How can I tell if I have introduced a fungus to my healthy mature Euphorbia cactus before it rots?

Asked January 26, 2017, 8:12 AM EST

Hi! Asking because I can't find a solution anywhere online. I have a 55+ year old (or what I at least believe to be) candelabra/milk tree/ euphorbia cactus that despite a recent move appears to be nice and healthy and growing normally. A few smaller arms were damaged in the move, so I trimmed those and attempted to root a few cuttings in the base of the mother plant to fill in some empty areas from growth. As backups, I used extra pots to root more than I needed.
None of the cuttings in the mother pot took, but I think it's because it was still too soon after its bi-yearly watering (Nov) and the soil wasn't dry enough. I transferred a few of the spare cuttings to the mom pot, but they rotted as well. At the time, I had thought that any rot in the cuttings was confined only to that cutting, and not that the fungus could potentially spread via potting medium and non-sterilized tools.
My main question: Now that I've realized that I may have introduced a fungus into the potting medium of my healthy mother plant, can I prevent any further damage by treating the soil with a systemic fungicide or should that not be used as a preventative? Will that potentially be more dangerous for the plant? It would be so awful to see something happen to this old plant that I could have prevented. I bought PlantDoc Systemic Fungicide Pt but have not used it yet. Is there any way to tell if the soil has been compromised without waiting it out?
I'm also curious if I should use this kind of fungicide on the (now healed) wounds where I originally took the cuttings with a non-sterilized saw.

New York houseplants fungus

5 Responses

I don't know exactly what procedure you took when trying to root the cuttings. Generally speaking, when rooting succulents, particularly ones that have a milky sap like these, the wounded area where they were removed from the parent plant should be allowed to dry out and callus for about a week before they are stuck in the rooting media. If not, they will tend to rot because there is this opening to the inside of the stem. The fungi or bacteria that caused it may not be injurious to the parent plant since it doesn't have open wounds, entry points for the disease to get in.

The fungicide you mentions essentially contains potassium phosphate as the active ingredient, not really a fungicide that I am aware of so I do not know how effective it will really be against any kind of pathogen. Its up to you if you want to apply it to your plant, but I don't think it will do any good, despite what the label says.

One other word of advice. Be VERY CAREFUL of the sap of this plant. It can be especially dangerous if it gets in your eyes. That happened to me and was extremely painful. My eye Dr. said it happened to another of his patients. It melted his cornea. I got rid of my plant after that.

Good luck.

I did allow the cuttings to callus over for two weeks, at least- I don't remember for sure now. I know about the prooerties of the milk/ sap, and am always careful to protect myself and clean spills, but thanks for mentioning. My question hasn't really been answered though. Do you think it is possible that cross contamination of the soil could have occurred in this instance, and if I may have a potential problem on my hands, what do you suggest? If you haven't used this fungicide, do you prefer another type or brand? Can you explain why? What do you suggest I do?

It is possible that the potting soil has some pathogens in it but I think they were already there before you stuck the cuttings into it. Fungal spores and bacteria are everywhere and can easily get into potting soil. If the saw you used had been previously used to cut through diseased material, then that could be a source of transmitting the disease but more likely it was transmitted to the cutting itself, not necessarily to the potting soil.

There are more powerful and proven systemic fungicides but most are not available to home gardeners. If you feel you want to use the one you have, go ahead but follow the application instructions. They are usually mixed with water and used as a drench to water the plant. Since you are only watering 2x per year, this should not create a suitable environment for a pathogen to proliferate in. People that over-water their plants have the problems with root rot disease pathogens.

I definitely wouldn't want to drench the cactus again for a while since the last watering was in November. I'm a little confused about the fungicide as you said the type I have wouldn't work, but anything stronger wouldn't be available for me to purchase? It sounds like you don't think I need to repot? I guess there isn't a way to test the health of the soil?

It's possible to look for things like Pythium or Rhizoctonia but there is almost always some of these organisms in the soil but they do not become a problem unless the conditions are right, and when it comes to disease pathogens, that usually means moist/wet conditions combined with a suitable host. I think as long as you keep the plant dry like you have been it should be OK.