Question for the Hive Mind: a Reader's Dieffenbachia

A reader e-mailed me to ask about a Dieffenbachia problem, which turned out to be something I hadn't seen before. Neither Google nor my books helped, and the few theories I came up with are mostly not borne out by the facts of the case, so I'm hoping someone out there has experience with this and can at least provide an ID, if not tell the person how to fix whatever it is.


The most alarming symptom is the weird bumpiness around the leaf midribs, often with a brown or tan slightly corky/woody top. This is reported on almost all the leaves, particularly the oldest ones, and are getting larger over time. It looks like the bumps mostly appear on the top of the leaf, but in at least one case, the reader provided a photo showing the woody/corky thing happening on the underside of the leaf too.


When I saw this initially, I thought maybe edema, or mechanical damage, but I couldn't find anything that talked about edema even happening on Dieffenbachia, much less an illustration of it, and anyway the environment would seem to be wrong for edema, since as far as I can tell, the environmental conditions are normal indoor temperatures and humidity.

My own plants have bumps like this on the midribs of some of the leaves, which I assume is mechanical damage of some kind (I've seen the same thing on my Anthurium "hookeri" and Zamioculcas zamiifolia), but in those cases, I already know the plant's been banged around a little, and the bumps are centered on the midribs and stems (particularly in spots where the leaf or stem has been bent, twisted, or stretched) and don't appear on other parts of the leaf. The reader says that the plant has not sustained any mechanical injuries while this has been happening, nor are there pets that might be causing this.




The reader also reports that the new growth is pale, yellowish, and sometimes shriveled, with margin burns on some of the leaves. (I didn't really see this in the photos, but bad-looking leaves could have been groomed off, I suppose.)

This has apparently been happening since spring, and is accelerating as it goes on.


There might be something like an insect or mite on the leaves, but it's hard to tell for sure; it could be dust or water spots. There's no webbing or stippling like one would expect from spider mites, and the specks don't look big enough to be aphids. The color and size is right for maybe an early mealybug infestation, but mealybugs don't cause leaf swelling and margin burn, in my experience. Thrips might leave streaky brown spots like this, but they wouldn't cause leaf swelling, and they're not known to be a particular problem for dieffs.

I did an on-line search for dieffenbachias with broad mites or cyclamen mites (which I at least know can cause weirdly distorted foliage and a dusty appearance), but didn't come up with anything there: if they infest dieffs, it must not happen very often. (Also, my understanding is that cyclamen/broad mite damage is mostly the result of the mites feeding on the foliage while it's still developing, not something they do to mature, developed leaves, as in this case.)

So I am stumped. Not only do I not know how to fix the problem, I don't even have any good theories about what the problem is. So I'm hoping someone else might have some theories. Even wild guesses would be better than nothing, at this point.

I asked the reader a few questions, hoping to find support for one of my theories (didn't get any), and the Q&A is below:

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Has anything fallen on the plant within the last few months, or has the plant itself been knocked over?
NO. I really am careful with them and I learned to give them time alone :)

Do you have a cat that might like to bat at the leaves, or is there anything else in the house that might pull at or twist the leaves sometimes?
NO ...and I'm a dog person :)

Is it in a spot where a lot of people passing by the plant would brush up against it?
NO, it's on a shelf in a dead-end balcony.

Have you had the plant for a long time, or is it fairly new?
Fairly new, I think this is the second year I got it as a little-thin-young stem, and the first year it grew thicker and stronger after repotting the upper-stronger part of the original plant.

Has it been exposed to any extremes in temperature?
Yes, I think so. There were some days last month when temperature approached 40C - 104F, but the problems started before that.

Does it stay inside year-round, or do you put it outside for part of the year?
Only in the closed balcony. No low temperatures in the winter.

Do you leave windows open during part of the year, and if so are the windows screened?
Yes, but not wide open. There is only glass-filtered sun, and yes I use shades when sun gets too strong (3-4 hours in the second part of the day).

How long ago did you first notice it, and has it gotten worse, better, or stayed the same since then?
This sping. At first there were only few bumps and small. Now there are present on almost every leafs (especially the old and mature ones) and seem to grow steadily.
The general aspect of the plant is worsening. Leaves wither and partially coil, the color is paler and yellowish, and almost all old leaves developed on their brims yellow-brown dead areas. (see the pic in the message before).

My impression is that there is a systemic problem that affect the whole plant. Something metabolic. Coming from an infection or infestation or the soil.
I have to mention that I used universal solid fertilizer from COMPO. One stick, once (made in Germany).

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Any of this ring any bells? Anybody?