Mold in the fruit?

Tonight I prepared a simple fried avocado dish. Essentially, I just sauteed slices of avocado in light oil and serve with lemon juice. Normally, this is a very easy and delicious way to consume avocados.

But the fruit I used tonight was a little older than usual. Certainly it was softer and the black spots of oxidization were very prevalent throughout the flesh. What was most worrisome was a large white area which seemed to begin at the stem and grow toward the seed area. I'd estimate it was approximately 1cm in length and about half a centimeter in width. It was shaped like a football and was hollow in the center.

I cut this area out and prepared the avocado as normal, but the flavor was bitter and I ended up tossing the dish.

Unfortunately I did not take pictures of this. What was it? Was it mold? How susceptible to mold are avocado fruits? How safe is this mold?