Ugly brown spots on oak leaves, I think it is galls of some kind

(Question)

I would like to chop up some oak leaves and use as mulch in the Rhodo… acidic bed, but with lots of the leaves covered with brown spots, will I be spreading a disease?

(Answer)

Galls are growths that occur on leaves, twigs, or branches.  They are frequently brown but can also be brightly coloured.  There are 1500 species of gall producers, the majority of which are insects and mites.  Some galls form where insects or mites feed or lay eggs.  The gall provides food and shelter for the larvae which reside inside.  Galls affecting leaves are seldom, if ever, a serious problem for the host plant.

However, some gall producers overwinter in leaf litter and garden debris, so don’t use your oak leaves to mulch your rhododendrons this year.  Rake and bag up the leaves for collection by the City.  Leaves collected by the City are subjected to higher temperatures than a home gardener can provide during composting, which will destroy the gall producers.

With some care you may well be able to use your oak leaves again in the future to benefit your rhododendrons, but not this year.