One of the downsides to my new location is it appears that I’ve moved into peach leaf curl country. From my own observations and amount of requests for help from clients, it’s pretty bad out there. Peach leaf curl is a disease caused by the fungus Taphrina deformans.
It causes the deformation of leaves (like the red bumpy leaf in the photo above)
on peach and nectarine trees. In really bad cases, it's hard to make out the shape of the leaf at all, it's that deformed. Now, while your trees are dormant, is the time to start treating your trees.
You will need to spray with a product containing copper ammonium complex like Monterey's Liqui-Cop Fungicide Spray . You can make this mixture more effective by adding 1% horitultural oil spray, I use Neem Oil . I spray once now, when I'm doing my winter cleanup and pruning. While doing the cleanup, it's important not to add the cut branches and dried leaves to the compost pile. This can spread peach leaf curl fungus to other places in the yard. I like to rake up all the debris, spray the tree until it's dripping, and then add a nice layer of compost around the base. If you have a particularly bad infestation, you can spray once more just before the buds on the tree begin to open. This takes a bit of observation, but it can be worth it. In my case, the infestation is pretty mild, so I will spray once.
Things to keep in mind when dealing with peach leaf curl. Once your trees have leafed out and you see it, it's pretty much too late to eradicate. There have been some studies showing that seaweed extract sprays can help but they aren't entirely conclusive. Also, make sure to keep your trees well fed with a good organic slow release fertilizer like Dr. Earth Fruit Tree Fertilizer. It helps them continue to grow while battling the fungus. You may also read about other chemical sprays on the internet, but they aren't available in California anymore. Right now, the copper mixture discussed is the most effective preventative available. More detailed information about peach leaf curl can be found at the UC IPM site