Forest Pests: Insects, Diseases & Other Damage Agents

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.

Identifying the Disease. - Symptoms vary with host susceptibility and disease progression, but in general, initial symptoms are reduced radial and shoot growth. Sustained infections can cause branch dieback, thin chlorotic crowns, epicormic sprouting, bark cracks, early fall coloration, and premature death. These decline symptoms, however, can develop from other causes. Witches' brooms are definitive, although inconsistent, symptoms of ash yellows.

Identifying the Pathogen. - Identification is based on finding MLO's in stained sections of phloem using a microscope. Tentative field diagnosis can be made by the presence of witches' brooms.

Biology. – Ash yellows is caused by MLO's that inhabit phloem tissue. MLO's are similar in nature to bacteria but lack cell walls. While infections are systemic, it is unclear how MLO's enter ash trees; insect vectors are one possibility. Infected white ash appears to be more susceptible to other stresses such as drought.