dying phalaenopsis plants

[Dottie]

I grow under lights. I have lost some of my plants and some are
effected by the same thing. They seem to have a rust like dust on the
leaves and then the base starts to blacken. The leaves are also rough.
I don't think it is spider mites as I have not seen any webs or mites.
I have dendrobiums and they are not effected. Thank you for your
time.

Dorothy Razukiewicz

[orchid doctor]

Dear Dorothy:

Rust disease can be found on orchids, but the spore cases are usually
very distinct (just like rust on roses, geraniums or snapdragons), and
you would never see an even dusting of red spores on the leaves. A
dusting of red powder is much more likely to be mite, or false spider
mite, of some sort. Some mites are barely visible to the naked eye,
some not at all, and many do not produce any kind of webbing.
Roughness of the leaves, especially with a silvery or gray appearence,
often becomming sunken and black is indicative of false spider mites,
which cannot be seen except with a hand lense though i suspect a heavy
infestation might produce a red dust (of droppings and shed skins
perhaps). They prefer leathery leafed plants such as Phalaenopsis or
Masdevallias and are less likely to attack Dendrobiums.

Growing under lights, conditions might favor their development on
plants: these being even warm temperatures (little difference between
day and night), very high humidity and weak air circulation. You may
control these pests with a horticultural oil spray, an insecticidal
soap spray (several applications at the recommended interval), or a
commercial miticide. Follow all directions and take all precautions,
and be aware that you may have damage to flowers and buds, or even
some leaf burn.

Running fans for air circulation helps prevent insect and mite
infestations, in part because it improves plant vigor. Frequent
inspection helps keep any that do occur from spreading. occasionally
bathing or washing your plants also helps prevent problems, but this
should only be done in the morning, and where air movement and
temperature assures plants will dry off by evening.

Sincerely,
Dennis