Ok great thank you. I am new to this about 18 mos in to a lifelong orchid journey of love, confusion, and some heartbreak. I would love to get better at feeding I just don't understand the numbers required yet. My tap water is straight up clorine and flouride. I won't even water my houseplants with it . I have a ph meter and a tds meter.

There must have been a reason for you to remove the sheath in order to see the 'rot'.

Did water get trapped in sheath and bulb surface?

It sounds like the fertiliser and mag-cal you apply is ok ---- as in the schedule.

If temperature and light requirements are good, and the nutrients/elements are good, then also focus on what's happening with the water. Does it get trapped in the sheath? It might not be the cause of the issue, but will be good to consider.

Also, is there gentle air-movement in the growing area all the time? That generally helps a lot to cut down on issues.

I have fans going 24/7. It makes sense with the water getting on this particular growth because of the way it was growing out at a 90 degree angle. The last growth that did this came from the grower and the spot was not under the sheath as this one was. I'm going to def going to be more careful when I water now that they are inside. It was only under the dried sheath I peeled off and didn't go to the base of the bulb or into the rhyzome. It probably would have if I hadn't caught it. Win win for me but no blooms and more knowledge 😁

I don't think you have a fungal nor bacterial rot problem. I think lack of calcium is more likely. Cattleyas are particularly susceptible to this, and lack of calcium causes exactly what you describe.

Systemic fungicides are only useful if you know which fungus your plant has and you also know whether that fungus is susceptible to the fungicide. Blindly using fungicides may not work. Plus, it can kill beneficial organisms that prevent worse problems.

I am asking about your tap water because I want to know how much calcium it contains. It would be cheaper than buying Cal-Mag to mix your tap water with rain to achieve the desired calcium. Chlorine and fluoride ion added to tap water are in such low amounts it doesn't harm most plants, including Cattleyas, so don't fear using it.

I mix some of my high-mineral tap water with rain, and add a little vinegar to adjust my pH. I also add a small amount of fertilizer. In this way I don't need to buy Cal-Mag and my plants get lots of calcium and magnesium.

Less expensive TDS and pH meters aren't usually that accurate. Your utility water quality report will have more accurate information.

ES could be right here. Anyway, if the orchid happens to have a serious element deficiency ---- then if its cells die as a result of it, then the death of the cells will lead to decomposing/rotting.

LMO ------- was that brown coloured region of the bulb soft and mushy?