AnonYMouse, I'm going with what I've read about Phyllosticta, as that's what I believe I'm dealing with. If it's a leaf problem and not a systemic plant problem, cutting off the infected area, isolating, and monitoring should save the plants. I hope.
"St. Augustine Orchid Society
St. Augustine Orchid Society - North Florida Orchid Growing
The Dreaded Thai Disease
Orchid Alert
Courtesy of Dr.
Martin Motes
, Motes Orchids
, from his book
Vandas
Phyllosticta capitatus
, also known as
Guignardia
is most commonly referred to in Florida as Thai
fungus. In fairness to the Thais, it is in fact cosmopolitan. Amateur growers have exceptional difficulty
controlling this disease largely because of a lack of understanding of its life cycle. An understandi
ng of
this fungus makes it no more difficult to control than any other of the leaf spotting fungi such as
Cercospora
.
Phyllosticta
seems to appear suddenly on otherwise healthy plants; usually after they have been
subjected to cold stress in the late fall
or early winter. The distinct, rough, often diamond shape lesions
are in fact the fruiting bodies of the fungus. The fungus has been present in the leaves for some time
and the sudden appearance of the lesions is analogous to the appearance of "fairy ring
s" of toadstools
after a heavy rain. Just as mystery surrounded the latter phenomenon for ages, many growers cling to
the mistaken notion that
Phyllosticta
is a systemic problem like a virus. Strictly speaking this fungus is a
disease of the leaf not the e
ntire plant. The disease spreads upward on the plant from lower infected
leaves to the newly formed leaves as they emerge. Only under the most adverse growing conditions is
this disease fatal to the plant. It is, however extremely persistent. To control t
his highly successful
organism requires equal persistence on the part of the grow"
http://staugorchidsociety.org/PDF/Alert-ThaiCrud.pdf
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