Yellow Spots Along the Midrib

[ChrisM.]

Hi everyone,
I need some advice from experienced growers. My Phalaenopsis has developed circular spots along the Midrib of two leaves over the past couple of weeks. To the eye they appear a light yellow-green color surrounded by a halo of grey-black. They appear to be flush or nearly flush with the surface of the leaves. The first picture is a leaf one or two years old and the second and third picture is of an old leaf that is beginning to yellow and fade.

[estación seca]

Welcome to the Orchid Board. The Phal experts will be by before long.

[DeaC]

In the meantime,check out staugorchidsociety.org

---------- Post added at 12:39 PM ---------- Previous post was at 12:32 PM ----------

St. Augustine Orchid Society - North Florida Orchid Growing Orchid pests and Diseases

[ChrisM.]

Thanks. There is a lot of great information on the St. Augustine website. I think it's most likely a fungal or viral infection. This new picture shows additional details. The dark centers look slightly fuzzy and yellow to my eye. Could this be an example of the plant walling off infection sites with dead tissue?

My current course of action is to isolate the orchid and provide it with a few doses of Physan 20.

[WhiteRabbit]

Hopefully some more members may have an idea

[ChrisM.]

Acting on the assumption the orchid is infected with a fungus, I bought Thiomyl (Cleary's 3336F) and administered a healthy dosage. Does anyone have any opinions on how many applications of fungicide might be needed to stop this infection? Preferably I'd apply as little as possible since this orchid is kept indoors and my options for quarantining are limited.