yes.. those are the bad leaves ... can we see the whole plant ...

tell us how you water ... do you fert ...

humidity is not a variable .. nor a requisite ...

how long have you had it ... could the leaves have been damaged thru rough handling.. and are just now showing the damage... how long have you had it ...

ken

The linear damage is mechanical, like a scratch or a bruise to animals, but it's difficult to say with great certainty what's causing the larger areas of necrotic leaf tissue. It's very likely due to over-watering and/or a high level of dissolved solids (salts) in the soil solution. They often go hand-in-hand, especially if you're watering in small sips to avoid over-watering, a practice which ensures all the dissolved solids in tapwater and fertilizer solutions remain in the soil where they build up to levels that can actually reverse osmotic flow of water into cells by "pulling" water out of cells via the same mechanism curing salt "pulls" water from the cells of meat like ham and jerky. Variegated plants usually first present symptoms of over-watering and/or a high level of dissolved solids in the lightest part of the variegation.

I too would like to know how you determine when it's time to water, What type pf grow medium you're using, if you've been fertilizing regularly and with what, and whether or not you flush the soil when you water; and, the pot has a drain hole, right?

Another consideration is sunburn. The pigment chlorophyll acts as nature's sunscreen for leaves. It helps to protect the plant from the harmful UV light frequencies and overexposure to light. Since the lightest fraction of the variegation has the least amount of chlorophyll, it significantly lacks the degree of protection enjoyed by the green tissue. Has it been exposed to full sun for more than 15-30 minutes within the week prior to you noticing the damage?

It's important to see a profile image of the plant and pot to see what's revealed. I'm particularly interested in the pot size and whether you're following the advice to "water when the top inch or two of the grow medium feels dry to the touch", which is a recipe for over-watering if your pot is greater than about 5" deep.

Al

Your plant is a Ctenanthe, not a Calathea; it is C. marantifolia (usually mistakenly labeled pilosa), and the variety is 'Golden Mosaic'.

This variety is very prone to this kind of spotting. It seems to be more of a problem when the plant suffers water stress, or cold temperatures.

You will need to keep it quite moist, and feed regularly. The variety can grow very quickly in good conditions, and when it has a good number of new leaves you can trim off the damaged ones.