New Plant Care

• You should always inspect the leaves of any newly purchased houseplants before introducing them into a room with other plants. If the leaves show signs of a fungal infection such as black spots, isolate the newly purchased plant and treat it with a fungicide. Remove any leaves that show signs of infection and dispose of them away from other plants in the home. Examine the plant closely to determine if it may harbor pests such as thrips, which can cause black spotting on the leaves.
  
  

Thrips

• Thrip insects are extremely small and difficult to see. The adult thrips will fly off the plant if it is disturbed. The feeding action of the thrips cause the leaf to exhibit areas of white discoloration. The leaf will also show small black spotting, which is caused from the insect's fecal matter. Pyrethroid insecticides control some thrips, according to the Colorado State University. Dispose of a heavily infected plant.
  
  

Fungal Infections

• Some plants will recover from fungal infections with little or no attention. Removing the infected leaves and increasing air circulation around the plant will help control a fungal infection. You should consider using a fungicide if a plant displays extensive signs of a fungal infection. Avoid misting the leaves of the plant and take exceptional care that water does not splash on to the leaves when watering. Fungus thrives and spreads in wet conditions and excessive humidity.
  
  

Other Causes

• A plant may have black spots from over-exposure to sunlight. The plant may also suffer an adverse reaction to aerosol sprays such as air freshener or cleaning products. Leaves that suffer an injury from a chemical spray or pesticide often exhibit spotting. Overwatering a houseplant will also cause leaf spotting as well. Cold water that comes into contact with the leaves of some plants such as African violets, will cause leaf spotting, according to the Purdue University.