Root rot: commonly occurs when a plant is placed in a pot that is too large for it's root system, the plant is being overwatered, the soil contains an active fungus, or any combination of those scenarios. The plants roots are deprived of oxygen because the soil is retaining too much moisture. This moisture imbalance in the soil causes harmful bacteria and fungi to overdevelop and further exacerbates the issue. Root rot causes the plants leaf to become discolored and start to droop or fall off. It can eventually cause the plant to die as the root system struggles to recover. The first step to identifying root rot is to remove the plant from the soil and check its roots. Healthy roots are firm and white/light in color. Roots suffering from root rot will be dark, mushy, and slimy to touch. Solutions to root rot include correcting over watering, applying a fungicide to the plant's roots, and repotting the plant in the correct size pot with new soil mix.
Powdery Mildew: this is fungal disease that presents as a white or gray mildew that covers the surface of a plant's leaf. It occurs in areas with high humidity and poor air circulation. For some flowering plants it can cause young flower buds to remain unopened and turn the plant's leaves brown. The plant's leaves eventually drop off.
Yellow leaves: Leaves will often turn yellow due to inconsistent watering or over watering. Inconsistent watering happens when the plant's soil dries out completely before being watered. Overwatering occurs when the plant is being watered too often. As a plant ages, some of its leaves may turn yellow over time before falling off.
Brown spots on leaves: brown spots on a leaf can be burn spots from too much direct sun exposure. It can also indicate the presence of a fungal disease, such as fungal leaf spot, or a plant pest.
Drooping Plant: a plant may droop due to under watering, root rot, or the presence of a sap sucking pest such as scale insects.
Leaf spot: spots on a plant's leaves can be caused by a bacteria or fungus. The spots disrupt the plant's ability to photosynthesize and the plant will eventually lose it's leaves and any fruit or flowers produced by the plant. Keep in mind that some variegated plants will naturally have spots on the leaves as a part of the plant's natural pattern.
Sooty Mold: sooty mold appears as black soot on a plant's leaves. Plant pests such as aphids and scale insects secret a honeydew that allows the sooty mold spore to grow and reproduce on the surface a the plant's leaf.