Origin

Candidatus Phytoplasma fraxini were first identified as causing the symptoms of ash yellows in 1971. The late identification was due to the difficulty of detecting these organisms, and the similarity of signs and symptoms to other environmental detriments to the tree such as drought, salt spray, poor soil quality, fungus, parasites etc. Ash yellows disease has only been identified in North America; its origin is otherwise unknown.

Life Cycle

The disease is caused by Candidatus Phytoplasma fraxini, a microorganism thought to be transmitted to the host tree by leaf hopping insects. The phytoplasma invades the phloem sieve tubes, a component of the tree’s vascular system.    

Little else is known about the life cycle of Ash Yellows. This is due to the difficulty of diagnosis, lack of documentation and inability to replicate to phytoplasmas in the laboratory setting. 

Ecological threat

The ash yellows disease causes the gradual decline of growth in a variety of ash tree species in North America. This disease affects twelve ash tree species, though it seems to affect the white ash species most severely. The impact on tree health varies. Some infected trees decline and die within 5-10 years. Others survive indefinitely, appearing healthy or showing only a loss in growth.