A. This type of sap, with all the scarring you see on the fruit as well, is caused by small puncture wounds to the skin of the fruit. When the skin is punctured on immature fruit, the sap oozes out of the hole and dries. Sometimes this dried sap looks like little squigglies coming from the skin.
The holes in the skin are caused by insects called thrips. These insects feed on the sap by pushing their long skinny mouth part, called a stylet, through the skin and rupturing it.
In wetter climates these punctures wounds can cause the fruit to rot due to infections. Here in our hot, dry climate the rotting doesn't occur as often.
To keep your nectarine fruit tree from blemishes and scarring, you must apply protective sprays to the fruit early in the season. The most effective organic spray contains Spinosad. You don't want to spray Spinosad or any chemical over and over again or you can end up with some problems.
You want to use two or three different sprays in rotation with Spinosad. Other organic sprays you could use in rotation with Spinosad include Neem oil, pyrethrum and insecticidal soap.
Remember that organic sprays do not last very long and may not cure a really bad problem that is underway. These chemicals are more effective when they are used to protect fruits when you see small numbers of pests.