This summer, the Napa County Master Gardener help desk received many calls about peach leaf curl, a fungal disease that affects the leaves and shoots of peaches and nectarines. It causes distorted, reddened leaves that eventually fall off the tree. See UC Integrated Pest Management page for photos and further descriptions of the disease.
Symptoms normally appear about two weeks after leaves emerge. The first signs are red spots on the leaves, which soon become thick and puckered. Whitish spores appear on the leaf surfaces, then leaves turn yellow and drop off. Healthy new leaves do emerge to replace the fallen ones unless subsequent showers cause the disease to spread to the new leaves.
If healthy leaves replace the damaged ones, why should a gardener be concerned? Because peach leaf curl affects the vigor of the tree. In severe cases, the disease can significantly reduce fruit production and tree growth. If peach leaf curl builds up and the tree is left untreated for several years, the tree may seriously decline and need to be removed.
Unfortunately, there are no effective treatment options between spring, when the symptoms appear, and summer, when the tree bears fruit. Although some gardeners pull off the diseased leaves, this technique has not been proven effective.
Taphina deformans, a fungus, causes peach leaf curl. Periods of cool, wet weather when leaves are beginning to unfurl create a favorable environment for the fungus. Maximum infection develops when the trees are wet for two or more days during this vulnerable period. Our spring weather pattern over the past three years has provided the conditions this fungus loves.
To control peach leaf curl, treat peach and nectarine trees with a fungicide in fall after leaves have dropped. In the past, the disease could be successfully treated with either lime-sulfur fungicide or a fixed copper fungicide with a copper compound containing at least 50 percent copper. Unfortunately, the most effective products have been removed from the market by the manufacturer. The only fungicides left for treating peach leaf curl contain lower levels of copper and copper soap.
You can make the copper ammonium complex products more effective by adding horticultural oil to the treatment mixture. The oil, which should be one percent of the mixture, also helps control some insects that affect the trees. Spray your trees twice, the first time in late November and the second time in early February. A handy trick for remembering those dates is to spray right after Thanksgiving and just before Valentine's Day. Spray trees until they are dripping.
Alternatively, you can prepare what's known as Bordeaux mixture. This treatment is effective against peach leaf curl, but it is not available for sale and you must mix the ingredients just before application. It takes longer to prepare and requires more knowledge and safety equipment both in the preparation and the application. Contact the Napa County Master Gardener office if you would like information on preparing and using Bordeaux mixture.
Another way to reduce peach leaf curl is to plant trees that are resistant to the disease. Resistant peach varieties include Frost, Indian Free, Muir and Q-1-8. Although frost is resistant, it does need fungicide treatments the first two to three years after planting.
In fact, all of the resistant varieties still benefit from spraying. While resistant, they are not immune, especially during years when the conditions are particularly favorable for the disease. My son, Sean, and I have both grown the Indian Free peach tree and found this heirloom to be vigorous as well as resistant to leaf curl. However, it is not self-fruitful, so it needs to be planted near another peach or nectarine tree for pollination. The nectarine variety‘Kreibich is also resistant to peach leaf curl.
If you experienced problems with peach leaf curl this year, consider pruning the tree this fall prior to applying fungicides. This practice can reduce the number of spores overwintering on the tree. Remove all pruning debris and dispose of it in the municipal yard-waste container. Do not water the tree with overhead sprinklers, as this could spread the spores. And don't forget to spray around Thanksgiving and shortly before Valentine's Day.
Napa County Master Gardeners (cenapa.ucdavis.edu) answer gardening questions Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 253-4221.