Understanding Mold Growth in Tubing

As I pull taps, I come across some drops that are way moldier than others. I could have a really moldy drop on one side of a tree and a really clean drop on the other. Both drops are the same age and got the same sanitizing regimen I use. When it's an older drop, I figure that something just got in there at some point and had a party, but I can find a lot of mold in drops that are one season old, too. It can happen to a single drop on a lateral of 4-5 drops. The rest are good but one is nasty. I've tried to deduce if it's a shade/sun thing, but I can find these bad drops on either side of the tree.

The most common place I see the most mold in any drops is in the 2-3" below the spout. The rest of the drop will be clean, but the top few inches will need to be cut off. I was thinking that what those situations all have in common is that is where there is an air pocket in the drop. Does mold thrive in the air? The flip side of this situation is where I have drops that are going into their 4th year when I plan to replace them and they are clean as a whistle - even right up to the spout.

So, back to the drops where the whole drop is gross - is the mold an indicator that the drop had a leak and more air was getting in?

The mainlines also have mystery pockets of mold, too. A couple that really stand out are near the saddle, but in these cases, the 2-3" on either side of the saddle are clean and beyond those clean bands, it is moldy upstream and downstream. The mainlines have consistent 3-4% slope. I'm careful about supporting sags, but maybe the little dip that happens when the saddle goes under the wire has something to do with it.

We had too many warm spells this year that made mold worse than usual, but I'm wondering what I can learn from where the mold is at its worst.

Air leaks will introduce bacteria. I find that drop lines where a spout was leaky for a couple days those are the drop lines that get filthy. I believe the reason why you see it closer to the tapholes is because the tree is darker and typically the bark will attract solar heat and this is where your bacteria will first grow.

Places where air is introduced unwanted and sap sits will induce bacteria growth.

I always wonder why some taps and trees are running hard while others are not I'll put in on the same day out of the same bag but clearly one tap hole has dried up sooner than the other. Some things are a mystery

Visual microbial growth has little relationship to the composition or quantity of growth in the tubing unfortunately.

A visibly clean drop can have or does have same microbal load as a filthy black one? I guess at the end of the season everything from the tree to the releaser is equally contaminated.

Too bad there wasn't a way to flush everything out halfway thru the season and sanitize while taps are in.