Question
Hello,
I have grown Cattleya orchids for 25 years with no problems. I have a sunroom with an ultrasonic cool mist humidifier that I use.  Two months ago I acquired 2 Intermediate/miniature Cymbidiums.  They are in spike and bud and seem to be doing ok except I have noticed in the last few days that there are black spots showing up on some of the leaves, especially underneath.  Some areas have bunches of tiny looking black specks altogether.  The spots seem to be part of the leaves instead of on top of them.  I have tried to wipe them off with a q-tip with soap and water but they don't come off.  Also there is black showing up on the pseudobulbs that resembles mildew. It does wipe right off with just q-tips.
What might this be and how can I fix it?  I do keep my sunroom at about 75 degrees daytime, 60 at night.
Thank you.
Darlene


Answer
Black spot is common on cymbidium leaves and is associated with high humidity. The fact that it is more common on the underside of the leaves is likely to reflect a potting mix that is too wet causing the humidity on the underside of the leaves to be higher than the upper part. Ultimately, the plants should be repotted in medium grade orchid potting mix which dries out more quickly following watering. Repotting during flowering is not advised so that must wait until after flowering.  The black spot is due to a fungus.  The damage cannot be undone, however, to prevent additional spotting, I recommend you obtain a copper based fungicide and mist the plant leaves once per week.  This may leave a green buildup on the leaves over the months. Try to avoid getting this on the flowers and buds. Cut back on the watering and increase air movement by these plants which should disperse some of high humidity in the vicinity of the plants.