Orchid Culture - Questions & Answers from This Month
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!
Silvering of Dendrobium Leaves
Q. My Dendrobium leaves are very silvered, is this from mites?
A. That looks like classic mite damage, and dendrobiums seem to be susceptible to mites. The two spotted or red spider mite causes a chlorotic spot or stipple at each feeding site as chloroplasts are sucked out along with the plant sap. Leaves eventually develop a mottled or stippled appearance with webbing under the leaf in severe infestations. Try spraying upper and lower leaf surfaces with the home cure mixture of 1 part rubbing alcohol, 1 part 409 or Murphy's Oil Soap and 2 parts water. Plants can also be sprayed with a miticide like Avid, Talstar or Kelthane following label instructions being particularly careful to contact all the undersides of the leaves. During warm weather, new generations mature every 6 days so repeat applications will be required, perhaps 3 applications at 4 day intervals.
Catasetum Bulb Rotted
Q. At our meeting, I asked you about my catasetum that had one of its bulbs rotting. Is this what you told me to do? I cut the black tissue out and doused the cut end with cinnamon, then put it sideways into sphagnum moss about half way deep. I have been keeping the sphagnum moist but not drenching. Or did I get this all wrong?
A. You did exactly right. You cut all the diseased tissue away and put on cinnamon to dessicate and seal over the wound. That sphagnum looks like the commonly available moss that rots within a few months. You might consider replacing it with some New Zealand long fibered sphagnum moss, look for the Premier or AAA grade on Amazon or eBay. If nothing happens by the end of November, the bulb may also be going dormant, in which case you should keep it dry. Sometime between January and March, you should see the beginning of a new plantlet from one of the nodes (joints) on the pseudobulb. When the roots grow into the moss and the new growth is about 5 inches tall, you can begin watering. If the bulb becomes severely shrivelled, you can consider placing the pot in a saucer to draw water up from the bottom.
Fungi on Dendrochilum
Q. Another difficult summer for us outdoor growers! Too much rain at
once and fungi developing. These pictures are of a Dendrochilum
magnum that seems to have fungal infections all the time. Any suggestions?
A. That looks like Anthracnose. See all the spores in the dead tissue? You'll have to remove the leaf to at least an inch below the discoloration to remove the source of innoculum, and then spray with Daconil or Cleary's (or Heritage or Pagaent if you have them) to prevent recurrence. The rains are great... until they aren't!