I am having a problem with my Christmas cactus (schlumbergera). Several months ago, one branch started turning yellow from the outer edges of the leaves inward and developed blackish spots along the edges. I took the plant to a local specialist. They said the plant was suffering from a nutrient deficiency (nonspecific). She repotted the plant, and for several months it has being doing okay. The existing damage/changes were not reversed, but no further changes were observed. However, recently (over the past month), the plant has been dropping the damaged leaves, and I am now seeing a bit of yellowing begin in one of the healthy branches of the plant. I am concerned that this may be an indicant of overwatering/root rot. I don't think the black spots are bugs since they seem to be within the leaves, but I could be wrong. The plant has been getting fertilized (Jack's houseplant special) with biweekly waterings over the summer.

Has anyone seen something like this before?

@purpleinopp

Thank you for the invite, Ham. : )

I have not seen this condition before and have no idea why it looks like that. If it was my plant, I might flush the soil gently but thoroughly the next time it is dry, so that a lot of water flows out of the drain holes. Preferably with rain water, if possible.

Looks a bit like this:
https://www.e-gro.org/pdf/2015...
Nutrient toxicity caused by low (too acidic) soil pH.
Just a guess...

Wow, I looked up Jack's Classic Houseplant Special fertilizer.
Label lists it as 15-30-15 N-P-K. This is REALLY heavy on Phosphorus.
I suspect your plant may have Phosphorus toxicity. Too much phosphorus interferes with the absorption and utilization of other essential minerals. This could be causing the symptoms you're seeing.
I recommend a thoroughly flushing out the soil as Tiffany recommends above. Then switch to very light feedings of a more balanced fertilizer. NPK ratio of 3-1-2 is a lot closer to what a plant can actually use. (As recommended by Tapla, among others.)
https://www.houseplantjournal....

And the stuff tapla recommends is like 9-3-6, so those numbers are wow!

I use a 1-0-1 orchid fertilizer on my orchids and holiday cactus.

Assuming you are using a dosage calculated according to the package label (50-100ppm N), you are not really adding enough phosphorous to cause toxicity. It is important to measure nutrients in order to achieve the desired results. The label instructions seem very reasonable to me.

When in doubt with nutrient issues, be sure to always water with an excess volume whenever you feed, so that some extra water comes out the bottom and flushes any salts that may be building up in the soil. Just water with a sufficient amount (ie. reasonable excess) every time, and flushing is already built into the program.

A fertilizer labeled 9-3-6 should behave just like a fertilizer labeled 3-1-2 when it is diluted to the desired dosage (usually determined by N concentration). They are functionally the same, given the same basic ingredients in those relative proportions.

I have used fertilizer rated 7-9-5 and 24-8-16 (equivalent to 3-1-2 when diluted) for all my cacti at about the dose recommended on the Jack's package, with good results.

It could be a pH issue with the water, I suppose. What kind of water are you using? Is it tap water, ground water, purified water, or something else?

Thanks everyone.

The nutrient hypothesis is a bit hard to digest as I have another of these plants (schlumbergera) that has been watered and fertilized similarly and has not had these problems. The affected plant gets more sunlight, but I don't think that is related. The light is indirect, and the plant was placed in its current location after the problem started. Additionally, if it was tied to nutrients, then I am not sure why half the plant has been largely symptom free. I would think root or stem damage or pests would be more likely to cause localized damage that spreads over time. Regardless, I can certainly try to fertilize with a 3-1-2 (or other recommended) mix.

In general, I water the plant using well water. Our water is a bit heavy on TDS. However, again, I don't think this is the problem. The same water is used for the other unaffected plant as well as most of my other plants. Every 2 years I'll repot due to deposits in the soil. However, I had the affected plant repotted after the problems emerged, so I don't think dissolved solids aggregating in the soil are to blame.

I have a product, Monterey Complete Disease Control, that I've used to successfully treat other plants suffering from root rot due to overwatering. Again, I have no reason to believe overwatering is the culprit here, but treating with the Monterey product might be one option.

Owh, you all are using very light fertilizer over there.
In this part of the world, the standard / general NPK is 13-13-13.

And I am using 13-31-40 or 19-25-25 for my tropical fruits. But I am not discipline in applying those, only when I think it is the time for the tree to produce / increase their productivity. Once a year or two years. From time to time will give them some banana peel juice.

As for the succulents, I pre-mix the 13-13-13 + slow release 6-13-25 into the media, and almost never apply additional fertilizer. So the C&S only get fertilizer during repotting.

Well, I confirmed it was likely a mold problem. As the yellowing of leaves was progressing, I decided to take cuttings to propagate new plants. I found that the soil was absolutely rife with mold (looked like cobwebs throughout), and there was lichen growing in spots and on the roots. I've never seen lichen in houseplant soil before. Regardless, root rot appears to have been the culprit.

😮