What is affecting my Pentas?

Question

About 5 days ago I noticed the leaves spotting, turning brown, spreading and eventually the whole leafs turns black with small holes. The underside of the leaf appears fine.

Of course, the follow up question is what I can do to treat them.

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Answer 1

This looks like a fungal disease. Pull back the mulch and check the soil. Excess moisture and lack of airflow can contribute to this issue.
Remove any dropped leaves from around the plant and discard.
Allow the soil to dry a bit between waterings.

Treat with a fungicide.

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
https://www.gardeningknowhow.com/garden-how-to/info/homemade-fungicide.htm