Diseases of Houseplants
Cultural diseases
Edema
Edema causes cork-like swellings on leaves of susceptible plants such as schefflera, ivy geraniums, pothos and philodendron. It is caused by high humidity and overwatering - cells absorb more water than can transpire into the humid air.
- Cultural control - increase light levels, so plants photosynthesize at an increased rate, increase air circulation, allow plant to become as dry as that plant can be before watering, then water well (not every Sunday!), do not allow water to stay in the saucer, check drainage hole (can be plugged by pebbles in bottom of pot).
Guttation
Guttation is also a watering problem - droplets of sticky moisture appear on uninjured leaves as a result of too much soil moisture and too much humidity in the air. Moisture is not lost as quickly through evaporation in very humid air as is being absorbed by the roots, so it oozes out of plant pores. It is often seen as a sugary-looking crystal at the base of a leaf of a weeping fig and assumed to be an insect.
- Cultural control - as edema above
Excess Salts
Excess salts can build up in soil from too much fertilizer over a long period of time, and not watering thoroughly. As the soil becomes dry, salt concentration in soil moisture is increased, and plant tissue that absorbs this moisture is scorched. Dark edges and blotches near edges on long, grassy leaves are symptoms, particularly on dracaenas and spider plants (which are susceptible specifically to fluoride in the water). Many more plants are harmed by over watering than under watering, but drought can cause leaf damage other than the expected wilt.
- Cultural control - water enough at one time to thoroughly soak the rootball, but do not leave water in the saucer. Increase humidity in the air (not practical in Calgary in the winter - it just freezes on the windows). Do not over-fertilize - if a little is good, a lot is not a lot better!
Poor Flowering
Poor flowering is caused by factors specific to each plant, and lack of fertilizer is not one of them! Each plant has its rule for flowering - some need a long day and short night, some a short day and long night, some a warm day and cold night, some need a great deal of sun to flower, some just have to become mature enough. There is no fertilizer that will make a plant bloom, although a healthy, well-nourished plant will produce more spectacular flowers than a sickly one. The only way to make a plant bloom is to learn what it needs and supply it - not always easy!
Infectious Diseases
Powdery Mildew
Powdery mildew is a fungus which causes a white, dusty coating on flowers and leaves of susceptible plants such as begonias and African violets. Leaf edges become brown and crisp, and flowers turn brown and die.
- Cultural control - increase air circulation, water the soil, not the leaves, make sure plant becomes as dry as that plant can be before watering, remove damaged leaves, maintain consistent temperatures (too cold can increase susceptibility). If flowers are affected, remove every flower and bud at the same time. If it is an inexpensive or short-term plant, consider throwing it out - the cure will cost more than the plant is worth.
- Chemical control - systemic fungicides registered for indoor use, such as Benomyl or Funginex, used as directed
Viruses
Viruses can cause streaks and strange patterns of discolored and abnormal growth of foliage and flowers. All viruses are not harmful - streaked tulips, variegated foliage of pothos, hoya, etc. are caused by a specific virus. Tobacco mosaic virus is a specific virus which causes yellow mottling on orchids, and cacti. Many viruses are spread by insects, so insect control is especially important if viruses are a concern.
- Cultural control - if the plant is unthrifty and degenerating, destroy the plant.
There is no cure
Black Rot Fungus
Black rot fungus causes soft, rotted areas, commonly on new growth, of orchid foliage. It spreads down into the rhizome, then upward into older leaves. Advancing edges are yellow as tissue dies.
- Cultural control - cut off infected areas, clean tools. Do not overwater - soft, succulent growth is more susceptible to infection.
- Chemical control - systemic fungicide such as Benomyl, used as directed.