What is Aspergillus alliaceus?
Growing cactus, whether in a pot or in the landscape, can seriously challenge a gardener’s wit and skills. They’re so different from most ornamental plants as to almost be a different creature entirely, yet there are a number of features that cactus share with other landscape choices. For example, they still get sick from much the same types of disease. Cactus stem and branch rot, for instance, is caused by a species of an already familiar fungal pathogen: Aspergillus, though the species particular to this cactus problem is alliaceus. Aspergillus alliaceus is fungus that has been a problem for ornamental cactus for a long time. Papers as far back as 1933 describe the pathogen, when it was fingered in a widespread infection of cacti including:
Acanthocereus Ancistrocactus Echinocereus Echinocactus Epithelantha Mammillaria Opuntia
In plant books, it’s more commonly known as stem and branch rot on cactus or pad decay, depending on the cactus type. Either way, it means sick plants that may very quickly collapse if left untreated. It can appear as small, depressed, irregular blue-black spots which can grow together to create large, water-soaked areas on the surface of cactus plants. Sometimes, however, it simply looks like part of a pad has been badly damaged, with a portion missing and the rest seemingly unaffected. Within a few days, you’ll know it’s Aspergillus alliaceus by the white to yellow fuzzy growth and large black, seed-like spore casings.
Treating Stem and Branch Rot
There’s no specific management suggested for stem and branch rot in cactus, but because Aspergillus is sensitive to fungicide, cutting out the affected parts (and into the healthy tissue), then spraying it with a fungicide may be helpful to stop the spread. However, be very careful when doing this because it’s easy to spread the fungus to other plants this way. A bleach wash can kill spores on tools, but if you drip infected fluids onto nearby plants, you may find yourself performing surgery yet again. In general, cutting out damaged parts of cactus tends to result in badly scarred or strange looking specimens, but sometimes that doesn’t matter, like when you’re preserving an uncommon cultivar. When practical, it’s probably best to simply dispose of the infected plant and purchase a new one, but you can also attempt to start a new cactus from a pathogen-free section of the old one. Cactus pieces tend to root fairly readily, though it can take a long time for any significant growth to occur. Protective fungicide treatments may help thwart future outbreaks of Aspergillus.
