The symptoms of blight may be brown spots, yellow soggy leaves, rot, decay or peculiar stunted growth. Unfortunately, they are very hard to control. The usual way is to try to change the environmental conditions what favor them. Chemicals are a last resort, and often don't solve the problem to our satisfaction. Fungi Often fungus disease and rot is caused by too much water and not enough air circulation. Another common reason is that foliage remains damp too long. It takes only 8 hours of moisture for a fungus to insert it's feeding tube into leaf cells. Sometimes, if the infection hasn't gone too far, correcting these conditions will stop it and the blighted parts can be cut off. However, certain varieties of plants (Reiger begonias for instance) are very susceptible to fungus infection if conditions aren't exactly to their liking. These susceptible plants can be sprayed preventatively with fungicides but usually they have to be treated repeatedly. The reason is that fungicides do not cure what's been damaged, they can only prevent future decay. There are many, many fungicides on the market and most are specific for each of the many many fungi that exist. The law requires that each fungicide container identify which specific fungi it treats and on which plants. Identifying an exact fungus is a very difficult job and repeated applications may be necessary. Which is why most people either change a plant's location and growing conditions or eventually throw it out.
Viruses Virus infested plants can't be cured. They grow stunted, have misshapen stems, twigs or flowers and just fail to thrive no matter what you do. The leaves may have strange discolorations or streaks, often yellow. The ends may grow twiggy like a "witches broom". Such infected houseplants will rarely again become attractive specimens, so just throw them out and buy something nicer. Don't reuse the soil because it may be infected with the virus.
Precautions When Using Pesticides Always remember, substances that kill bugs can't be good to breathe in or get on our skin. Wash hands after using any pesticides, even safe ones. And, ideally spray outside. When spraying indoors, a good safety measure is to put a big plastic or paper bag over the plant with holes in the sides into which to spray. Leave it on for several hours until the spray has settled. Then air out the room because chemicals are absorbed easily by the lungs. (Keeps dried spray spots off the walls too.)