Huh, okay. I will give the pot a good flushing out and see if it stops developing the marks, in case it's due to built-up salts. I have several other spider plants - none of them variegated - and none of them show the same discoloration; they're all getting the same water, but they've all got newer soil compared to this one.

As it looks, it doesn't seem to be a big problem. But flushing the soil sounds like a good Idea. I reccomend waiting for someone else to throw their 2 cents in

It could be caused by a deficiency or mechanical. Spiders are very heavy feeders. Check my post on keeping plants healthy during winter, it lists symptoms and what to do.

Thanks. It does sound like it could potentially be one or two of the listed deficiencies, and since I haven't fertilized that plant ever, that's not impossible. However, since it's also winter, would you recommend leaving off feeding the plant until it's likely to be more active with foliage? Or would small amounts be okay to help correct the likely deficiencies? I'm not sure if this plant is actively growing right now or not; several of my houseplants decided in the last 2-3 weeks to put out new growth, and I'm perplexed - I guess they didn't get the "winter is for dormancy" memo.

It IS a cultural issue and is most likely related to over-watering or a high level of dissolved solids (mineral salts from tapwater and/or fertilizers) in the soil solution. It's also possible that it's a reaction to the presence of fluoride, a chemical the effects of which accumulate at the same rate fluoride accumulates in leaf tissues. Fluoride generally takes the rap for issues that should be attributed to the first two possibilities listed.

While you can't fix the damaged leaves, you can help prevent subsequently forming leaves from displaying the same symptoms. The KEY lies in your choosing or making a soil that can be water correctly. If you can't water your plant to beyond saturation w/o your soil remaining soggy so long it severely affects the plants ability to take up water, and in doing so causes the symptoms your plant exhibits, you'll be constantly battling your chosen soil for control of the plant's vitality AND appearance. Also key is fertilizing frequently but at low doses. Spider plants don't like high EC/TDS (fertility levels) and let you know it in no uncertain terms - by displaying the symptoms your plant exhibits.

In the end, you want to employ strategies that don't limit the plant's ability to efficiently move water from roots to parts most distal, which would be leaf tips and margins.

BTW - it's extremely common for variegated plants to first display symptoms of unfavorable cultural conditions in tissues largely absent of chlorophyll - the white/light part.

Following, you'll find something I posted to a thread a while back and saved because the issue you're asking about comes up with such regularity:

Some info from something I
posted previously:

While necrotic leaf tips or
margins can occur in this plant from over/under-watering, in fact, it's much
more common for the actual cause to be a high level of soluble salts in soils.
It's also commonly reported that this plant is particularly intolerant or
fluoride, but it's still more common for the cause of leaf burn to be a high
level of solubles, to which fluoride can be a contributor, than it is to be
fluoride itself. WHEN there is a high level of salts in the soil, low humidity
can be a contributor, but low humidity alone rarely presents an issue, it must
be in combination with a high level of soluble salts in the soil or either
over/under-watering.

Of course, you cannot correct
the already burned tips (they won't 'heal'), but you can take steps to keep it
from happening:

A) Most important is to use a
soil that drains very freely. This allows you to water copiously, flushing the
accumulating salts from the soil each time you water.

B) Fertilize frequently when
the plant is growing well, but at low doses - perhaps 1/4 the recommended
strength. This, in combination with the favorable watering habit described
above, will keep soluble salts levels low, and keep levels from rising due to
the accumulative effect we always see when we are forced to water in sips when
plants are in water-retentive soils.

C) When watering, using
rainwater, snow melt, water from your dehumidifiers, or distilled water also
eliminates the soluble salts in your tap water and will go a long way toward
eliminating or minimizing leaf burn.

D) If you make your own soils
and use perlite, be sure the perlite is rinsed thoroughly, which removes most
of the fluorides associated with it's use.

E) Allowing water to rest
overnight doesn't/won’t do anything in the way of helping reduce the amount of
fluoride (the compounds are not volatile), and it only helps with chlorine in a
very few cases, depending on what method of
chlorination was used to treat your tap water; but nearly all municipalities
are currently using chlorination compounds that are entirely nonvolatile, which
means they won’t dissipate into the air.

Al

That is fantastic information, thanks! On my plans for next spring/summer are to do a big repotting of plants - most of them have been in the same potting medium for a couple years, for one, and I definitely want to get them into something better-draining than I have in the past, for another.

Just as Al stated, variegated leaves of spider plants are more prone to the effects of fluoride and/or chlorine in your tap water. As you've probably noticed, only the white portions of the leaves are getting the brown spots. That's a good indicator of the problem being the tap water. I was having similar problems with my Hawaiian spiders, several years ago. Once I stopped using tap water, and only used rainwater that I've collected, the problem almost immediately went away.

I doubt it’s fluoride in this case – the city doesn’t add it
to the municipal water, and whatever levels are in it naturally, it’s too low for
dental health. (There was a BIG political deal about this a couple years ago,
and I can’t believe I
forgot that until now.)

I did a little more digging into what the city DOES add to
the water, and what it naturally has in it, and it could be the
chlorine/chloramine (the water gets treated with chlorine and then ammonia, so
we end up with stuff what won’t just evaporate from the water. Bah.). Or maybe sodium
build-up? Our water is also naturally very soft (so why am I still getting
white mineral residue built up on things??) which I suppose might also
contribute to problems if the plants need more calcium or magnesium than what’s
possibly in the potting mix and/or make the sodium more of a problem.