Peach leaf curl is caused by a fungus that gets into the tissues of young leaves when they first start growing in the spring. It only affects peach and nectarine trees. While it does not kill the tree, it can severely weaken it to the point that it dies of other causes. In less severe cases, it can cause the tree to be unable to ripen its fruit, and rarely may cause the fruit to be deformed. The most obvious symptom is deformed and discolored foliage. At first the foliage is “bubbled” and may be white, pink, red, or even purple. As the disease matures, the foliage starts looking dusty or powdery, which is the spores emerging. These spores spread all over the bark of the tree, where they wait for the next spring to infect the new foliage. This fungus needs cool, wet conditions to do the most damage. It is dormant below 48 degrees and above 79 degrees and in dry weather. Once the leaves are fully opened out, they are no longer able to be infected. Leaves that grow after the spring rains have ended and the weather has warmed up are much less likely to be affected.
The best way to combat peach leaf curl is to prevent it by spraying during the dormant season. The best products for this have recently been taken off the market, but there are still two products which do a reasonable job. Kop-R Spray from Lilly Miller, and Liqui-Cop by Monterey Lawn & Garden both have enough copper in them to be fairly effective. Copper soaps have such a small concentration of copper that they really don’t help much. Another possibility is Bordeaux Mixture, but that involves buying copper sulfate and hydrated lime, and mixing them together, and then mixing with water at the time you are going to spray. It cannot be mixed up ahead of time, so cannot be sold ready to go. Whatever you use should be sprayed three times to insure as many of the spores as possible are killed. One good spraying should do it, but if we have rain, fog, or heavy dew before it has done its work, you need to repeat the spray. For best effect, you should spray in the fall before the leaves have all fallen, again after you prune during complete dormancy, and again when the buds begin to swell, but no leaves are showing yet, in the spring.
It is very important to clean up all fallen leaves, preferably as soon as they fall. Also any leaves that don’t fall and mummy fruits that remain in the tree must be cleaned up. They should be bagged up and thrown away, not composted.
Once the symptoms have appeared, it is impossible to get rid of the disease for that season. The best you can do is take good care of the tree to help it resist and recover as much as possible. Some research has shown that spraying the tree with liquid kelp can help reduce the effects of the disease. You need to spray twice, about 7-10 days apart. After the second spray, the diseased leaves should look burned and fall off, whereas the healthy leaves should look good, and new leaves will be clean. Be sure to fertilize the tree generously and make sure to keep it watered. A stressed tree will suffer from the disease much more than a well-cared-for tree. If a lot of fruit sets, be sure to thin it, and the worse the disease is, the more you will need to thin.
After a season with peach leaf curl, be sure to do a good clean up, spray appropriately, prune after complete dormancy, and take good care of your tree to reduce spore build up and minimize disease the following year.