Sick Cactus - Black, semi-soft spots forming on top
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CactusSCHilary
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Sick Cactus - Black, semi-soft spots forming on top
I bought this cactus, affectionately called Herb, on my honeymoon in Nevada. I have had him for about a year and half. I re-potted him into a larger pot shortly after the purchase. I give cacti/succulent food in the soil (not on the plant) seasonally. It sits in a sunny window where other cacti are thriving. However, after this past winter these black spots formed on the top; they are semi-soft and some are a little sunken-in as if collapsing. They are not present on any other portion of the plant. Any ideas what this may be or tips for treating? I should note that I'm not sure of the exact type of cactus this is.
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Re: Sick Cactus - Black, semi-soft spots forming on top
Your cactus is most likely a Saguaro (Carnegiea gigantea) plant. If you spent a good amount of money on it, then the identification is probably correct. 
Now to the blackish streaks above and below the areoles on the ribs of your plant. I do not think, that you do have to worry about a disease, insect, or other problem. It does look like a physiological response of your plant to the environment. Are the ribs with the dark streaks facing toward the window or away from it? They seem to be present only on one side of your plant. Making a slight adjustment or turning the pot every few weeks should make "Herb" happy, I hope.
I do worry a little though about the growing medium your cactus was planted in. It does look a bit too rich to me. If it does contain too much organic matter (compost, humus), your plant is more likely to go into the dreaded problem of root rot. However, whatever you are doing seems to be fine, because your plant does look healthy. You obviously have done a good job, keeping the cactus alive for one and a half years.
Harald
Now to the blackish streaks above and below the areoles on the ribs of your plant. I do not think, that you do have to worry about a disease, insect, or other problem. It does look like a physiological response of your plant to the environment. Are the ribs with the dark streaks facing toward the window or away from it? They seem to be present only on one side of your plant. Making a slight adjustment or turning the pot every few weeks should make "Herb" happy, I hope.
I do worry a little though about the growing medium your cactus was planted in. It does look a bit too rich to me. If it does contain too much organic matter (compost, humus), your plant is more likely to go into the dreaded problem of root rot. However, whatever you are doing seems to be fine, because your plant does look healthy. You obviously have done a good job, keeping the cactus alive for one and a half years.
Harald