Mexican Fence Post Cactus-Pachycereus marginatus
Hi, I have 3 cactus and am having some issues with one of them. I can’t tell if this is too much water, too little water, a fungus or what. The little one (2ft) has a strange coating on it. I tried to wash off but it appears to be part of the skin. I picked it off and made a scab on part of it. There’s also what looks like maybe nibble scabs towards the top. This was living outside but moved indoors about a month ago. The lady I purchased from said water twice a year. I’ve been watering once per month per online resources. I used my water meter and it’s registering wet (I watered 2 weeks ago). It was recently repotted in a mix of coarse sand and potting mix and watered at that time. The tan exterior film has been there now for at least the last 3 months. Any guidance on what this is and how to better care for them is much appreciated!
Washington County Oregon
Cacti are succulents and do best with little water. I would listen to the person from whom you purchased the plants. The online resources may be focused on AZ and the Southwest where the climate is much different from ours. There is little research for cacti in the PNW, but I found this article from Arizona https://apps.cals.arizona.edu/arboretum/taxon.aspx?id=20 You should let the soil dry out completely between waterings, and let it go completely dry during the winter for a dormant period. Other notes indicate this plant doesn't do well with humidity. The latter is more of a challenge in our climate.
What I see from your pictures does look look like fungal lesions. You can try a broad-spectrum fungicide, but that could also contribute to humidity.