If your plants have brown spots on the leaves, they need your attention. House plants and outdoor plants are all subject to disease, insects and climate sensitivity. Once you're able to determine the cause of the spotting, you'll be able to help your plants return to their natural healthy state.
It is possible that your plants are affected by scales, a common parasite. If you suspect infestation, but do not see bugs, apply double-sided tape on various parts of the plant. Using a magnifying glass, check the tape for tiny crawlers. In their mature stages, scales are embedded under the leaves and secrete a waxy substance. The tops of leaves will turn yellow and may fall off prematurely. The secretions attract a dark fungus that will cover the top of the leaves. Scales are hard to get rid of, especially in their mature stages, but using horticultural oils or insecticides will reduce or eliminate the pests if you dedicate time and attention to the problem.
When a plant's leaves have circular or irregular brown or black spots with raised centers, it may suffer from a fungus. Remove the affected leaves with clippers, and collect leaves that have fallen below the plant. Burn or bag them so the fungus does not continue to spread. Clean the clippers with a mixture of water and bleach to prevent infection from spreading. Spray the plants and others in the vicinity with a fungicide. If you are unsure of the appropriate chemical for the plant and climate, put a few leaves in a clear sandwich bag, bring them to a garden shop and ask for advice.
Bacterial diseases are common to many plants. Symptoms may include yellow to brown spots on leaves, tip burns, blights, rot, unpleasant odors, and wilting. Change the soil, fertilize, and accommodate the plant toward its natural environment as much as possible. If you can move the diseased plant away from other non-affected plants, it will help prevent the spread of the bacteria. Do not overwater or mist the infected plant, as bacteria will thrive.
Gas Leaks or Fluoride
Another problem which may cause brown spotting is too much fluoride in the water. If you suspect your water may contain fluoride, try using rain water or distilled water. Your leaves will show marginal and burned tips.
Brown spots can also be caused from a leaking natural gas line. Tropical foliage leaves, such as philodendrons, will have dark brown spots around leaf margins. Have your furnace checked immediately if you suspect a gas leak.
Plants may spot due to overwatering. Feel the soil before you soak the roots. Follow guidelines for the plant type, regarding how often you water and mist. For outdoor plants, summer months will strain your plants if they are not watered frequently. Indoor plants might be affected by heating/air conditioning, lighting, crowding, and stress due to poor air circulation. At the first sight of leaf spotting, introduce your plants to different locations throughout the house, but do this over a period of time to assess which areas best suit them.
About once every six months, repot indoor plants in fresh soil. During warmer months, fertilize approximately every eight to twelve weeks. Stay on schedule with appropriate watering. To prevent disease, or pests from infecting healthy plants, use a preventative pesticide as directed on bottle or package. Healthy plants are more apt to fight off disease and insects than plants in distress.