Friday, July 13, 2012

A London Planetree in Pain?

London Planetree leaf (Platanus x acerifolia) showing symptoms of Mildew infestation.
I found this sitting on my desk with a note wondering what was wrong with the tree. No identification but the soft maple like leaves coupled with the the problem itself ruled out anything but the leaf being from a London Planetree (California sycamore leaves are similar but usually much larger). The problem is a fungus known as mildew. London Planetrees are susceptible to mildew, and the recent long period of cool weather has increased the amount of mildew I'm seeing on everything. Mildew is what is known as an obligate parasite, in other words it needs its host to survive so it will not kill it...but -it can make the tree look pretty bad, especially if its a variety that is not resistant to mildew. If the tree were in the wild this would not be a big deal, but here the life and death of trees is dependant on how we like them as ornamentals and a tree that is constantly coated with leaf curling fuzzy-white mildew could irk someone enough to consider removing it; thus curtailing its changes of surviving.

So what can be done to alleviate this blotchy leaf-deforming parasite? The best thing to do is wait -time will heal this as long as we have a summer that is somewhat close to normal. Hot temperatures and dry weather will desiccate the mildews branching, threadlike vegetative state and cause its existence to be less noticeable by us human beings who don't appreciate its leaf distorting fuzzy presence.
What about fungicides? Applying fungicides to large trees is, unfortunately, inherently expensive and dangerous. A spray service could do this for you but you'd be literally spritzing your money away.

There are varieties of London Planetree that are more resistant to mildew and another leaf disfiguring fungus called anthracnose; these include 'Bloodgood', 'Liberty' and 'Columbia' but unless the extended June gloom that we've been having that caused this problem in the first place becomes a permanent climate feature here I would just keep the tree you have and wait out the fuzzy invader. Also, remember to keep the old leaves picked up off of the ground -they could be the source of other fungal problems if you allow them to rot on bare ground. Many leaf attacking fungi are spread when infected leaves touch the bare ground and stimulate the production of fruiting bodies (the term for the mushroom like structures that grow off of the leaf that would be roughly analogous to a plants flower).
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