I suspect pests like aphids. They usually prefer new leaf growth so these leaves may have been "new" when the infestation took place and the apids left since then. If the aphids are still around, you could try addressing the problem now and, for sure, next year when new growth leafs out in your zone.

One way to do this is by releasing beneficial insects (like ladybugs, etc.) now. To prevent future infestations, release the ladybugs about 2-4 weeks before the date when the problem started this year (so if the problem started today, release them next year around the middle of May). Once you have a high enough level of these beneficil insects, you will not need to repeat this every year of course. To maintain these guys around, provide them with some water and reduce (or eliminate) the use of insecticides as insecticides will also kill beneficial insects. And if the number of beneficial insects gets too low, that can allow aphids to get out of control in future years.

You can also control them with insecticial soaps or insecticidal sprays. As the leaves mature, the aphids still around will not be able to cause as much of a problem.

Luis

Thank you so very much, Luis!