"There are no houseplants in nature" is a quote used by a former colleague when talking about interior plant care. Over the years, horticulturalists have selected certain species of plants that are able to withstand the austere conditions characteristic of indoor settings and designated them as "houseplants." Since the winter months force most of us to conduct our plant-growing activities indoors, January is a good time to review some of the problems houseplants can develop.
Occasionally, leaves of seemingly healthy houseplants begin to develop brown tips and margins, dead spots and yellowing. A common assumption is that a disease caused by a pathogen such as a fungus or bacterium might have infested the symptomatic plant. More often, however, these symptoms are the result of an unfavorable environment. Low relative humidity, insufficient light, over or under watering, or too much or too little fertilizer can lead to leaf damage that mimics disease symptoms. To make the problem even more difficult to diagnose, leaf problems often are due to a combination of these environmental factors.
How, then, can one determine the actual cause of leaf damage when there are so many possibilities? While an accurate diagnosis of a fungal or bacterial disease infestation is best made by a trained plant pathologist, there are a number of characteristics that can give any avid indoor gardener a clue to the possible cause.
First, we need to differentiate between plant problems that are biotic in nature (diseases) versus those that are abiotic (disorders). A disease can be transferred from a plant showing symptoms to a previously healthy plant. In time, the healthy plant will begin to show the same symptoms of the disease. Disorders, on the other hand, cannot be spread from one plant to another. Their symptoms are caused by non-living factors, most often environmental in nature.
If the plant showing symptoms has been indoors for several months (or years), the appearance of leaf damage is more likely the result in a change of environmental conditions rather than the development of a disease. In most indoor situations, diseases do not develop on leaves of established plants, since high humidity, wind or splashing water are not present. The former are conditions favorable for fungal disease infestation and are rarely encountered in the average home. Therefore, if leaf diseases actually exist, they may have been present before the plant was brought into the home either from a retail outlet or from placing the plant outdoors for the summer.
The pattern of leaf damage also can provide clues relative to the cause. In the case of damage caused by a disease, the progression of symptoms often transitions from healthy (green) tissue, to chlorotic (yellow) and, finally, to necrotic (brown) tissue. Additionally, when true diseases attack leaves, the dead or dying spots often are scattered on the leaf and occur closer to the growing medium or in the center of the plant where relative humidity is higher or where water got on leaves when the plant was watered.
In the case of damage caused by abiotic factors, the yellow transition zone between healthy and dead tissue is greatly reduced or absent. Additionally, necrotic tissue often appears on leaf margins and tips.
Among those factors that lead to abiotic disorders in houseplants, excess fertilizer ranks high on the list. "If a little is good, more is better" certainly does not apply to fertilizing plants, especially if they are growing indoors. During the winter months when days are short and light is poor, most houseplants make little, if any, new growth. Attempting to encourage new growth by applying fertilizer is counterproductive.
Excess fertilizer salts applied to the growing medium are taken up by the plant, where they tend to accumulate along leaf margins and tips. This causes salt "burn" which can lead to marginal or tip necrosis. If fertilizer excesses are very high, root damage also may result which tends to worsen the problem.
The afore-mentioned symptoms also can appear (and for the same reason) if plants are watered with softened water. Most water softeners replace relatively insoluble salts (e.g. calcium carbonate) with highly soluble salts such as sodium or potassium chloride. This results in creating the same saline conditions in the root environment as applying too much fertilizer.
Another common abiotic cause of leaf edge and tip necrosis of many popular houseplants is fluoride in the water. The dracaenas as well as most members of the Lilaceae plant family are especially sensitive to the element fluorine. Upon being taken up by the plant, this toxic element is translocated to the edges and tips of it leaves where it soon reaches a lethal concentration, resulting in the death of the leaf tissue.
Additionally, temperatures too low for the houseplant species in question as well as excessively dry soil for extended periods of time also can cause leaf edge and tip necrosis.
Given the cause of leaf damage has been determined to be abiotic, the solution most often involves improvement of the environment. If the cause cannot be determined, then attempting to provide a more favorable environment can be helpful. This involves more uniform watering, better light, higher humidity and fertilizing properly.
Alternatively, leaf damage caused by disease organisms can be managed by removing symptomatic leaves, increasing air circulation around the plant and attempting to keep foliage dry when watering. Labeled fungicides can be used if the identity of the disease organism is known. Extra care should be taken when pesticides are applied to plants in the home. Should the use of fungicides be deemed necessary, it is advisable take the plant(s) outdoors (weather permitting) or into a garage or other structure away from living quarters when application is made. The REI listed on the pesticide label should indicate when it is safe to take the plants back into the home.
Even with the best conditions, recover of damaged plants may be slow. If the damage was severe, it may require the longer days and improved growing conditions of spring and summer to bring a plant back to its past state of attractiveness.
REVISED: February 21, 2017