Ash yellows

Taxonomy

Domain	Bacteria
Phylum	Tenericutes
Class	Mollicutes
Order	Acholeplasmatales
Family	Acholeplasmataceae
Genus	Candidatus Phytoplasma

Scientific Name

Candidatus Phytoplasma fraxini

Common Name

ash yellows phytoplasma (AYP)

Compiled by Ned Tisserat, Colorado State University:

Hosts

Many ashes (Fraxinus species) are hosts to ash yellows. From most to least susceptible are white ash (F. americana), green ash (F. pensylvanica), and black ash (F. nigra). A similar disease caused by a related phytoplasma strain occurs on lilac (Syringa).

Importance

Ash yellows causes substantial growth reduction, decline, and mortality of white ash in the Northeastern United States. Green ash appears to be more tolerant of ash yellows. Symptoms similar to those of ash yellows have been reported in Georgia and Louisiana. Trees of all ages and sizes in landscape plantings, hedgerows, and forests are susceptible.

Diagnosis and Damage

The only reliable diagnostic field symptom is the presence of witches' brooms on the trunk and major limbs. Leaves on the brooms tend to be small, simple and chlorotic. Branches of brooms do not have a dominant shoot but are made up of many shoots (it is important that normal sprout growth on the trunks and large limbs called epicormic sprouts not be mistaken for witches' brooms). Small roots or leaves collected from symptomatic trees can be tested for the presence of the ash yellows phytoplasma. Over time there is a permanent reduction in tree growth. After several years of minimal growth, leaves often will only appear in tufts at the ends of branches. Leaves may fail to attain normal size and appear chlorotic. Branch dieback may occur following dormant periods. Infected trees do not recover. Ash yellows apparently reduces the ability of white ash to recover from other stresses, such as drought, stressful sites, etc. and often exhibit-slowed growth and dieback. However, infected green and black ashes tolerate infection in many instances without progressive deterioration. Many other factors may be involved with declining ash trees, including insect borers, over watering and other environmental stresses.

Biology and Disease Cycle

The causal organism for ash yellows is unnamed, because it has not yet been isolated and characterized. It is known that the disease is caused by a phytoplasma, a prokaryotic organism, similar to a bacteria, but lacking cell walls. Insects such as leafhoppers, planthoppers and psyllids vector most phytoplasms. Infected white ash appears to be more susceptible to other stresses such as drought.

Control

Conditions that cause stress to susceptible host trees will favor disease development. In forestry, merchantable white ash with ash yellows dieback should be harvested. Infected landscape trees without severe dieback may respond favorably to fertilization and irrigation.

Gallery

References

• Solomon, J.D.; Leininger, T.D.; Wilson, A.D.; Anderson, R.L.; Thompson, L.C.; McCracken, F.I. 1993. Ash pests: A guide to major insects, diseases, air pollution injury and chemical injury. Gen. Tech. Rep. SO-96. New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station. 45 p.