Diseases That Affect Houseplants

Harmful micro-organisms are always present to some extent in the soil, the air, the water and on plants themselves, but a healthy plant has natural defenses against them.

* Always use sterile potting soil or rooting medium.

* Clean and disinfect old pots with bleach before reusing.

* Keep any gardening tools clean.

* Water plants carefully. Water standing in the crown of a plant encourages fungal or bacterial growth, which may cause stem and crown rot. Water left standing on leaves, flowers and buds may encourage the development of botrytis blight or other diseases.

* Do not overwater plants. Roots which cannot get enough air will die, weakening the plant and making it susceptible to disease.

* Provide adequate ventilation and avoid overcrowding so that each plant receives fresh circulating air.

* Protect plants from cold drafts or temperature fluctuations.

Anthracnose: This disease is caused by the fungi Colletrotrichum and Gloeosporium. The leaf tips turn yellow, then tan, then dark brown. The browning may extend completely around the leaf. The leaves eventually die. Wounding enhances penetration by these fungi.

Anthracnose

Prevention & Treatment: Pick off and destroy infected leaves. Do not mist leaves.

Fungal Leaf Spot: Several fungi can cause leaf spots. Symptoms include small, brown spots with yellowish margins on the leaves. Spots may have a concentric ring or target pattern. Small black dots (fruiting bodies of the fungi) may be visible in dead tissue. Sometimes the lesions run together and the entire leaf dies. The fungi survive on dead and decaying plant matter in the soil.

Fungal Leaf Spot

Prevention & Treatment: Remove and destroy infected plant material. Provide good air circulation around the plants. Avoid splashing water on the foliage since this spreads the fungi.

Bacterial Leaf Spots: Plants infected with bacteria have water soaked spots, sometimes with a yellow halo, usually uniform in size and sometimes with sticky ooze. The spots enlarge and will run together under wet conditions. Under drier conditions the spots do not enlarge but dry out and turn reddish brown, giving a speckled appearance.

Bacterial Leaf Spot

Prevention & Treatment: Remove all diseased plant material. Avoid low temperatures, crowding plants, and spraying or splashing water onto the foliage.

Root Rot & Stem Rot: The fungi Rhizoctonia, Pythium, Botrytis, Phytophthora, Alternaria, and Sclerotinia cause these diseases. With root and stem rot, leaves and stems show a noticeable wilt. Stems may be girdled at or near soil level by a ring of brown or black tissue. Infected roots are brown to black and may be soft. The fungi survive in the soil or on infected plant debris in the soil. Their spores can be spread by wind, splashing water or the moving of infested soil.

Root Rot	Stem Rot

Prevention & Treatment: Use sterilized soil and pots. Do not overwater the plants, since too much water increases the occurrence of root rot. If only a few roots are infected, cut out these roots and repot the plant in sterile soil. Fungicides are available, however most indoor gardeners will find that these chemicals probably cost more than a new plant.

Powdery Mildew: The fungus Oidium species causes the formation of a white, powdery growth or dry, brown, papery leaf spots. Initial infections usually come from fungi surviving in dead and decaying plant materials or from airborne spores from wild or cultivated hosts out-of-doors.

Powdery Mildew

Prevention & Treatment: Since the disease develops most rapidly under humid conditions, proper ventilation and not overwatering will help control this disease. Remove severely infected leaves.

The most important element in avoiding diseases on your houseplants is to keep them healthy. Many problems are caused by cultural issues such as too much water, not enough light, too much light etc. Here’s a table with common symptoms and the most probable causes.

Common Houseplant Diseases and Their causes

If you notice in the chart above, 6 of the 8 causes list overwatering as contributing to the problem. More houseplants are killed or damaged by overwatering than any other cause.

A few other things that I do to keep my houseplants healthy:

Stop fertilizing during the winter months; the reduced daylight slows the plants need for food.

Once a year slip the plant from the pot, if it's root bound repot it. When repotting move up one size; for example, if it's in a 4 inch pot move it up to a 6 inch pot.

Keep it away from heat ducts during the heating season.

If your home is extremely dry, place some pebbles or marbles in a shallow dish, add water and set your pot on top of the pebbles. This added humidity will keep the leaves from drying out.

Every 6-8 weeks put your plants in the shower and let tepid water flow over them for 5 minutes. This will flush any salts or minerals that tend to build up in the soil as well as flush any dust from the foilage. I like to capture rain water to use on my plants, this eliminates salt or mineral buildup.

Photos courtesy of:  Ohio State University, Clemson University and University of Maryland