What element are we missing? Well that's the questions as I would assume that different plant species will be somewhat different. But folks are noticing improve results after applying mancozeb. Bjorn's with his seedlings/flask (randsii come to mind) and Philippe on his plants. I know of another grower who uses those on adductums too and we all know its hard to establish those. So what are are the ingredients on dithane/mancozeb? Boron, Mn, Zn or does it also contains Fe?
I have been feeding my bonsai with a soybean meal/blood and bone cakes for years and have excellent results. The cakes are made by mixing dry ingredients with water and then leaving on trays to ferment and harden. They are then stored and used as needed.
I mention this because some growers in Japan use similar types of fertilizer for their orchids, especially Cymbidium but also Paphs. In Japan they traditionally use a rapeseed meal/bone meal mix. Recently I have found a source for organic Canola (rapeseed) meal and I have ordered some to try. I will mix this 75/25 with blood and bone as bone meal is unavailable here.
According to Tanaka, you must be careful to not over-do it and he suggests one piece of cake the size of the tip of your thumb per 4'' pot once per month or so.
I have included a link to a rapseed mineral analysis, (page 999).
http://www.journalofanimalscience.org/content/58/4/996.full.pdf
Have a look at the Mn Zn and B contents.
Perhaps the synthetic fertilizers we use are not the best choice? I intend to experiment with this organic approach on a few plants the entire growing season and will report back. I have taken ''before'' pics of all the plants.