Question
Looks like beaded styrofoam on stems and underside of leaves - what to do?

Answer
Sounds like an insect problem.  If there is edible fruit on the tree I would use Safer's Insecticidal Soap as a treatment.  You will have to do this several times in order to get rid of the bugs.  If there is no fruit on the tree to be eaten this year, I would treat the plant with Bayer Advanced Tree and Shrub Insecticide.  This product is mixed with water and poured on the soil around the base of the plant.  It is taken up systemically and will kill the bugs within a week or so.  It also provides protection from reinfestation for up to a year.  The other option is use heavy mineral oil and coat each bug mass with the oil using a Q-tip.  This can become tedious, but will work quite well.  Good Luck with the problem.