troublesome disease of water lilies is crown rot, caused by a Phytophthora fungus that can kill off entire clumps. Again, the only control is cultural. Affected plants turn yellow and produce few flowers. Remove the plant from the pond and discard rotting stems and compost. Replant healthy pieces of rhizome in clean compost. If you grow more than one cultivar, it is best practice to keep your lily in a separate pool until you are sure it has recovered (a bucket of water makes an adequate nursery pool).
The rusty film on the surface of the water is probably rust from the seams of your galvanized tank. You'll have to drain it to make sure. If you see rust spots, you may have to line the tank to keep water from direct contact with the metal.
Best wishes with your waterlilies!
Q&A Library Searching Tips