I'm quite new to both bonsai and hydroponics, but I've been keeping a couple Ficus, Willow, and Serissa alive for a few months in an ebb and flow setup using lava rock gravel as the medium and supplemental fluorescent lighting.
I change the water about once a month, but recently I've noticed a problem. A fine white fluffy mold has started growing on some of the roots and leaves. This is really perplexing me, because I spray each plant with a fungicide every few weeks. The specific brand I'm using is "Fungicide 3" by Garden Safe, a pesticide/fungicide mix. I lost my first bonsai, a juniper (not using hydroponics), to a combination of spider mites and insufficient light, so I've been keeping a much closer eye on my current bonsai.
So far, the plants aren't show any signs of distress, but I've only just noticed the problem. How concerned should I be about the mold? If it's harmful, what would be the best way to get rid of it and stop it from returning?
I don't think you have mold, sounds like mealy bug. Your spray probably won't get rid of it. Here's a link and some info:
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of the most highly visible and familiar pests of hydroponics plants is mealy bug. Growers who have not come across these pests before often mistake the first signs of these pestsÑa white, waxy or floury looking depositÑas some sort of disease rather than an insect pest. While this white cottony growth does look similar to a fungal growth, it is in fact the waxy covering of the mealy bug. The adult female is about 3 mm long, oval and wingless, with short antennae and a fringe of filaments around the body. One of the most common mealy bug species found in hydroponic systems is the long tailed mealy bug, which has two very long filaments at the tail end of the insect. Underneath the white waxy covering the insect is usually yellow or grey and may have a darker strip running down the middle of the back.
Most mealy bugs lay eggs, up to 600 at a time, in a loose cottony waxy deposit on the undersides of the leaves. As the young mealy bugs settle down to feed they begin to exude the white waxy material that soon forms a covering over the whole body.
Some common types infesting hydroponic systems can produce live young, and populations can build up rapidly. Each female long-tailed mealy bug, for example, can give birth to as many as 200 crawlers, with as many as 3-4 generations per year, depending on climate. Under cooler winter conditions, the insects tend to retreat under bark, into crevices and other places to hide until temperatures warm up.
These insects have piercing and sucking mouth parts which they use to suck sap from the plant. A heavy infestation can suck a plant dry, stopping growth and yellowing foliage, leading to plant death.
Mealy bugs feeding will often disfigure foliage and fruit. These insects also excrete honeydew, which forms a sticky deposit on plant leaves. Sooty mold may eventually grow on the honeydew, resulting in a black tacky mess that inhibits plant photosynthesis.
The best method of control is regular inspection of plants and taking action as soon as mealy bugs are detected. Mealy bugs are difficult to control with conventional sprays because the adult is protected by a water repellent waxy covering. In addition, they tend to hide in leaf axils, buds and other places where sprays don't penetrate for adequate control. For an individual plant, its often recommended that mealy bugs be brushed with a solution of alcohol or methylated spirits, which dehydrates the pests. However, this can be extremely time consuming and often doesn't provide adequate control.
Thanks, that's good to know, and I'll keep an eye out for those. However, I'm pretty sure it's not a mealy bug infestation. While the mold is cottony-looking, I'm not seeing any "waxy" or "floury" deposits. There's also not really anything directly on the leaves, only on the roots right near the lava rock and a few low-hanging leaves, where it stays moist. And at 3mm long, it sounds like this bug would be pretty easy to spot (unlike spider mites), but I don't see any on close inspection.
Also, the main reason why I'm convinced it's actually a fungus is that the lava rock itself is speckled with bright white dots of mold.
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