Answer: Because you have an identical problem on two unrelated plants, I'd suspect an insect is causing the problem. Sucking insects exude a sticky substance known as 'honeydew'. If allowed to remain, the substance can be host to fungal diseases. Check the undersides of the leaves and the stems for aphids, mealy bugs, or scale insects. Aphids are easily seen because they fly when disturbed, mealy bugs can be identified by the little tufts of cotton around them, but scales look like little bumps and can easily be overlooked. Try to identify the critter responsible for the damage and treat your plants to eliminate the bugs. There are chemical sprays that are effective, (depending upon insect), or you may be able to dislodge and destroy them one at a time with a cotton swab dipped in plain water.
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