Answer: Your Canna plants are infested with an insect known as the Canna leafroller. These larvae "stitch" the rolled foliage together with webbing and then feed on the interior of the leaves. The goo and dirt is the excrement from the insect.
The safest control is to remove the webbed foliage and bury it or throw it in the trash. Then spray the plants with a product containing B.t. (Bacillus thuringiensis). It is sold in numerous brand names including: Dipel, Thuricide, Ortho BioWorm Killer, MVP Caterpillar Killer, and many more.
Repeat sprays on a weekly basis for a while to make sure you have broken the life cycle of the insect. Continue to remove webbed foliage promptly whenever it appears, and retreat with B.t.
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