Need help from some cacti experts please!

Hi all,

Need some help from some experts with identification and disease. Earlier this year I moved to a new house that has 3 big beautiful columnar cacti. They are growing in rocky well drained soil, in a subtropical climate. Back in February they bloomed, at night time the beautiful flowers would open. They lasted a couple of weeks before the flowers dropped, and they did not fruit. I'd love some help to figure out what type of cactus they are, and also 2 of them have are sick and would love some help on what to do to help them out. They are so big (3m - 4m in parts) and beautiful and I'd really like to save them.

Hmmm looks like a Cereus species to me. Have they produced any fruits?

Sometimes it is best to cut off the badly damaged columns and let them grow back. With really large cacti like these ones, wood boring insects can get into the stems. Not saying it is but something to consider. Also a regular spray with some confidor and a fungicide will go a long way towards keeping the parasites at bay.

The second photo looks like there is some lichens and moss growing on it. This is a common problem in cool wet climates. Many of the big ones on the upper Atherton Tablelands have this issue. You can try various sprays or just a good scrub with a 'softish' dish brush. I'm no expert but I hope this helps.

It does look like a Cereus. When it flowers again you could key it out to species. Does the flower tube fall off cleanly leaving the ovary and style? If so, then look into Cereus.

With those damaged columns I'd give them a tug and see how strong they are to make sure it is safe to be around them. With the discoloured ones you should have a close look at them (or upload a close-up picture!) to see what's going on with them before taking any action. If it is just lichen or moss then they aren't a direct problem to the plant, but if it is a scale infestation then something has to be done about it. There also seem to be a lot of spider nests in there (not directly a problem either but when they leave the nests will become traps for debris and turn into homes for other things).

That Syngonium (I think) in the last picture might get annoying later when it decides to climb.

Possibly Cereus Hildmannianus http://cactiguide.com/cactus/?genus=Cereus&species=hildmannianus

For the sick/diseased specimens, I would take a cutting from an unaffected limb as a precautionary measure. Some pests this may not help with but given good conditions many of the cereus are considerably fast growing and it is good to keep the plant going if the original doesn't recover.

Thanks everyone for your replies.

@Sneaky Cuttlefish - no they are yet to product any fruits, I have been watching closely for 6 months, and thought it was meant to happen after flowering but we got nothing. Thanks very much for your advice, I will give them a spray and a brush and see how it goes.

@Cereoid - I'm not sure what an ovary and style are, but I can tell you that yes the flowers dropped cleanly, leaving behind a small dark round bit with a stringy bit hanging out. Hehe is that an ovary and style? Thanks so much for your advice and input, I will take a close up over the weekend and will post a photo of the green/yellow patches, would love to get your thoughts. I will also give a tug to the badly damaged columns - thanks! Good thinking...

@Supergirl - thanks for your link and for the advice, I wasn't sure how quick they grow but would like to do what I can to save them. I will give your suggestion a go. I get the impression that cacti can be difficult to ID properly so I really appreciate all input.

Does anyone know how frequently ones like this tend to flower? Also how quickly they grow?

I had a cutting of one very similar to this. It grew over a meter in a single season. As for flowering there are two huge tree sized ones up here and they flower two or three times a year. This will vary from species to species and climate conditions.