Keeping your plants healthy is the best defense for fighting off house plant diseases. A plant that's under stress from underwatering, overwatering, too much or too little sun, chills etc is much more vulnerable to diseases than a healthy plant. Fortunately, many problems are treatable if you catch them early. But remember, prevention is always better than cure. There are a few extra things you can do beyond the basic water, light and temperature. Promptly remove any dead flowers from your plant. Don’t allow any fallen leaves to remain in the pot as fungus thrives on dead plant tissue and moist conditions. A perfect breeding ground for diseases which can also attract insects.
Like all plants, houseplants will occasionally come under attack from pests. These insects can be just as voracious as their outdoor counterparts but have the added benefit of developing and reproducing in near ideal conditions. As a result, houseplant pests can multiply very quickly, so you have to be diligent about checking for symptoms. If you notice a plant that suddenly begins to look ill, take a closer look — chances are an insect is responsible. Infestations can be very severe and plants that have had more than half of their leaves damaged are probably not worth saving. For this reason, it is important to identify and control indoor plant pests as quickly as possibl
Aphids
If your plants do suffer from Aphids use an insecticide to control them.
Black Leg
This attacks cuttings during the propagation stage in unhygienic conditions. There is no cure so the cuttings must be burned.
Leaf Spot
Examine the plant periodically for signs of disease and use the recommended fungicide as well as destroying the affected leaves. Correct application and timing are crucial.
Mealy Bug
The bugs are easy to recognize, as tiny pale lumps with a cottony casing, sitting on the leaves. These can be troublesome on some plants as they get older. Spray insecticide heavily and directly onto them. An even better remedy is to sponge the leaves with malathion.
Mites and Scales Insects
Spray the plant with a diluted horticultural oil soap.
Red Spider Mite
Red spider mites will cause discolouration. To cure, spray with malathion.
Root Failure
Due to wet and cold conditions, this can cause the plant to collapse. The same can happen in too dry conditions which can be avoided with care.
Root Rot
There are two possible causes;
One is a long period of overwatering. This can cause some of the roots to die back due to a lack of oxygen. The leaves start to turn dull and yellow while the whole plant seems to be dying. Even if the soil conditions return to normal the rot can still keep spreading.
The other is fungus in the soil. The fungus may lay dormant in the soil until over watered.
If you’re not sure whether the problem is root rot you can check by removing the plant from the soil, the roots will feel mushy and look black, some may even fall off when you touch them. Wash away as much soil as you can, being gentle with the plant, then cut away all the affected roots. You may need to cut away most of the roots and then a third to half of the leaves of the plant too, just make sure the blade is cleaned, preferably with alcohol. If possible, dip the remaining healthy roots in a fungicide solution to kill off any remaining root rot fungus. Replace all the soil in the pot after washing it out thoroughly with a bleach solution. While the plant is regrowing its roots, do not fertilize the plant, as this may stress it.
Scale Insect
To start with, they mostly trouble the reverse sides of the leaves. These should therefore be examined carefully as a matter of routine. Chemical treatment is best, but check carefully first to make sure the necessary insecticide will not damage the plant itself. Treat a small section first to see if there is any reaction. Strong leaves can be wiped with a cloth to remove the insect.
Slugs
These can be a particular problem for plants with young tender leaves. Use a recommended slug repellant around the base of the plant to eradicate them.
Symphylids
Symphylids are a cream or white insect with two antennae and 12 pairs of legs which eat roots and tubers. Young plants can be destroyed by this pest. This shouldn’t be confused with the common garden centipede which is orange and eats other insects. They can be killed, if necessary, by drenching the soil with malathion.
Sciarid Fly
These pests, better known as fungus gnats, are found in the soil if it is very wet. They are not at all harmful but can be unsightly. If you wish to get rid of them, this is easily done by soaking the soil in malathion.