Rust disease on opuntia

[cperreault]

Hi all,
One of my opuntia in my garden in Arizona has developed these rust color patches. Any idea what it may be? It also had cochineal bug but I've never seen them doing this kind of damage. It doesn't come off without scaring the cactus, like scale would. Any suggestion most welcome. Photo attached.

Best,
Charles

[mikethecactusguy]

I would think they are mealy bugs not rust. When the bug is disturbed it bleeds red. Hose down if you can with a water spray. Treat with rubbing alcohol. After that chemicals.

[cperreault]

Thank you Mike. The cactus did have cochineal bug that bleed red when squished, but I've never seen those bug leaving this kind of scaring. My other opuntia also have cochineal bugs but don't show these kind of rust diseased patches.

[hegar]

Hello Charles,

those are interesting looking lesions indeed. It would be interesting, to look inside those blisters, in order to find out what caused them.
While rust fungi do produce blisters, I would not agree with that diagnosis. Going through our extensive database - going all the way back to 1985 - I was only able to find one rust pathogen mentioned. That one occured on a Hylocereus sp. stem. I did try to find out more about it, but was unsuccessful. There is no image shown anywhere, only a verified diagnosis to the genus level.
There are a number of causes for reddish-brown lesions on cacti, but it would be rather rare indeed to be able to find a rust pathogen. I cannot rule out another kind of fungus though. However, with most diseases, there is also a discoloration, most of the time a bleaching or yellowish halo or a darkened, water-soaked perimeter, visible around the lesion. Your plant does not show that.
I would go with feeding damage judging by the image you posted.
Would it be possible for you to get a close-up of one of these splotches? That could perhaps give me a clue.

One thing I always ask people is: Are these lesions increasing in size and in number? Did they appear all at the same time?

Harald

[cperreault]

Here are some more photos. Thanks for the help! I knew I came to right place why signing up for this forum. I’ve search the web for so long for examples of similar disease but couldn’t find any.

To answer your question, I’m not sure how quickly they appeared. This is how the plant was when I noticed it. I would say that this wasn’t here when I planted it, which was 4 months ago.

[cperreault]

Photos were too large to post. Here is a link to them:

https://imgur.com/a/DImHgtR


Thanks for your help!
Charles

[hegar]

Hello Charles,
I did download your latest images and tried to magnify them, in order to see more features of the lesions. Unfortunately, that did not help much either, because the images became too blurred when electronically enlarged. So, a diagnosis of this problem cannot be done with any degree of correctness.
I do have a pretty good collection of various prickly pear species in my outdoor planting. However, I have not encountered any lesions on those that match what your plant(s) is (are) exhibiting. Images that I was able to find on the internet also were not good enough to see fine details of what the lesions actually look like. For example, there was a photo of feeding damage by a leaf-footed bug (Family Coreidae). The shape and color of the lesions were similar, but I did not see the blistery borders. Your plant exhibits a water-soaked margin just outside of the raised, blistery border of each lesion. That could be a sign of an edema, which is a physiological condition caused by overwatering and/or poor ventilation. Other possibilities would include two fungi, one that does cause "charcoal rot" and is caused by Diplotheca wrightii and the other which causes a condition called "scorch" or "sunscald" and is caused by the fungus Hendersonia opuntiae. To be positive with a valid identification, I would need to see the fungal fruit bodies and the spores these fungi produce. There are of course other pathogens, which do infect Opuntia spp., including the ubiquitous Colletotrichum sp. (anthracnose) and a very distinct one named Phyllosticta. However, your lesions do not match those caused by these fungi.

What are the physical characteristics of these lesions? Are they hard and dry or soft and as succulent as the rest of the plant? Do they multiply and/or grow in size. Is the color of the lesions changing, i.e. do they become darker when more mature? Do you see tiny dots, cushion-shaped raised specks on the lesions or mycelial threads on those lesions?

You could go ahead and try to apply a fungicide, but that may not be very effective. Fortunately, Opuntia spp. are generally fast growing and inexpensive plants. They are also easy to propagate by using a cutting. If all fails, you can go ahead and cut off a healthy looking part of your plant and place it either upright in sterile soil or have it lying flat in the pot. The cutting should form roots from one or more of its areoles.


Harald

[ohugal]

I've had edema on an Opuntia Ficus Indica grown indoors (indeed because of bad watering habits, poor ventilation and high humidity). In my case the patches were white and less structured than those of Charles. If you pricked one of those white patches with a needle it felt like puncturing a hardened blister. In my opinion I don't think it's that.