West Midlands,
United Kingdom 
Has anyone seen this effect on their Buddlejas?
The leaves become very narrow and stems congested, and the flowering is reduced. I think it may be a bacterial infection called either Little Leaf Disease or Witches Broom caused by a phytoplasma. Anyone familiar with these diseases?
Do you think there might be a different cause?
20 May, 2018
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Sadly, those are the likeliest possibilities, unless someone in your neighborhood uses a growth regulator as part of their landscape maintenance program, and sprayed on a windy day.
20 May, 2018