A peach tree disease that I frequently hear about is peach leaf curl. We happen to have a lot of peach trees here in Austin, Texas, so I thought it’d be a good idea to get more familiar with this common disease and dive into some research. Here’s what I found about leaf curl and how to treat it.
Leaf curl is a common and highly infectious fungal disease that affects peach, nectarine, almond, and apricot trees. The disease attacks the leaves of the tree, causing them to discolor and curl. While infected leaves can’t be saved, spraying after leaf drop in the fall is the best method of control.
So, while peach leaf curl can be discouraging and devastating, how can we identify it, and better yet—what are the most effective treatments? Let’s take a closer look.
How to Identify Peach Leaf Curl
|Peach Leaf Curl
|Initially red-purple and wrinkled, followed by yellowing, browning, and/or dropping early. Grey powder can develop from spores.
|Grey powder from spores.
|Wilted. Drops shortly after infection. However, blossom infection is unlikely.
|Red, shiny, and bumps or warts. Drops shortly after infection. However, fruit infection is unlikely.
Generally, you can identify peach leaf curl by leaves that become red-purple and wrinkled. Over time, these leaves develop grey spores before turning yellow or brown and then dropping. While rare, some symptoms may show on the fruit and blossoms. However, curled and discolored leaves are the better indicator.
Leaf curl requires the following conditions.
- Cool and wet weather
- Bud break or leaf emergence
- 10-11 hours of constant wetness
Peach leaf curl first occurs in cool and wet weather, often in the early spring. These conditions help the fungus spread from leaf to leaf. Normally, rain spreads the spores from the top down, while irrigating spreads the spores from the bottom up.
Temperatures between 50ºF to 70ºF contribute to the most spread of leaf curl. However, temperatures 45ºF and lower have been shown to weaken the fungus to a dormant state. 45ºF is also the same temperature required for peach tree dormancy (also called chill hours).
After meeting these temperatures, and once the tree goes through bud break or leaf emergence, their immune system becomes compromised. At this point, leaf curl typically requires about 10-11 hours of constant wetness or moisture for infection to take hold (source).
Once the tree is infected, the spores spread from leaf to leaf, damaging them and weakening the tree. The tree then has a hard time photosynthesizing energy from the sun, weakening it further and reducing leaf and fruit growth. This gets progressively worse and can result in the tree dying if left untreated.
Damaged peach trees are also more susceptible to other diseases and winter injuries. This is a reason why pruning or wounding the tree further isn’t a good idea (more on this later).
Let’s now take a look at the treatments for peach leaf curl (both organic and chemical).
How to Treat Peach Leaf Curl
The most effective method to treat peach leaf curl is by applying a copper fungicide after leaf drop in the fall. Since the spores survive winter by staying dormant on the tree’s trunk and branches until spring, spraying the entire surface of the tree in the fall has the highest rate of success.
The bad news is that if your peach tree is currently infected, there’s nothing you can do for its current leaves.
The good news is that any mature leaves that have not been diseased have missed the infection window and will not become infected.
Also, peach trees are deciduous, so they naturally drop their leaves in the fall. As long as the tree stays alive, this gives them a good chance to grow new leaves in the spring without the disease.
To help with this, spraying your peach tree’s trunk and branches entirely in the fall (after leaf drop) with a fungicide will provide the best chance of avoiding leaf curl.
Leaf curl is not difficult to control. Since the fungus survives the winter on the surface of twigs and buds, a single fungicide spray, thoroughly covering the entire tree, will provide control.Michael A. Ellis, Department of Plant Pathology, Ohio State University
However, if you’d like to be extra safe, applying another round of spray before bud break in the early spring can help kill any spores that many have survived the first spray in the fall.
Before we look at the most effective sprays and methods of control, let’s first address the common myths when treating peach leaf curl.
Since leaf curl is such a common disease for peach trees, there’s a lot of growers doing trial and error. While this method helps rule out ineffective methods and uncover effective ones, some solutions haven’t stood the test of time.
Here are the most common myths when treating peach leaf curl:
|Prune diseased leaves
|Pruning doesn’t help, and can be harmful
|Collect infected leaves off the ground and burn them
|Other leaves are already infected
|Infected leaves can be saved
|Infected leaves cannot be saved
|Fertilize the peach tree with a high nitrogen fertilizer
|Fertilizer might help
The biggest myth out there is to prune the diseased leaves off of the peach trees. It’s thought that by removing these leaves, we’re protecting the leaves that have not yet been infected. However, several sources from Universities say otherwise.
The leaves are infected in the bud; once they have emerged they are no longer susceptible to infection.Mark Longstroth and Bill Shane, Michigan State University Extension
Since the leaf curl infection occurs when the leaves are young and budding, pruning and removing any infected leaves generally has no effect. This includes picking up and discarding fallen infected leaves.
Some people remove diseased leaves or prune infected shoots, but this has not been shown to improve control. Normally, diseased leaves fall off within a few weeks and are replaced by new, healthy leaves, unless it is rainy.J. C. Broome, Plant Pathology, UC Davis/UC Cooperative Extension, C. A. Ingels, UC Cooperative Extension, Sacramento Co.
There’s also a myth that applying a high nitrogen fertilizer will provide peach trees with a boost of nutrients and help fight off leaf curl. In my research, I didn’t find conclusive evidence for or against this.
Some sources said the high nitrogen fertilizer helps prime the peach tree to grow new leaves faster in the spring, growing fast enough to avoid most of the infection window (since infection occurs when budding). Other sources said the heavy nitrogen can stress the tree more, leading to other complications such as killing the tree and the soil, and promoting fungal growth.
Given the increasing trend of growers are switching from chemical fertilizers to organic, and the evidence against chemical fertilizers, I personally would steer away from using a high nitrogen fertilizer on peach trees.
