Is there a treatment that can be used on freshly pruned cuts of apple trees to prevent disease?
Question:
This is a question regarding fruit tree pruning.
Is there a treatment I can use on freshly pruned cuts to prevent apple scab from taking advantage of the fresh wound?
I’m a beginner, hobbyist fruit tree grower, mainly apples. I have some 2 to 3 year old trees myself, but am also trying to reclaim some very overgrown old trees. I’ve been gradually pruning them back to a more accommodating shape, but I think they’re burdened with scab. The cuts I made last year have a mild to moderate blackened appearance. Regular spraying as an orchard would do is probably not an option, but an early season spray or two is. Removing the old trees are not an option. I believe they can be salvaged.
Do I even need to worry about it? Should I paint the wound? Spray with a fungicide? I’ve read that cobalt fertilizer can systematically reduce scab burden. Or maybe spot treat with a cobalt spray?
Answer:
Abi Griffith, Horticulture Community Education Assistant
In general, it’s best to leave the tree unpainted. Here’s an excerpt from our bulletin Pruning Woody Landscape Plants: “Tree paints and wound dressings should rarely, if ever, be used by a homeowner. Recent research has shown that these materials are rarely beneficial. In fact, they may sometimes prevent plants from successfully compartmentalizing wounds. Sometimes arborists use wound dressings if they are pruning at a time of year when a specific insect or disease organism is active.”
Additionally, apple scab is a disease of the leaves and fruit, not of the twigs and branches.
Below is a description of its life cycle taken from a fact sheet on Apple Scab from the PennState Extension:
“The apple scab fungus (Venturia inaequalis) overwinters in infected leaves that have fallen to the ground. Fruiting bodies are produced within the dead leaf tissue. As spring approaches these begin to mature and produce spores (ascospores; primary phase) that are discharged into air currents and carried to developing apple buds. The fruiting bodies in the fallen leaves must be wet for the spores to discharge. The ascospores are not all discharged with the first spring rains, for they mature over a 4- to 6-week period. This period usually coincides with the time that elapses from ¼ inch green until 2 to 3 weeks after petal fall. Mature ascospores peak from late pink through petal fall, which is the most critical time for protection during the primary phase of the disease. especially if ideal environmental conditions favoring disease are present.”
So, keeping this in mind, the most important cultural control for this disease is preventing fruiting body formation in fallen apple leaves, where the fungus overwinters. Rake up and burn or bury or shred up to encourage leaf decay now if you hadn’t already in the fall (additional cultural controls can be considered here: fact sheet on apple scab disease – scroll down).
If you are seeing dead tissue where the old pruning cuts were, it is from something else. My colleague has suggested you consider fire blight, as we’ve been seeing a fair amount of it the last few years. That could have easily been spread from cutting one infected branch to other cuts. Review the photos in these fact sheets about apple scab and fire blight to see if you’ve noticed any of the fire blight symptoms on any of the trees. It’s likely you could also have apple scab, unless you have been spraying or they’re resistant varieties. Cleaning the pruners between cuts is recommended if there is fire blight in the trees. For management info, see this fact sheet, Fire Blight (PDF) from Colorado State University Extension.
This site from UMaine Extension has additional links to information about apple pests and management.