This could be an infectious disease (we’ve included a link about this at the end of this response), but its more likely marginal leaf scorch caused by high salt content in soils and/or water. If watering is done for brief periods of time, this can cause a build-up of salts at the root zone, which results in brown scroch of the leaves, usually at the edges and tips. Fertilizer that has not been watered-in very well can also cause this damage. We recommend watering for longer periods of time (but less frequently) to push the water and salts further into the soil. (We’ve included two links about this). If you think you’re already doing this and salt burn/leaf scorch is not the cause, we recommend you check out the link about bacterial leaf scorch.
https://www.extension.purdue.edu/extmedia/bp/bp_25_w.pdf
http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/environmental/scorch.aspx
From the link: http://soilplantlab.missouri.edu/plant/diseases/scorch.aspx
“Bacterial Leaf Scorch in Landscape Trees
Cause: Xylem-limited bacterium, Xylella fastidiosa. This bacterium is transmitted by leafhoppers. The bacterium grows in the xylem of the plant and physically clogs the vessels. This in turn creates water stress.”
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