This is a tree question I was recently posed with:
[Friends want] to spray their fruit trees with dormant oil to prevent the leaves from curling especially the peach tree leaves. What are your recommendations? Will this spray help prevent this problem, should they do this, is it organic or toxic? Will it hurt other plants, do the growing beds need to be covered? We are having a meeting of garden reps. on Wednesday and would like to give them some info at that time, thanks, Erica
Thanks for writing. This is a really good question.
The quick answer is no, this is not the right time and that is not necessarily the right treatment, especially for peaches.
Peach leaf curl is a fungal disease. The fungus itself is called Taphrina deformans and as your [friends] know it can cause the leaves to curl, turn red perhaps, and ultimately fall from the tree very prematurely. Peach trees in their determination to survive tend to put out a second of flush of foliage which might look better but under such stress the tree doesn't have the same vigor or have the energy for strong flower and fruit production. The problem with fungal diseases is that you have to know the life cycle of the fungus or at least the right time to treat the plant. To control peach leaf curl the tree has to be treated with a fungicide in winter, and I am sorry to say now is definitely too late. Most guidelines say that the window for treatment is in fall after the tree has lost 90% of it's foliage or BEFORE buds have swollen in spring. With all this abnormal heat and the accelerated spring we have had I am sure the tree is too far along to still treat it - for in my own garden the crabapples are blooming a good 3 weeks early! The other catch is that many of the fungicides that one uses to combat this fungus have to be applied by a registered New York State pesticide applicator. Though we always keep herbicide/fungicide/pesticide use as the last resort it is very helpful to have an applicator in one's contacts, especially for some of these more serious disease issues. Luckily for us lovers of all things organic there is a product called Lime Sulphur which is supposed to be a reliable fungicide to use for peach leaf curl and is considered organic. I have attached a PDF about leaf curl you can pass along with this email and hopefully that will help educate people. We will try and be prepared so we can treat this problem next winter. For now your friends can only nurture the tree as best they can - water during dry spells, remove and garbage any affected leaves that fall, perhaps some light pruning to remove really bad areas or limbs that are too heavy and causing the tree additional, unnecessary stress.
Now a quick word on "dormant" oils. Horticultural oils and other products of the sort, often called dormant oils because of the time of treatment, are used more to treat pest issues and not diseases so it is important to make the distinction of what problem you are treating. The bad analogy is you don't want to waste the money or time going to a dentist when you should really be seeing the eye doctor. A lot of oils sold for gardeners are actually petrolium based, which many of us are not crazy about, but they are not really toxic to the garden. Others like Chrysanthemum oil, Neem oil, etc. are derived primarily from plants oils and are just as effective so you can find more organic or eco-friendly options out there. Obviously you always want to try and understand the ingredients of what you are buying and follow the directions explicitly. Either way the goal of the oil is to coat the insect so that they are suffocated and unable to live and keep doing damage, thus killing them and knocking them off the plant to decompose back into the soil. You can see that this type of product wouldn't be appropriate for most fungal diseases because it would only coat and help sustain unwanted spores and not necessarily help remove them.
With all this said, you mentioned other trees. Now, we all know that there are plenty of insect pests that cause leaf curl on different kinds of trees and shrubs so it is tricky making sure you have matched the right diagnosis to what can be similar symptoms. If some of the other plants are carefully inspected and found to have aphids, mites, etc. then perhaps a horticultural oil could be effective. But certainly you want to make sure it is a pest causing the damage if you can before treating. Most insects do go after the more fragile new growth and can themselves hide in the newly deformed leaves so encourage your friends to break out their hand lenses or magnifying glasses and keen observation skills.
I hope this helps. Keep me posted and we will be in touch.