Peach Leaf Curl - it's time to apply dormant spray
I hope you enjoyed a wonderful Thanksgiving day. Did you know that Thanksgiving weekend is unofficially the start of the dormant season and the time to start the dormant spray season? So if you have peaches or nectarines, this is the time to apply your first of three applications of a dormant copper fungicide. I will try to outline this process as simply as I can, however, I suspect this may be one of my longer notes for you.
What is peach leaf curl?
Peach leaf curl (plc) is a disease caused by the fungus Taphrina deforman. Plc affects most notably the leaves, causing a blister effect that will also discolor the leaf, as shown in the photo above. Plc can also affect the flower blossoms, the fruit, and the new branch shoots. When severe, it can reduce fruit production. Its reddish, distorted, puckered leaves first appear in the spring on the newly developing leaves. These distorted leaves may stay on the branches or fall off and are then replaced by a second set of leaves that will develop normally unless the weather is still cold and rainy. Here in California plc is extremely common because of our cool, wet springs, which are optimum conditions for this fungus to grow. Because of this, it is advised to apply a dormant copper fungicide three times throughout the dormant season; at Thanksgiving, at New Year’s, and then around Valentine's Day.
One thing to note, something I hear often enough, is that folks see puckered leaves on their apple, plum, or pear tree and think it is plc, when in fact is it aphids, as shown in this photo here. This is aphid damage and not peach leaf curl damage. A fungicide will not offer any control for aphids, however the horticultural oil will. Two very different problems, thus why properly identifying the pest is so important.
What is a dormant spray?
A dormant spray is typically referred to as a dormant oil, such as horticultural oil, or a dormant copper, such as a copper fungicide. The dormant oil is for managing the insects that are overwintering and the dormant copper is to manage any fungus that may be overwintering. Dormant sprays are applied at a more concentrated rate, a stronger mixing rate, onto a tree, shrub, or rose while it is in its dormant period — meaning in the winter months with no leaves on it. In some areas of the Bay Area, the temperatures can stay relatively mild, enough so that the leaves may stay on a plant while it is in its dormant stage. I have seen this many times with roses. If this is the case, then simply remove the lingering leaves by hand and dispose of them in the green waste can. With stone fruit trees, the leaves typically drop without fail, regardless of any mild temperatures. Applying a dormant spray is an extremely effective way to use pesticides as a preventative, understanding that you will be using less product since there is less surface area -no leaves on the tree.
What is a copper fungicide?
A copper fungicide is a very effective pesticide for controlling a variety of fungal problems commonly seen by gardeners, one being peach leaf curl. When buying a copper fungicide, there are different formulations of copper, different active ingredients, and with very specific application rates. Whenever we look to use a pesticide, always confirm that the pest, in this case the disease is on the label, and understand the mixing rate and application rate. This can be a bit tricky to decipher on the label so I will offer a few tips. The two most common copper fungicides that I see around are Liqui-cop by Monterey and Copper soap (copper octanoate) fungicide.
Liqui-cop by Monterey: this is copper ammonium complex, a fixed copper, and from my experience is typically only used as a dormant spray to manage diseases. According to the UCIPM website, it can be more effective if you include 1% of a horticultural oil with it. It will act as a sticking agent, helping the copper to adhere to the branches, as well as the additional benefit of controlling any aphids, scale insects, or mites that may be overwintering on your plants. This is the one and only time that I know of when you can mix two pesticides.
Copper soap (copper octanoate) fungicide: apply for dormant application prior to bud swell as a preventative. Copper soap fungicide does not need to be mixed with a sticking agent. Copper soap is sold in concentrated form, which is used as a dormant spray by mixing a concentrate with water according to the label or in a ready-to-use bottle for the spring through fall growing season. Common brands containing this fungicide include:
Copper Fungicide by Monterey Lawn & Garden
Bonide Cpt Jack’s Copper Fungicide
Ferti-lome Copper Fungicide
Another option is to use products containing a biofungicide.
Bacillus amyloliquefaciens strain D-747: this is a biofungicide that is applied as a curative at bud break and throughout the growing season. There are a few brands that have this biofungicide in it:
Complete Disease Control by Monterey
Bonide Cpt Jack Revitalize Biofungicide
Ferti-lome Fungicide 5
Arber Bio Fungicide
When mixing a concentrate
It is important to only mix what you are going to use. There are a few reasons for this; one, concentrates lack shelf stabilizers, so will not last or store well once mixed. It is intended that you use what you mix for best results. For another reason, it is not safe to store a pesticide in an unmarked bottle. This can be quite dangerous.
Here is a fantastic tip for mixing a concentrate pesticide
If you have no idea how much pesticide to mix up, start with water. Fill your sprayer with water to the measured volume mark. For instance, let's say you have a one-gallon tank sprayer. The label on the pesticide bottle lists a mixing rate of 2 tablespoons per gallon. You only have one tree that needs to be sprayed, yet you have no idea how much to mix up. Fill your one-gallon sprayer with just water. Spray the tree. Note how much water you sprayed. Let's say it was one quart that you applied. Now you know to only mix up one quart of pesticide. You refer to the label, it says 2 tablespoons per gallon, so you will need half of a tablespoon for a quart of water. Mix accordingly, and apply until you have used all that you have mixed.
When applying a dormant spray
Never apply a pesticide when there is rain, frost, or excessive heat within 24-48 hours. Never apply when there is a breeze of more than 5 miles an hour. Never apply when beneficial insects or pollinators are present (which on dormant plants you will not see, another benefit). And always wear PPE. Remember, even less toxic pesticides are not risk-free. Please protect yourself.
When is dormant season over?
The minute the flower buds crack and show the petal color. We have had mild winters when bud break was before Valentine's Day, so many were unable to apply a third dormant application of copper fungicide for plc. Once there is bud break the plant is no longer dormant. If you apply a dormant pesticide at this time, the flower buds could be damaged.
For pests or diseases other than peach leaf curl, you will simply apply a dormant spray once or according to the label. These instructions are specifically for peach leaf curl.
I hope you find this helpful. I will discuss what happens when we see peach leaf curl on our trees after the dormant season this next spring when it is relevant. If you have any questions about applying dormant sprays or your fruit trees, please don't hesitate to ask. I'd love to hear from you.
In the meantime, take good care and stay well!
~Suzanne