How do I treat a poinsettia disease in my beloved, rapidly dying plant?

August 30, 2018 8:13 PM

I am not a gardener but I have a beloved poinsettia plant that I've managed to keep alive for almost four years now. It has caught a mysterious disease which is rapidly killing it. Would you help me confirm which disease my poinsettia has and understand the treatment?

My previously quite happy poinsettia is dying rapidly and I can't figure out which disease it has. Please hope me!

Details:
-Disease is most definitely killing the plant
-I live in an area which is both hot (average ~95F for the past four months) and very humid (64% at the moment)
-This is the first summer that poinsettia has spent outside (it's almost four)

-Pictures are on Flickr but in case they don't come through: the plant has about 50% of the leaves it had in June; infected leaves get large tan/brown spots on them and gradually they lose almost all green color/go into chlorosis, wilt, and turn yellow before dropping
-This seems to be a disease of the foliage (leaves) not the roots or branches
-Whatever it is hasn't spread to my Meyer lemon tree a few feet away and I'd like to keep it that way

My best guess is that it's Xanthomonas leaf spot but I'd appreciate confirmation

Finally, I am struggling to decipher the treatment instructions from university Ag Extensions because they're written with the farmer in mind and I can't find any treatment instructions from gardening sites--so I could really use some help understanding how to treat whatever it's got

posted by librarylis to Home & Garden (5 answers total)

I think you need to destroy your sweet plant; I'm sorry. Commercial growers do not treat bacterial/fungal infections; they destroy affected crop immediately, and treat adjacent crop. Home gardeners can use copper, sulfur, or bio-fungicides. These methods focus on prevention, not treatment. Periods of high humidity can tip the balance on these diseases. If these diseases were more easily treated, there would be a slew of effective products on the market. Don't put the plant/soil on your compost pile if you have one. Clean the pot with a bleach dilution.
posted by missmary6 at 7:50 AM on August 31, 2018 [1 favorite]

Have you tried calling your county Master Gardeners? It's a program through Ag extension offices for home growers. They may ask you to bring infected leaves in.

In the meantime, remove infected leaves and throw them away in the trash. Plants can lose a lot of leaves and survive. I'd also pot it up into a larger pot, or at least add more potting soil to the one you have and apply a fertilizer to help your plant grow new leaves.

Moving plants around is stressful for them, their leaves are optimized for the amount of light they get when they grow them (and people can't really eyeball light intensity very well). You may also be losing some leaves from the change in light intensity.
posted by momus_window at 7:52 AM on August 31, 2018

You might try applying a general disease control/fungicide. I have had some success treating some ailing outdoor plants with Dr. Earth's Fungicide Spray.

There are other fungicides on the market if you are able to diagnose better and this one isn't as chemical laden.
posted by countrymod at 8:41 AM on August 31, 2018

Response by poster: Thanks, all! I will try contacting the Master Gardeners for my area but will go into it knowing that they may not be able to help (even though I'm hopeful that a magical bacteri/fungicide exists, it seems unlikely).
posted by librarylis at 5:45 PM on September 2, 2018

Response by poster: A quick update for anyone who finds this later: the poinsettia is still hanging in there (it got better this winter and then sick again this summer). It probably has a slightly different fungus than I first guessed. The lemon tree isn’t thrilled with life (though hopefully it’s just greedy for nutrients for the baby lemons) but it’s doing its thing. I still need to contact the Master Gardeners.
posted by librarylis at 5:09 PM on August 28, 2019