Orchid Culture - Questions & Answers from This Month
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by Sue Bottom, from the St. Augustine Orchid Society Newsletter.
Email us with any orchid question. If we can't answer it, we'll find someone who can! Send photographs too!
Blackening New Growth
Q. I've noticed that new growths on some of my cattleya species have been turning black. Do you think this may be caused my fertilizer burn?
A. If you are growing outside or watering from overhead, it's possible that water is pocketing in the new growths above the papery sheath that covers the new growth, and then a bacterial infection destroys the new growth. The problem isn't with wetting the leaves, it's that a pocket can form between the emerging pseudobulb and the protective sheath, and water can easily be trapped in there. I either score the sheath or gently pull down the sheath to remove the pocket.
Sunburn and Bacterial Infection
Q. We had a blustery day and my shade umbrella fell down. I forgot to put it up the next day and I thought I had saved my phals from the sun.
My local orchid FB page suggested that the problem was sunburn plus some infection.
I’ve never seen the circular spots before with sunburn.
A. I think your local FB page is correct. That looks like the sun burnt those thick fleshy phal leaves and that created an opening for bacteria to invade the tissue. It probably happened really fast. You should cut away all the infected tissue to prevent the bacteria from reaching the crown of your plants.
Good luck with rehabbing them!
Yellow Spots on Leaves
Q. I have a couple of orchids with yellow spots. One is an antelope type dendrobium, the other an Epidendrum nocturnum. I have Physan, Sevin, Banrot, and what used to be Bayer’s (in the blue bottle). Will any of these work or must I buy something like Pageant?
A. That looks like the beginning of one of the cercosporoid infections, so you'd use thiophanate methyl. It used to be sold as Cleary's 3336 or Thiomyl, and is one of the fungicides in Banrot. I find the fungus is almost impossible to get rid of; it's a blight on dendrobiums. You can spray for it, but once you see the damage, the fungus is inside the plant, beyond the reach of the chemicals. The best advice/hardest thing to do is remove all the damaged leaves and then spray. That chemical is also available as a granular, which is much easier to use. You can find it by searching for thiophanate methyl granular on https://www.domyown.com.