Question
Hi, I have several geraniums that grow and blossom like crazy (1 is over 6' tall). Lately I've noticed that some of the leaves are turning yellow but others turn almost black and turn to dust.  The plants look good overall and continue to bloom, and grow new leaves and buds.  There are no signs of bugs.  All of my plants are on my porch in San Francisco (Sunset zone 16).  We get full sun, it never freezes, and I water about once a week unless it's really hot and then sometimes a bit more.  I do sometimes spray the plants which I just found out Geraniums don't like.  I could also be over-fertilizing as I do hit the Miracle grow pretty hard.  What do you think?  Thanks!!!!!!!!!!

Answer
You have a Fungus problem caused by spraying the leaves. You seem to know already what you are doing wrong. Now let's see if we can correct it.
Pick off, and destroy any infected leaves as soon as they appear. Inspect the plants daily. If any of the plants are seriously infected, destroy them as well. They will only serve as a source of more problems with your other plants. Keep the area clean where the plants are. Disinfect the tools that are used on the plants. Before reusing any of the infected pots for other plants disinfect with bleach water. Do not reuse any of the infected soil. Discard it. Be careful when watering to not get water on the leaves, or splash infected soil on the leaves. Flush the soil well with clean water, then let the soil dry between watering. Stop fertilizing. Spray all of the geraniums with wettable sulfur or bordeaux mixture. Read the label for instructions on how to apply. Watch for leaf discoloration with the bordeaux.
Shelter the plants from the rain.  Increase air circulation by moving the plants farther apart.
Try to clear up the problem quickly, before Winter.
When Winter storage time comes, let the plants dry out, and place in a dry, darkened place with good air circulation, and prune them back to the new growth.
Hopefully, the fungus will be completely killed, and not return in the Spring.
Geraniums are fairly heavy feeders. I only fertilize mine in the Spring, after I see new growth emerge. Then later in summer when the temperatures drop, because of clouds or rain. I water when the soil feels dry, and the containers are light. My plants bloom, and grow like crazy. I keep them dead headed, so that they will keep putting on new buds. I sometimes need to remove a yellow leaf, or two, but the plants seem to produce better with neglect, then with too much attention.