That’s black spot. A common ailment among elms and exactly what I am fighting by applying fungicide (Rose clear 3-1).
Don’t mist elms.. it’s just my opinion and from research but don’t do it.. watering elms on an elm will encourage black spot. Especially on fresh new leaves that have just sprouted. Once the leaves harden off in summer, then you can get them wet.
I would have thought it's an unnecessary step at this point now. Foliage would have hardened off enough that a dip in temperate overnight every now and then wouldn't do any harm I would think, but won't do any harm either... extra peace of mind I guess!
Oh Boy! more fun with hardening off. The most misunderstood term in bonsai....why? because it should never be in the bonsai vocabulary.
Like all anthracnose fungus in maples OR elms and most all deciduous trees the disease is there the year previous, just like in maples of which I have spoken ad nauseum, but no one listens. I'm gonna change my screen name to Ryan Neil and see if things improve.
The name of the fungus is
Stegophora (syn. Gnomonia) ulmea. It is set up in the previous years growth and shows up in the current year when the buds open. It has nothing to do with whether the leaves are hardened or not. In fact it is during the summer when the humidity is low and things are drier that the fungus goes into hybernation and comes alive in the fall when the conditions are met again. In the summer you may think that the leaves hardening off has controlled the fungus but it will be back and ready to show it self next spring unless you change your ways.
Controlling it is simple. Use common sense. Look in a nursery or the nursersy section of your favorite store. There products called DORMANT DISEASE CONTROL. They are meant to be used while the tree is in dormancy. Most usually it is dilute lime sulpher. I have no idea about these products in the UK.
In the spring when buds begin to open continue the control by weekly applications of a fungicide. Copper or daconil work well.
AGAIN PEOPLE FUNGUS HAS NOTHING TO DO WITH LEAF HARDENING, IT AFFECTS NEW LEAVES CAUSE ITS ALREADY THERE!!!!
http://entoweb.okstate.edu/ddd/diseases/elmblackspot.htm
http://cemarin.ucanr.edu/Programs/Custom_Program97/Tree_Deseases/Chinese_Elm_Anthracnose/