These are galls, most likely Poplar bud gall. This is a manifestation of an insect problem, not a disease. Recommended control is pruning - removing ALL the galls, as any which remain can further the problem in subsequent years. If the infestation is severe, spraying is recommended in early spring before bud break. This link may help:
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pam - gardengal
Our poplar, the only one in our yard has developed growths, usually on a
> leaf stem. They crack and open revealing a dry grey chalky like powder. >
> The pic below shows, at the top right, how the growth will spiral and the
> other shows how they can be aprt way up a stem. About 1 in three, maybe
> more, stems/leaves are affected. The tree is about 40 feet high. The
> weather gas been in the upper 20c to mid 30c with very little rain -
> typical for this time of year - but the cankers(?) are not.
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> Tips - hints appreciated - please, and thank you.
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> More pics can be viewed atformatting link
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