This looks like abiotic damage (not caused by a pest or disease), though it's hard to tell from what cause. Is the container being kept moist and not allowed to wilt or get too dry? (Lobelia isn't very tolerant of drying out.) The damage looks a bit like scorch, which can be tissue damage from desiccation. Scorch can also occur on plants that prefer semi-shade (like Lobelia) which are grown in too much direct sun, or which are moved from a shaded location (or indoors) into a sunnier one without a transition period during which they can acclimate.

Was anything sprayed on or dripped on or near the plant? (Neem oil, soap, liquid fertilizer concentrate, glass cleaner on a nearby window, etc.) The leaf injury also looks a little bit like it could have been caused by phytotoxicity (plant tissue damage from chemical exposure).

In either case, the damage itself is permanent in that those affected leaves can't heal, but if the situation that caused it doesn't repeat itself, then at least the damage should not spread or worsen with new symptoms, and new growth should emerge and mature normally. You can clip off the affected leaves if you prefer for aesthetics, but it's not necessary.

Keep an eye on the plant for now and just monitor it for watering needs as the weather trends warmer. No intervention with a pesticide is needed or recommended, but if the plant declines or develops new symptoms, feel free to send us more photos for assessment. If a fungal issue like Botrytis does appear at some point (a fairly common leaf- and petal-blighting infections), it tends to thrive in conditions of high humidity, prolonged leaf wetness (like from watering that gets the leaves wet, especially in the evening), and poor air circulation (from crowding or a wall that blocks the breeze). Botrytis can cause browned, mushy, shriveled leaf tissue, but it will tend to eventually produce a gray mold-like coating on the infected surface, which helps to distinguish it from other causes of leaf damage.

Miri

Thank you very much for the information; I will keep an eye on the plants to see how they do.

You're welcome.