Peach Leaf Curl Control Tips (2024)

Peach leaf curl causes leaves of peaches and nectarines to discolor, thicken, pucker, curl, distort and eventually fall off. The fungus overwinters in these trees as spores, usually in the new buds. The rains of late winter and early spring - or in the case of 2024, the abnormally wet weather of February - splashed these spores onto the emerging leaves, causing more problems. Emerging shoots can die; fruit production can be reduced in severe infestations. Only rarely do reddish, wrinkled areas develop on fruit surfaces; later in the season these infected areas become corky and tend to crack.

The good news is that a second set of leaves soon emerges and can develop normally when the rains cease and daytime temperatures steadily reach into the 80's.

Studies at UC Davis have shown that nipping off infected leaves of peach and nectarine trees doesn't do much good this time of year.

If your peach and nectarine trees are showing signs of peach leaf curl now, here are the best things you can do is to assist those trees through this stressful period.

• Rake up any fallen leaves and pull weeds that are growing beneath the drip line of the trees.

• Fertilize the area thoroughly, if you haven't yet done so.

• Before the weather heats up into the 90's, spread four inches of fresh organic mulch beneath those fruit trees. Organic mulches, such as compost, shredded branches or the fallen leaves of healthy shrubs and trees will help conserve soil moisture, hold down weeds and add nutrients to the soil as that mulch breaks down.

• According to Steve Zien formerly of the Sacramento-based organic garden consultancy firm, Living Resources, there now appears to be a method for control after the leaves have developed peach leaf curl. His evidence indicates strong foliar applications of quality seaweed fertilizers (containing Ascophyllum nodosum) on distressed foliage can, in some cases, stop the spread of peach leaf curl.

"Our findings indicate that foliar applications of seaweed (Maxicrop brand seaweed) can result in approximately 80% control of peach leaf curl," says Zien. "A mixture of one part Maxicrop liquid concentrate to ten parts water was applied two to three times during early spring. Following treatment new leaves developed normally. Meanwhile, unsprayed trees and trees sprayed with water, continued to develop disease symptoms. Leaves damaged prior to treatment will remain distorted. However, the leaf thickens, becomes greener and remains on the tree for a longer period of time (compared to untreated trees). This provides the tree with vital nutrition until new healthy leaves develop. One theory is that the seaweed thickens the cuticle (leaf skin) creating an environment not suitable for the fungus to develop."

(Note: Test results at the Fair Oaks Horticulture to control peach leaf curl during a subsequent winter showed the ineffectiveness of Maxicrop. "Compared to untreated branches, Liquicop-treated branches averaged about 70% control," said the late Chuck Ingels, Sacramento County Farm Advisor. "Copper soap was slightly better at 80% control, Agribon (row cover material) by itself was less effective at just under 60% control, and both Agribon + Liquicop and lime sulfur (late fall) followed by Microcop (late winter) resulted in nearly complete control. Maxicrop (sea kelp) did not work at all and seemed to increase the severity on some of the branches.")

The experts at UC Davis advise pruning infected peach trees in the fall before spraying with a copper ammonium complex product with 1% horticultural spray oil added to the mix.

And now here's the bad news about sprays: the copper sprays available currently are weaker (about 8% concentration). Lime sulfur has been removed from the market. Bordeaux mixtures are expensive and wasteful...and potentially caustic.

In the good old days of fruit tree sprays (2009), 50% copper concentrates were the recommended course of action. Not any more.
The UC Davis Integrated Pest Management information on controlling peach leaf curl says, "Fixed copper products include tribasic or basic copper sulfate, cupric hydroxide, and copper oxychloride sulfate (C-O-C-S), but currently only liquid products containing copper ammonium complex products with 8% MCE (e.g., Kop R Spray Concentrate [Lilly Miller brands] and Liqui-Cop [Monterey Lawn and Garden]) are available to consumers. The most effective copper product, 90% tribasic copper sulfate with a 50% MCE (Microcop) is no longer available to retail outlets, because the manufacturer withdrew the product in 2010."

One of the reasons for that removal: repeated annual use of copper products over many seasons can result in a buildup of copper in the soil, which eventually can become toxic to soil organisms, and if it moves into waterways, can harm some aquatic species.

The removal of lime sulfur products was prompted by a rash of self-inflicted deaths in Japan in 2008 called "Detergent Suicides", which then spread to the United States.

Bordeaux mixtures, a combination of copper sulfate, hydrated lime and water, are effective in controlling peach leaf curl, but come with their own set of warnings. According to the UC IPM Guideline entitled"Bordeaux Mixture": "When applying Bordeaux, be sure to wear protective clothing, including goggles, because the spray deposit is corrosive, can permanently stain clothing, and is difficult to wash off." They also recommend wearing a dust and mist-filtering respirator when mixing in the hydrated lime. And that mixture can discolor anything it touches, including buildings and fences.

And that brings up the cost and waste involved: copper sulfate and hydrated lime are usually sold in large quantities, much more than the average homeowner needs for the backyard peach and nectarine trees. Storage involves mixing the leftover individual ingredients separately in water and storing in their own sealed jars. That UC IPM Bordeaux Mixture Guideline warns: "Be sure to clearly label both stock solutions and store them where children can’t get into them, since these materials, especially the copper sulfate, are very toxic and corrosive."

The synthetic fungicide chlorothalonil is the only non-copper fungicide available for managing peach leaf curl in the backyard orchard. Although one fall application may help prevent a spring outbreak of peach leaf curl, a second application in January or February, as the buds begin to swell, can be beneficial, as well.

But be sure to read and follow all label directions if you choose to use chlorothalonil, including this:
"This product is toxic to aquatic invertebrates and wildlife. Do not apply directly to water or to areas where surface water is present or to intertidal areas below the mean high water mark. Drift and runoff from treated areas may be hazardous to aquatic organisms in neighboring areas."
Or this: "May be fatal if inhaled. Harmful if swallowed or absorbed through skin. Causes moderate eye irritation. Avoid contact with eyes, skin or clothing. Do not breathe spray mist.”

No matter which spray method you choose, several days of dry weather must follow for the products to work. And that is one of the reasons the peach leaf curl breakout this year in Central and Northern California is so vexing: even though gardeners may have applied a spray, there were very few consecutive 6-day windows of opportunity (non-rainy, sunny and windy).

There are peach varieties that are more resistant to peach leaf curl. The downside: they may not be as flavorful as you might like. Peach varieties reported to be more leaf curl resistant include Frost, Indian Free, Q-1-8 and Muir; among nectarines, only the Kreibich variety is resistant, says UC Davis.