Geraniums can be very loosely divided into two general groups: those that are winter hardy and those that aren't. Plants in the first group are usually marketed as "hardy" geraniums. The latter are usually called "annual" or "common" geraniums and are often sold as bedding plants. Annual geraniums will not survive freezing temperatures. If your geraniums are turning mushy and brown, and the weather has been cold, they are probably annual geraniums that need to be brought inside for the winter or discarded. In addition, geraniums need a lot of sun -- at least 10 hours a day. If your geraniums are getting too much shade, they will not produce flowers or the buds will drop before they can fully open. On the other hand, too much heat can cause some geraniums to wilt. If these geraniums overheat, they will not produce flower buds.
Fungal diseases can cause discolored leaves, rotting stems and overall decline in geraniums. Most fungal diseases thrive in overly wet soil and spread on water, including rainwater or splashing water. For these reasons, it's important to plant your geraniums in well-draining soil. In addition, water at the level of the soil to avoid getting water on the leaves, where it can sit and eventually cause leaf rust. Yellow and brown leaves on the lower part of the plant are often a symptom of a fungal disease. If your geraniums have been prone to developing fungal diseases in the past, spray them with a preventive fungicide in the spring.
Hungry insect pests can destroy the leaves and buds of your geranium. It's usually rather easy to tell if this is what is wrong with your geranium, because you can see the bugs. Aphids, which are small green or white bugs, tend to cluster on the undersides of geranium leaves or where the leaf stem meets the flower stem. Scale insects attach themselves to the stem of the geranium. Spider mites can cause the leaves to turn black and fall off. All three insects feed on the plant's juices. To rid your geranium of a minor insect problem, spray it with a strong stream of water then set it in the sunlight to dry. If your plant is heavily infested, hit it with a dose of insecticidal oil.
Geraniums can suffer from an unusual problem called odema. Often nicknamed "dropsy" because it causes the leaves to fall off, this situation arises from overwatering. When this happens, wet lesions appear on the leaves. Over time, they turn brown and corklike. Then, they drop off the plant. Only water your geraniums once a week, and only if there is no rain. In addition, if your geranium seems to be languishing for no apparent reason, consider fertilizing it. They are heavy feeders and need a lot of nutrients, especially if grown in containers. Use a 6-12-12 blend and apply it every month. Follow the instructions on the package as per the size of your geraniums.