Keeping your doodles' ears clean and dry will help avoid yeast infections. Yeast are present all the time, but when conditions are favorable (warm, moist environment) they can overgrow and cause irritation and discomfort. Some dogs are particularly susceptible to yeast overgrowth for all sorts of reasons. If your doodle's ear canals are hairy it's important to keep them plucked. This isn't as painful for the dog as you might imagine. Rinsing the ears with a vinegar and water solution can also help because it makes the ph of the ear canal less desirable for yeast growth. In my experience, though, by far the most effective control for yeast is to powder the ear canals (after they dry ) with an anti fungal powder - the same stuff people use to prevent athlete's foot. If your dog is prone to yeast infections, it's important to establish a routine of powdering the ears lightly every few days. It also keeps their ears from getting smelly.
Yeast infections can also become a problem if your doodle is having to take antibiotics for any reason. The "good bacteria" that normally control yeast overgrowth are reduced by antibiotics, so if your doodle needs to take antibiotics it's especially important to keep a close eye on the ears.
One more note: none of this advice applies if the infection is bacterial. So until you are sure you can identify yeast, it's best to have your vet check it before you do anything.