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!
 
Dendrochilum with Brownish Discoloration
Dendrochilum with Brownish Discoloration

Dendrochilum with Brownish Discoloration

Q. This mounted Dendrochilum glumaceum has leaf changes that began about 3 months ago on two leaves which I removed. It is now on older and some younger leaves, mostly the undersides but then progresses to the tops; the pseudobulbs are not affected. It receives infrequent overhead water, being watered every other day after the moss has dried and has good air circ. I sprayed with hydrogen peroxide twice early on, but nothing else since.
A. That looks and sounds like a fungal infection, rather than bacterial or viral. It sounds like one of the leaf spotting fungi. There are common fungal leafspot diseases in ornamentals from Alternaria, Cercospora, Septoria, and Phyllosticta. Something like Pageant or Heritage are often recommended for leaf spotting fungi.
    It sounds like you are doing everything right culturally. I wonder whether this cultivar is just more susceptible to the fungus. If this is the case, you can accept it, try routine applications of one of the fungicides, or perhaps consider the radical option to completely sanitize the plant. Remove each and every leaf that shows signs of the disease to the top of the pseudobulb. The hope would be to remove all the spores from the area so the fungus is stopped. Then, apply a protective spray on the plant, cross your fingers and hope for new growth and leaves.   (May-21)
 
 
Black Diamond Shaped Lesion on Vanda

Black Diamond Shaped Lesion on Vanda

Q. Is this Thai disease on my vanda? Is Physan good for treatment, or is there something better?
A. It looks like the fungal infection Guignardia, or Thai disease. Run your fingers across the lesion and if it feels like sandpaper, that's what it is. If so, maybe invest in some Thiomyl, otherwise known as Cleary's 3336, with the active ingredient Thiophanate Methyl. Monthly preventative sprays are recommended in Motes' vanda book. (May-21)
 
 
Green Pointy Bumps on Cattleya Leaf

Green Pointy Bumps on Cattleya Leaf

Q. My catt has some bumps and strange growths, should I be worried?
A. I don't think it's anything to worry about. I've had what looks like extra tissue growing on the upper side of my cattleyas from time to time. I don't think it's edema, which is a blister that forms on a fully developed leaf. This is some sort of mutated tissue that forms as the leaf is developing. Courtney thinks it is a genetic issue, probably triggered by environmental conditions. Hopefully, the next leaf will be normal. (May-21)