Hey everyone, so I have a problem with one of my Monstera Thai plants. The upper 5 leaves (the lower leaves seem to not be affected by this) are getting yellow around the edges, and eventually, turn into crispy brown. The backside of the leaves exactly where the yellowing is, also have some markings, that I cannot pinpoint what they are. On some spots, I even see tiny orange dots?
This started happening approximately 2 months ago and at first I thought that maybe it is some sort of salts burn up so I have been trying to "wash" the soil with clean water but this did not solve the problem and for the past 2 weeks, it is spreading more and more. Does it look like nutrient deficiency? Or maybe abundance? I am so lost lol.

A bit of background of the plant: it was repotted in October last year, it is potted in a very well draining potting mix (approx equal parts bark, perlite and potting soil). I am watering it once per week on average (some times every 5 days, some times every 10 days - depending on the weather and how fast the plant gets dry). The plant did have a thrips infestation last year (it was the one that had the most damage - although the other monsteras are butt to butt close to it, it seems that the thrips had a preference for this one Monstera in particular) but has been clean of them since October and I have not spotted any new pests on it. It has 5 more monsteras next to it, all growing at the same conditions. The yellowing is affecting only this one plant. They do not leave in a greenhouse, so the humidity is not controlled, aka sometimes it drops to 30% sometimes it is in the 60%. They usually get bright indirect light but whenever the weather is too gloomy, I turn on grow lights on top of them.

See photos attached. Does anyone have any idea what this could be? The plantie is currently unrolling a new leaf and I am dreading those yellowing things appearing on it too
@Gina1960 since you have so much more experience than pretty much anyone I know, maybe you might have an idea? Thank you in advance!

What are you feeding, and how often?

Gina1960 said:What are you feeding, and how often?

Thank you so much for your reply, Gina! And thank you for your time in advance.

Usually, I am going with the following schedule as this is available in Europe (during the growing season)): 1 time per month, they get 1/4 dose of https://www.amazon.de/BioBizz-..., then they get a 1/4 dose of https://www.amazon.de/BioBizz-..., then they get clean water with 5tsp 3% Hydrogen Peroxide per cup as I read that it gets extra oxygen to the roots and prevents root rot, afterwards they get 1/2 https://www.amazon.de/BioBizz-....

Whenever I add something to their water, I always try to adjust the PH to be between 5.8-6.2.

I am attaching photos of the contents of each. The company (BioBizz) claims all is gentle on the plants as it is created from natural ingredients, hence, why I opted for them. They are also quite pricey.

The first one:


The second one:


The Fish stuff (two photos as with one I was not able to catch fully the ingredients and their concentration):





Also, last year (October) when he had thrips and after I repotted the plantie, I gave him these pesticide sticks which supposedly are 2-1: fertiliser and a systematic acetampirid and were supposed to work for 3 months, of course, during that time, the plant did not get any extra fertilisation but only water and a 1/4 of the first juice (with the micro nutrients that supposedly is for the immune system of plants) approximately 1 time per month as I did not want to overfertilise him (his name is Sparky btw): https://www.amazon.de/Celaflor... Here is a photo of what these sticks contain:
The rest of the Monsteras around him did NOT get these sticks as the damage from thrips was the worst on this plant. However, some other plants (epipremnum) got the same sticks and they are not exhibiting the same yellowing on their leaves.

Sparky is really the only one that is having this exact problem at the moment and I have no clue what to do with him or what to change so that he would feel better.

If it is relevent, I forgot to mentioned that the plant and all the ones surrounding him are in a clay (unglased) pots as to allow their roots to breathe.

Gina1960 said:What are you feeding, and how often?

I was just reading and going through possible deficiencies. Does this look maybe like Potassium deficiency? Because of the orange dots at the edges on some places. It would also explain why it started increasing rapidly as the temperatures here have started increasing as by what I read, evaporation is reduced in the case of Potassium deficiency and the temperature at the edges of the leaves increases and cells rupture.

It is a bummer how I was not able to find how nutrient deficiencies look on a Monstera so I am still unsure