Hi Mike,
I really don't know much about Monsteras. I look after the garden next door and they have two very healthy plants (one bearing fruit this year). This one is under shade cloth and seems to do better than the one which is exposed to the elements more.
An article I read suggests that black marks indicate they have been exposed to temperatures which are two low. Brown marks are usually from sunburn and pale marks are from not enough fertiliser or a ph which is too high or low, which is stopping the plant from taking up the nutrients properly.
From the photo, I got the impression it was not just colour but that the surface was affected slightly. Is there any way they could have been damaged before they actually opened up?
I hope you find the cause of your problem...good luck!
Cheers,
Dee.

Hi Dee

Thanks for your reply.

I have been growing monstera for several years, both from cuttings and from seed. In the tropics they need some shade, or a permanently overcast climate, to prevent sunburn -- that causes a general yellowing.

I see the mottled condition repeatedly, sometimes more pronounced and widespread than at other times. There is some indentation in the leaves. I have thought that drought during a leaf juvenile phase might cause the indentation and yellowing. Also I have thought that herbicide use might contribute. Soil pH is an interesting question. These plants do GREAT under ironwood trees (Casuarina equisetifolia) which cover the ground with needles, and basically NOTHING else grows, no weeds, just a few ferns. I assume high acidity, though I have not tested. We get average 130" annual rainfall, so one assumes acidity everywhere without treatment.

Mike

Hi Mike,
You are right, the casuarina needles do lower the ph levels. It might be worth testing the soil, although it would seem strange that you hadn't had the problem before if this was the cause.
The other thing I was going to suggest was posting in the Tropical Plants forum on this site. There seem to be a number of very keen growers there.
Cheers,
Dee.

Hello Mike,
since you are a commercial grower, I wonder if you have a Dep't of Primary Industries (or similar) in Hawaii that might be able to help culture some samples for you to determine if its a viral problem?
You might also try posting in the US Gardenweb Aroids forum as I think there could be some knowledgeable folk there.

Here is a link that might be useful: Aroids forum

Mike, I tend to lean toward it being a viral problem, simply because I have 2 plants, grown from cuttings of the same plant, both potted at the same time and in the same mix, kept side-by side in the same location.

One plant is showing similar symptoms to yours, and I had been wondering about it, so was pleased to see your post. In light of your post I might perhaps bin it.