Following from the previous post.

A few common houseplant diseases include:

Powdery mildew: this appears as a thin layer of powdery white fuzz on the leaves and stems, which later dry up and go brown. It likes to attack new leaves, which then grow distorted. Unusually for a fungus, this one actually takes hold when the conditions are warm and dry, but then spreads when things get more humid or moist.

Botrytis/grey mould: this looks just like the latter name says: a light, slightly fluffy mass of grey filaments growing on the surfaces of leaves, flowers, and stems. In addition to poor air circulation, it’s also encouraged by overfeeding and overwatering the plant.

Sooty mould: another one that’s what it says on the tin. It looks like patches, or a dusting, of tiny black specks. Unlike the others, this one doesn’t really affect the plant directly. In fact, it’s a symptom of something else: one of the pests I mentioned in the previous posts. Aphids, scale insects, and mealybugs suck sap from the plants and excrete a thick, sugary liquid called honeydew. This rains down on the plant parts below and creates a shiny, sticky buildup; this is what the sooty mould is growing and feeding on. Treat to get rid of the pests, then wipe off the sooty mould and honeydew with a damp cloth. Leaving it will only encourage more, and when it gets too thick it will inhibit photosynthesis. If the plant doesn’t have those pests, check the plants around it.

These three are among the most common and most cosmopolitan houseplant diseases. Individual plant species may be affected by their own particular ones; for example black spot on roses (which has happened on roses I’ve brought inside).

For people starting seedlings indoors, another fungal condition often rears its ugly head, called damping off. This is when a seedling suddenly develops a “soggy” weak spot on its stem, droops, and dies. There’s nothing you can do about it except prevent it by using clean materials, good air circulation, and perhaps a light sprinkling of sulphur or cinnamon.

Finally, I’ll talk about bacterial leaf spot, especially common on orchids. These look like little sunken chocolatey brown dots on the leaves that expand into larger spots or streaks, often with yellow haloes. I find spraying with hydrogen peroxide (from the pharmacy) helps greatly, but if it gets bad, then prune and burn.

I think that’s it for this series of posts. If it seems that I’ve repeated myself sometimes, well, I did. I’ll excuse that, however, by emphasising that any living thing is affected by the components of its environment as a whole, not as unrelated factors working in isolation. Moisture, soil, humidity, the container – these things all work in tandem to affect the plant’s health directly, and indirectly by influencing the plant’s resistance (or susceptibility) to disease and pests. Only light works independently of those factors, but it too is affected by (and affects) the plant’s surroundings.

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