I'm a bit of a newb at growing cactus. I've had this guy for about a year now. It's grown a good amount since I got it.
When I first got it it had some white patches and had a bit of a twisted spot in the middle where the patches were. I've been mostly letting it do it's thing with some watering more often now that it's summer and occasional fertilizing with a 20-20-20 formula.
Just recently it really started to get a bend in the twisted area and I noticed it looks like the patches have spread. Is it a parasite? what should I do here?
Your plant most likely does have an infestation of spider mites and/or mealybugs. Those elongate shapes visible could be mealybugs. They do produce also a fluffy whitish mass, which does hold eggs from which the offspring (crawlers) hatch. Spider mites doe produce silk strands, like spiders. However, the webbing is finer. You could go ahead and, using a soft brush. transfer some of the material from those lesions, around the spines, areoles, etc. onto a white sheet of paper. With a decent magnifying glass (hand lens) of around 10x magnification, you should see the culprits.
Thrips also do cause damage to the epidermis of cacti. They too are small insects with an elongated body.
The bending of your plant is normal. Columnar cacti like yours will start to lean toward the direction from which they receive light. For a potted plant, you could turn the pot around, giving it a twist clockwise or counterclockwise and the plant should stop leaning as much. Also, your cactus is becoming a bit top heavy. Most likely, that is due to it having been exposed to better lighting and fertilizer treatment. Just make sure, that you only apply 1/2 or 1/3 of the dosage recommended for regular plants. Also, try to apply a fertilizer, which does have a small first value and larger middle number.
