Apple Tree disease and treatment?
Hi there. We have two dwarf Honeycrisp Apple trees that bloom prolifically, but produce only a smattering of fruit, most of which the squirrels get to before the apples are even ripe. We’ve been good about pruning, but not so disciplined with dormant oil treatment, which we are committed to remedying this year. When I examined one of the trees today, I see several areas on the trunk that appear to be diseased (photos attached). Can you ID these growths for me please and let me know how/ what I can do to treat the problem? Thank you! Lise
This intriguing growth is called burr knot, and is a form of adventitious root growth. (Adventitious means root tissue growing from plant tissue that doesn't normally develop roots.) Many of our common fruit trees are grafted, and certain rootstocks have been known to trigger this over others. Nothing can be done about this growth and it may just have to run its course; because it is not a disease, no treatments will be effective. Some sources say this afflicted wood can become weak and more prone to breakage in stormy weather, but we suggest not pruning this off as it will remove too much of the tree's growth and may recur anyway since it can be triggered by the rootstock.
If Honeycrisp is the only variety of apple you are growing, consider adding another variety, as all apples need cross-pollination (even "self-pollinating" varieties) to set good fruit. This would be the simplest explanation for a poor harvest; weather conditions not conducive to pollination during blossoming would be another. Make sure the trees are receiving good care (not too much fertilizer, as too much nitrogen can interfere with fruiting, for example), protection from wounding (such as from lawn equipment or improper pruning cuts, as this invites disease), and irrigation during dry spells. Dormant oil is a good preventative treatment, but timing may depend on the disease or insect pest in question and weather conditions. Knowing what ailments the tree suffered last year will direct treatments for this season.
Keeping squirrels away from ripening fruits is harder. Sometimes they are simply seeking a moisture source during dry weather; in that case, providing even just a birdbath as an alternative may help. Squirrels are notoriously troublesome, however, and this may not be enough. You could try one of the commercially-available fake owls or snakes as scare devices, though they would need to be moved around the yard often to avoid the squirrels realizing it's not a threat. (Even this may not work after they get accustomed to it.)
Subsections of this page may help outline proper apple care and preventive measures throughout the growing seasaon: https://extension.umd.edu/hgic/topics/apples