Apple proliferation might sound like a bumper crop of Granny Smiths, but it’s far more sinister than that. Sadly, I couldn’t find any images I could use.
Apple proliferation (AP) is a devastating plant disease that can result in crop losses of up to 80%. It is spread by apple proliferation phytoplasmas (Candidatus Phytoplasma mali). There are quarantines for this pest in Canada and the U.S. While AP mainly attacks apple trees, it also occurs on Asian and European pear, hazelnut, and plum trees, along with dahlias, hawthorns, magnolias, and roses.
What are phytoplasmas?
Phytoplasmas are a type of bacteria known as mollicutes. Mollicutes are unique in that they do not have cell walls. Instead, they have a multi-layered membrane. Phytoplasmas were discovered in 1967. Most phytoplasmas contain a single mysterious protein. Studying these 1 μm life forms has been difficult since no one has been able to grow them in a lab to date. For reference, the plastic wrap in your kitchen drawer is probably 10 μm thick.
How the disease spreads
Phloem-feeding insects such as cherry leafhoppers, planthoppers, and psyllids are responsible for spreading apple proliferation. AP can also spread through natural root grafting. Natural root grafting occurs when the roots of different trees end up pressed against each other. Over time, the vascular tissues of both root systems come into contact with each other and begin sharing nutrients. There is debate about whether or not this is a survival mechanism. We may explore that another time. For this discussion, those natural root grafts can spread disease.
Research has also demonstrated that infected trees emit unusually high amounts of a chemical that actively attracts psyllids, increasing the spread of disease to neighboring trees and shrubs.
Apple proliferation symptoms
Symptoms of apple proliferation change with the seasons because the phytoplasmas migrate within the tree. In the winter, they head south into the root system. As temperatures rise and the sap moves upward into twigs and leaves, so do the phytoplasmas.
Symptoms also vary depending on the species infected and how long the infection has been present. This condition may appear in only some branches. It may disappear altogether for a couple of years before returning. No one knows why, but it may be related to the fact that there are more and less virulent strains of this particular phytoplasma.
The most common symptoms of apple proliferation are small tasteless fruits with longer stems, witches’ broom, leaf rosettes, enlarged leaf stipules, shortened petioles, and dwarfism. Other, less common symptoms include increased suckering around the tree base, chlorosis, and downward leaf cupping. Leaves may also be smaller than normal and brittle. In autumn, the leaves of infected apple trees will turn red rather than yellow.
There is no known cure for apple proliferation, so prevention is the only option. Infected trees must be removed and destroyed. Luckily, there are APP-resistant rootstocks, so look for that when tree shopping. And if apple proliferation is occurring in your region, do your best to control those psyllids and leafhoppers.
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