This is the pest that I believe plagued my first A. utahensis v. eborispina. I noticed the rings of spores forming near the base of the older leaves on the plant, following by increasingly rapid leaf retirement & eventual progression of the infection through the core. The ringed spores actually weren't obvious at first - it almost looked like the plant had sunburn near the bases of the leaves. Then the rings would appear after the leaves were completely done for. Below is a quote from the original description of Colletotrichum in A. utahensis by the Missouri Botanical Gardens:
.In a short time many plants were found dying from a disease which first attacked the older leaves... The disease often has a striking appearance. The conidia of the fungus apparently germinate on the surface of the host and gain entrance through either the stomata or wounds. The tissues are penetrated by the mycelium in all directions, forming either a circular or elliptical spot which at first is darker in color than the adjacent tissues changing as the tissues die to either a brown or gray color. The adjacent tissues of the host under moist conditions turn brown and rot rapidly, the mycelium penetrating to all parts of the leaf
I recently acquired a larger A. utahensis v. eborispina with a small pup. I first noticed that the pup did not seem to be growing. The leaves of the core looked disproportionately small compared to the rest of the plant, but nothing else was amiss. Then I noticed the bleached, "sunburn" appearance near the base of some of the central leaves. In another few days, I noticed a small amount of brown liquid in the crown of the plant. It was on the top shelf and we had had no rain recently, so I figured something was up. The fluid was turbid & viscous when removed by a paper towel. Over the next day or so, there was obvious circumferential rot in the crown of the plant. I was able to remove the central leaves without any resistance whatsoever, and the entire core was mushy and rotted. The remaining leaves retired rapidly and the rings appeared (photo attached). I treated the plant with sulfur powder. I did not remove the pup because I wanted to disrupt the plant/spread spores as little as possible, as the mother plant appears to be doing very well without signs of infection. Also, a bulk of the infection appeared to be flush with the soil in the remaining crown of the plant and I fear doing more harm than good. So I dumped about a centimeter layer of sulfur powder in there and called it a day.
With some investigation, I identified the culprit as Colletotrichum anthracnose thanks to this tell-tale photo from the University of Arizona (pdf attached). Even more interesting, it sounds like this particular type of fungus primarily affects plants in cultivation & is particularly virulent in A. utahensis, as described by the Missouri Botanical Gardens (pdf attached).
The plants of the species A. Utahensis were the first to be diseased, and it is thought that the fungus was introduced into the greenhouse with this species. After a short time the disease spread to other species, none of which suffered so much as the original host.
This does appear to be fairly rampant among the A. utahensis I've seen in the nursery. I've only seen it in eborispina. The kaibabensis and nevadensis I have appear as yet unaffected.
Fortunately, the mother plant continues to have active growth without leaf retirement & no signs of infection. However I am very nervous & to be safe am planning on purchasing a copper fungicide ("Bordeaux Solution") which is what the authors recommend. Open to any suggestions ya'll might have. I know I probably need to remove the pup, but I'm just so scared to accidentally wound the mother plant or somehow promote the spread of infection. Maybe I'll remove it after treating?
A Disease of Cultivated Agaves due to Colletotrichum - Missouri Botanical Gardens
Problems and Pests of Agave, Aloe, Cactus, and Yucca - University of Arizona