Gardeners are often quick to blame insects or infectious disease when their plants develop leaf spots. But, in many cases — especially with houseplants — those spots may be due to environment and cultural practices.
House plants that experience extremes in soil moisture often develop spots on their leaves, called “oedema” (sometimes spelled “edema”). The spots may first appear as a blister or raised spot, particularly on the undersides of leaves, but may occur on the top side as well as on the stems. Eventually, the blister develops a rust-colored, cork-like scab.
Oedema occurs when plants have been subjected to overly dry conditions, followed by abundant moisture. The plant cells take on too much water too rapidly, causing them to burst. Oedema can also occur on outdoor plants. The blisters appear when soils are abundantly moist, coupled with cloudy, humid weather.
Oedema spots do not spread, but the scab will remain unsightly. The dead tissue may dry and fall away on thin-leaved plants, leaving a “shot-hole” appearance.
To prevent oedema from occurring, avoid extremes in watering practices. Feel the soil of houseplants before you water to be sure the soil needs additional moisture. Mulching outdoor plants will help avoid extremes in the soil moisture supply.
Gardeners often become alarmed when their prized African violet develops leaf spots. Although there may be other possibilities, these spots are usually just the result of splashing water. African violet leaves are sensitive to cold water and will form spots where the water contacts the leaf tissue. Watering from below is often recommended as a remedy.
Placing water in the bottom dish and allowing it to be drawn up into the soil should avoid the splashing of water onto the leaves. However, continuous watering from the bottom can lead to a buildup of salt residue in the soil. Most tap water has some degree of mineral salt content, and some sources may have a considerable amount. Fertilizing the plants will also add salts to the soil. As water evaporates from the top of the soil, the mineral salts are left behind. This is the white, crusty material that is often seen on houseplant soil and containers.
An occasional heavy watering from above will help flush those salts out of the pot. About every fourth watering, apply enough water from above so that some runs out of the drainage holes at the bottom of the pot. Be sure to discard that excess, so that the salty drainage water is not re-absorbed into the soil.
Spotting of African violet leaves can also be avoided by using room-temperature water. Then water can always be supplied from above, preventing the salt buildup. You will still have to ensure that the drainage water does not become reabsorbed. A simple way to prevent re-absorption is to set the pots up on rocks, bricks or other support so that the bottom of the pot is out of the reach of the drained water.