Likely possibilities: Over-watering/compacted soil. Allow soil to get fairly dry between waterings.

Roots getting too hot. Shade containers or move to shade (if not too huge) during hottest parts of day.

Fungal issue. Try watering only the soil & not leaves. Be careful not to splash soil onto leaves when watering. Make sure air can move freely where containers are located.

Al

Sounds like botrytis, a fungal disease. I stopped growing geraniums for that reason - they always got botrytis. I don't know what you can do about it. The baking soda spray might help if you want to bother spraying annuals. You have to be sure and reapply after a rain.

Dunno the cause, but I have found potted geraniums should be watered sparingly. I have my 12" pots in full sun on cement and water them when it rains and whenever I think of it which is not more than once every couple weeks. I don't think they like being watered much.

This may not be true for your zone: if you have overwintered your geraniums then they may be getting old and need renewal.

In addition to the new leaves, are they also producing new sideshoots? If they are, long and vigorous or small sidegrowths at the ends of the main stalks?

If you're in love with the colour, or they have sentimental value, take some cuttings. They can root both in water and in a damp sandy mix. If you do the sandy mix let the cuttings dry for a couple of days to form a callus over the cut. They are less likely to rot.

When they're happy in a mix geraniums have an extensive root system. It may be worth removing one of the plants and seeing just what is happening in the tub.

One possibility is that the water you add just isn't soaking through - or is whizzing by so fast that it's leaving a big ball of dry stuff around the roots. Some potting mixes are really bad for this. You might need to add a wetting agent to your watering process to ensure that the mix really is getting wet.

If your tubs are up on feet - water until there's a flow from the hole. Go away for fifteen minutes - and come back to water again. It can take that much time for a dry mix to rehydrate.

Once it has soaked up a reservoir of water then , depending on your local climate, you only need to water once a week to ensure there's enough damp to keep growth going without flooding/drowning the roots. Geraniums like plenty of air to go with their water.