Etiology of Arizona ash decline

The epidemiology and etiology of a disease of Fraxinus velutina Torrey (Arizona Ash) called ash decline (AD) is discussed. Symptoms of AD include a progressive dieback of stems and branches as well as yellowing and necrosis of leaf tissue. The incidence and severity of AD in F. velutina cv. 'Modesto' trees in Tempe, Arizona has been monitored since 1985. There has been a 30% mortality rate in the study group over an eight-year period; symptom severity has increased dramatically during this time. Etiological studies determined that a mycoplasmalike organism (MLO) was the causal agent of AD. MLO's were detected in the phloem sieve tube elements of symptomatic plants using both the Dienes' and DAPI staining techniques. MLO's were detected in stem tissue from healthy branches, branches with dieback symptoms, witch's brooms, and roots, but they were present in the highest level in branches with dieback symptoms. The polymerase chain reaction (PCR) was also utilized to detect MLO rDNA in ash tissue. Ash DNA extractions were subjected to PCR reactions at 35 cycles in order to amplify a 558 bp fragment. A comparison of restriction fragment length profiles of the 558 bp product indicated that the MLO which causes AD was not related to the MLO which causes ash yellows in F. americana L., a disease in the eastern U.S. with similar symptomology. The AD MLO was closely related to the western X-MLO group. A statewide survey of cultivated and native F. velutina trees found that AD symptoms and MLO infected trees occurred throughout the state of Arizona, but only trees of the F. velutina 'Modesto' cultivar were severely affected by AD.