That does look like powdery mildew. It’s also usually one of the easiest issues to get rid of. One of the best steps is to be diligent about clean orchard management every year. If you’re going the organic route, wettable sulfur can be used for powdery mildew. Many fungicides used for brown rot control are effective against powdery mildew as well. If the infestation isn’t too bad, you can clean off the fruit, and still eat it. I’ve cleaned my fruit with vinegar to get rid of the powdery mildew and then left it on the tree to ripen.Copper fungicide can also be sprayed on every part of your fruit tree, including the fruit. Nectarines, in my part of the country are one of the harder fruits to grow. Regular rotational fungicide management is a must for us. Below is a solid list of fungicides that work for the issue you have. Good luck!

https://ipm.ucanr.edu/agriculture/nectarine/powdery-mildew/

Hi. Thanks for the reply. How did you go about cleaning your fruit with vinegar? Did you use a spray or water it down? Are you sure you can put copper fungicide on the fruit itself? Ive been advised against that elsewhere, and also told it wouldnt really help it anyway.

There is so much misinformation and bad advice around proper use of fungicides.

At least that’s been my experience. There are many different types of copper fungicide and of course you always want to follow the label. Here’s an example from the one that I’ve used in the past. It’s OK if it gets on the fruit, because that is often the only way to prevent powderly mildew and brown rot from occurring on the fruit itself. The link below is from the label of a copper fungicide brand.

https://www.domyown.com/when-can-eat-vegetables-after-spraying-bonide-liquid-copper-fungicide-concentrate-qa-35025.html#:~:text=Answer%3A,to%20wash%20well%20before%20eating.

Nothing is better than a homegrown, peach or nectarine, and in my case, I didn’t want to lose any fruit and have to wait until next year’s harvest. I just took straight vinegar, sprayed it on the fruit and gently cleaned and rubbed it with a paper towel. After some applications of fungicide and cleaning up the fruit, I was good to go. Depending on how bad the infestation is, I’m sure it won’t work on every piece of fruit but you’ll likely be able to salvage several. I hope it works for you and good luck!