Several years ago I was going on vacation for a week. To keep my Orchids from drying out, I put up plastic around my light stand. I do not know if this was the cause of what was to come, but I will never do this again.
I soon started to notice spots on my Phals. These were smallish sunken spots, that were dark in color, but dry. Both the top and bottom of the leaf was sunken, but it was more obvious on the top. The spots expanded and multiplied and grew together until there were large areas of the leaf affected. The infection usually, but not always started near the edge or tipof the leaf and worked its way to the base. It takes 1 to 2 weeks for the whole leaf to be completely dry.
After reading every book I could get my hands on, I still could not identify the problem. It sounds a bit like a fungus, a bit like bacteria and a bit like everything else. The closest description was a phal spider mite - but there were no mites present.
My plants were going fast. In desperation I plastered the leaf with a fruit tree spray that contained ferbam (phosalone 4% ferbam 15% - CIL Insecticde/fungacide for fruit trees. It was a fungicide and insecticide.The stuff is very black and I mixed it with water (1:5), a lot stronger than recommended, but then I had little to loose. It worked. I do not know how the plant survived, with the leaf covered in black ie little light would getthrough, but the disease stopped spreading.
My plants started putting out new leaves and they were clear as can be. I felt like I had won. When the new leaf was almost full size, the problem started again. (If anyone out there knows why the growing leaf is not affected until almost mature, I'd like to know.) Suddenly (it really goes fast) my plants looked quite bad so I again plastered them in black. I even tried Benomyl - a wettable powder, but I found this stuff hard to work with, being indoors and having only a few plants - I think it did work.
At an Orchid show, one of the top Phal breeders in the US, told me the problem was a microfungus. It was becoming common, no one knew what it was and that the only solution was to discard all of my plants - there was no cure. He said several growers lost large collections of Phals. I was not going to give up without a fight so I kept looking for a solution. In talking to others, I found that quite a few people had this problem, but did not know what caused it. They lost lots of Phals and then decided not to grow Phals again. No one felt that it affected any of their other plants. This may be the result of throwing out anything that did not grow.
I was reading a great book on Phals (Culture of the Phal Orchid) by Bob Gordon when I read about a 'microfungus'. The symptoms matched those I was seeing. In fact when Bob came to our Orchid society meeting he confirmed that it was microfungus. The book did give a chemical control for the problem, but I could not find any of the ingredients in Canada. No one hasbeen able to tell me what a microfungus is, but the key is that it is onlyaffected by a systemic fungicide. A non-systemic fungicide will not work.
My treatment should have worked, but I made one big mistake. I only treated the Phals. I never saw the same symptoms on other types of Orchids so there was no need to treat them. It is possible that I was curing the Phals, and that other plants were reinfecting the Phals. I had a vicious cycle going were I would cure the Phals and then they would get infected again.
I started to use Funginex 6.5 (6.5 % Troforine) a water soluble systemic fungicide, at normal strength, about every two months. But I treated every plant I had. As long as I sprayed, new plants did not get infected. But once infected, a plant never recovered. In my case, most of my phals were infected and close to death, before I found a solution, so eventually I just gave up and threw the sick ones out. Every phal that I had before I started using funginex, died or was thrown out. New introductions have never shown the problem.
I am now free of the problem for 5+ years, and I spray with Funginex about twice a year.
It seems to affect Phals the most, but I think it also affects other plants. Oncidiums (especially equitants) seem to dry up, but without the sever spotting. B. nodosa got very black sunken spots but complete leaves never dried up. Cats and Laelia seemed to be mostly unaffected. BC and BLC plants had some sunken black spots, but some only showed very mild brown spotting. Paphs seemed immune (I only have a few), but a large Zanzibar got quite bad with brown spots on the leaves. Dens seemed to be unaffected, except for aggregatum which had brown sunken spots and lost most of its leaves. All of these other symtoms were different from the Phals, so it is possible I just happen to have other fungal problems on these at the same time. Are other Orchids or other types of plants carriers?
Conclusions: Do not give up with your plants. Experts can be wrong. I am not convinced the above treatment will cure a plant, but I am fairly sure it prevents the spread of the disease.
I have received a number of emails about Microfungus but none of them have ever indicated that they cured the disease. Several people have told me that the concept of a "microfungus" does not exist. It is either a fungus or it's not. In any event, in the lay persons world, people call this disease, microfungus.
The above descriptions of the symtoms seem to be fairly consistent when I talk to other growers. Probably the most identifying characteristic is the fact that it does not affect growing leaves, until they are almost fully grown, and it does not affect fully mature leaves.
( I'd be interested in getting more information from other growers on this topic.)