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!
Sunken Spots on Dendrobium Leaves
Q. My dendrobium has developed spots on the leaves and I'm unable to identify the cause. I have been spraying with a systemic fungicide.
A. I would say that is bacterial rather than fungal, particularly if it happened pretty quickly. Copper is a very effective bactericide but of course you can't use it on dendrobiums because they are so sensitive to it. You can pour or spray the leaves with a fresh bottle of hydrogen peroxide. There are not many great options for dendrobiums. The systemics you are using are good fungicides for the leaf spotting fungi, but not for bacterial infections.
The question is, how did the bacteria get a stronghold in the plant. Low air movement and excess leaf moisture are conditions conducive to bacterial blighting, particularly if you are growing outdoors where the plants are watered by Mother Nature, so the leaves are wet at night.
Center of Cymbidium is Dead
Q. I am located in Melbourne, Australia. When my orchids bloom mainly in our winter, I get a healthy number from each. However, there are a large amount number of dead looking growths in my pots. Should I be re-potting to remove them or leave things as they are?
A. Harry, the Cymbidium Man, advised Nigel to repot: If cymbidiums are not repotted every 3 or 4 years, the center part does tend to die and should be discarded when repotted. Potting media tends to break down and decay over time and encourages rotting of the plants as well.
Remove all rotten material
including roots. Pots should just large enough to allow for 2 years growth.
I like to divide my plants when I repot and pot two or three
good growths and a nice firm back bulb. Two years growth for a division is 2 inches between the plant and pot rim all the way around.
Not sure what material they use for potting in Oz but fine bark should be good, like the Pinus radiata bark from New Zealand.
Some of the Australian growers I know have mentioned that they use it.
Black Streaks in Flower
Q. What is causing this flower blighting? The white cattleya flower opened in perfect condition and developed the black streaks after a few days, and the pink cattleya is starting to show some streaking. These plants are being boarded and are now in a more humid environment.
A. When you get that necrotic streaking in the flower about a week after it opens, and if it seems to follow the veining in the flower, I would be very very afraid that it is virused. If it is, I would suspect it is brown necrotic streak which some believe is caused by a strain of Cymbidium Mosaic Virus and others believe is from a dual infection of CymMV and Odontoglossum Ringspot Virus. The white cattleya is almost certainly virused. The pink cattleya may just have some bacterial blighting, but if in a few days that necrotic streaking appears through the midribs, then you would be afraid of virus. Do you have any test strips to verify whether or not they are virused?