What is causing most leaves on my old hackberry tree to be covered on their undersides with little whitish, hard lumps?
You have described nipple galls caused by attack of a psyllid, Pachypsylla celtidismamma. At least ten species of these related tiny insects, which resemble miniature cicadas, are known to attack hackberries. Other than premature leafdrop, they cause no permanent damage to their hosts. Thorough cleanup of fallen leaves helps to control next year’s infestation.
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