Following from the previous post.
Moving on to diseases. I’ll be blunt: plant diseases, especially indoors, can be difficult to control and even harder to eradicate. Prevention is definitely better than cure here.
Most common winter houseplant diseases are caused by fungi. Fungi are generally encouraged by still, moist air, so good circulation goes a long way toward keeping your plants healthy. As I said in a previous post about humidity, maintaining good humidity is a balance: too dry and the plant suffers; too moist and you risk fungi taking hold.
I don’t want to give the impression that fungi in general are bad. In fact the vast majority of them are beneficial or inconsequential. It’s just that we notice the bad ones more and take the good ones for granted. The same goes for bacteria: it’s not about how much are present so much as which ones. If you constantly use antibacterial products on everything in the house, you’re actually doing yourself a bad turn in the long run. (As a biologist, I have quite a long rant about this, but I’ve strayed from the topic at hand…)
You can improve air circulation by not crowding your plants together. I know (from experience) that if you have a lot of houseplants and limited space, the temptation is to jam as many into the window or under the grow-light as can fit. Well, you have to do what you have to do. Individual plants (depending on the general growth habit of the species) can be pruned to have a more open, airy shape, and this also benefits air circulation.
In addition to good air circulation, fungi can be controlled by spraying with baking soda and water or dusting with powdered sulphur. Baking soda, however, is a prophylactic: it helps prevent fungi from taking hold, but won’t do anything to cure the problem. Sulphur and garlic spray can both help to control fungi once they’ve shown up, but even these aren’t always reliable, especially if the problem is bad. I’ve also heard good things about cinnamon. However, spraying and dusting are not always practical in the house.
Often, the best way to get rid of a disease is to prune out the affected parts and burn them. Do not put the prunings into your compost bin/heap; if burning is not practical, then put them in your municipal green waste pickup and let the city deal with it. Such facilities can kill disease spores; backyard composters and worm bins usually don’t. The same goes for anything that was affected by disease and fell off the plant. With houseplants, I would recommend pruning and prevention rather than dusting or spraying.
Use clean, sharp tools to prune (standard advice), but make sure to sterilise the blade(s) after every cut, and I mean every cut. Dip the blades in rubbing alcohol and then pass them through a flame. A bleach solution works too, but it needs to soak for fifteen minutes to sterilise the blades, so…that’s kinda boring. Leave that for when you’re finished. Don’t just prune out the bits that look diseased; prune well into healthy growth to maximise the likelihood of removing affected tissue.