If you’d like more information about chemical vs organic fertilizers, I recently wrote a post that goes into more detail: Is Manure Good for Fruit Trees (And Which Kinds Are Best)?.
Another busted myth is that infected leaves can be saved if we spray enough fungicide on them. The truth is that once the leaves are infected, no fungicide treatment will effectively work (source).
So, given all of these myths, what can we actually do to help combat leaf curl?
Copper Fungicide Spray
Copper is used in many applications for disinfecting bacteria and fungi, and not just for gardening. Since copper is the active ingredient in copper fungicide, the higher the copper, or MCE (metallic copper equivalent), the more effective the spray will be.
While copper fungicide is the most effective method to treat leaf curl and it’s technically “organic”, it still has its downsides.
Copper fungicides work to kill pathogen cells by denaturing enzymes and other critical proteins. However, copper can also kill plant cells if absorbed in sufficient quantities.Gordon Johnson, Extension Vegetable & Fruit Specialist, University of Delaware
Aside from potentially causing damage to the plants, copper fungicides are known to be toxic and should be used with caution.
High amounts of copper are also known to damage:
- Other Trees
Repeated annual use of copper fungicides and products is not recommended as it can result in a buildup of copper in the soil. This can become toxic to beneficial soil life, plants, and livestock, and get carried through the waterways—potentially harming aquatic life.
If you do use a copper fungicide, avoid applying it while the tree’s leaves are wet as they’re more likely to absorb the copper and become damaged (source). Also, avoid spraying close to any rainfall as the rain can wash the spray off the leaves.
Bordeaux is a combination of copper sulfate, lime, and water, and, when mixed in the correct order, provides long-term protection to plants against bacterial and fungal diseases. This spray sticks to plants better than copper fungicides and can even be used when there’s rainy weather.
While Bordeaux mixtures are effective, they also come with their fair share of disadvantages.
Cons of using Bordeaux Mixture:
- High pH
- Salty Deposits
- More Phytotoxic Than Copper
- Incompatible With Most Pesticides
- Highly Corrosive
Overall, copper sprays are easier to prepare and have fewer consequences than Bordeaux mixtures (source).
However, even with their downsides, both copper and Bordeaux sprays are classified as organic and can be used in organic commercial operations (source).
Lime Sulfur Spray (Restricted Availability)
Lime sulfur spray is an effective treatment for leaf curl, but in 2008 it was discontinued for home growers due to being highly dangerous and corrosive when in contact with skin. It has since been available to commercial growers with pesticide licenses, with some brands being on and off available for home growers over the years (source).
Now, let’s take a look at the natural solutions to reduce and prevent leaf curl.
Resistant Peach Varieties
Getting peach trees that are naturally resistant to leaf curl is one of the best and most natural methods of preventing this fungal disease.
Here are the recommended peach tree varieties that naturally resist leaf curl:
- Indian Free
- Kreibich (Nectarine)
Keep in mind that while each of these trees is resistant to leaf curl, it doesn’t mean that they are immune to it.
Essential Oil Spray
I’ve always been skeptical about the use of essential oils (it probably doesn’t help with all of the MLMs), but I’ve been seeing universities and other studies backing their effectiveness on plants, so I had to take a closer look.
Here are just some of the studies that I found:
- Black caraway, fennel, and peppermint essential oils were confirmed to inhibit fungus on plum fruits (source)
- Dozens of plant oils were shown to have anti-fungal properties and promising uses in reducing fungal infections when applied to other plants
- A peer-reviewed study from 2003 tested the effectiveness of essential oils on tomato plants with fungal infections. They found that oregano, thyme, lemongrass, and cilantro completely inhibited the growth of the fungus (source)
- Essential oils reduced fungal contamination on cheeses by 50%, and are encouraged to be used agriculturally (source)
I also wrote about how essential oils can help treat the fire blight fungus. You can check my post out here: Fire Blight Treatment: Non-Organic & Organic Solutions.
Since essential oils have gained interest only recently, other studies are still in the works. Because of this, even though I couldn’t find information regarding the use of essential oils specifically for leaf curl, many current studies are supporting the use of essential oils for fungal infections on plants.
However, while essential oils may seem promising, one of the biggest downsides is that they have a limited shelf life and don’t last long after being sprayed on the plant. This makes their production and uses as a natural commercial fungicide difficult.
So, if you’re not a commercial peach tree grower and instead have a small orchard, spraying your peach trees with essential oils after leaf drop in the fall and before bud break in the spring can likely work to reduce or prevent leaf curl.
As with other sprays, avoid spraying when the tree is wet, or shortly before the rain. Oregano, thyme, lemongrass, and clove have been consistently shown to have anti-fungal properties and are likely good choices to use on your peach trees.
For example, I’d recommend using 6-7 ml of oregano oil mixed in a 16 oz bottle of water then spraying the tree from the trunk to the top as well as each branch.
Whey (lactoserum) is a slightly acidic liquid that is made when the milk is curdled for cheese and is a safe and effective treatment for many fruit tree diseases. Whey is natural and sticky, so it adheres to the surface of leaves and bark easily.
To see whey spray in action, and to learn more about it, check out this cool video by Stefan Sobkowiak – The Permaculture Orchard.
More Tips to Prevent Peach Leaf Curl
- Avoid splashing soil when watering peach trees. If splashed, any potential fungus in the soil can be introduced to the leaves and spread from there. As a best practice, only water the soil, not the tree. Watering with drip irrigation and providing 2-6 inches of mulch, such as straw, works well.
- Provide compost instead of chemical fertilizers to reduce the chance of stress and chemically burning the tree’s roots. Applying 2 inches of compost on top of the soil every 1-2 months is a good practice.