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Ash Yellows

Phytoplasma fraxini

Ash Yellows

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Ash Yellows is a plant disease caused by a bacteria called Candidatus Phytoplasma fraxini. The disease affects Ash, impacting White Ash the most severely. In addition to this, the bacteria can cause Lilac Witches' Broom in several Syringa species. Currently, not much is known about this disease and how its life cycle plays out. It's proposed that leafhoppers and spittlebug insects play some role in its transmission, but more search is needed to confirm whether that is the case. Ash Yellows can produce symptoms in trees which may easily be mistaken for other plant problems. Before a tree is diagnosed with Ash Yellows, mechanical damage, drought stresses, insect infestations, and parasitic fungi must be ruled out. Ash Yellows can also make trees more prone to secondary infections, making tree health deteriorate more rapidly.
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Identification

White Ash is most severely affected. Leaves turn yellow, and twig growth is reduced. The crown may experience overall thinning, ultimately leading to significant branch dieback. The trunk-crown and branches may develop witches broom. Symptoms in Green Ash are less severe. Trees will develop witches broom at the base of the crown.

Symptoms

Witches broom
Leaf yellowing
Branch dieback
Reduced rooting system
Susceptible trees die within 1-3 years

Biological treatment

It's advised to remove the tree once 30-50% is lost through dieback.

Lifecycle

The life cycle of this bacteria needs more research. It's thought that spittlebugs and leafhoppers may play a part in the transmission between plants.

Prevention

Grow resistant species. The more tolerant the tree, the longer it will survive following the infection. More resistant species include Fraxinus pennsylvanica ‘Bergeson,’ ‘Dakota Centennial®’, ‘Patmore,’ and Fraxinus americana ‘Autumn Applause.’ When tending to trees, ensure they're well-watered during periods of dry weather. Try to avoid planting Ash in areas which experience severe drought. Plant Ash with other genera.

Affected plants

Ash

Fraxinus spp.

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