Ash and Elm Yellows in Utah
Few people are aware of yellows diseases in these important urban trees. This esoteric group of diseases gets its common name from the general yellowing of affected plant foliage.
Yellows diseases are caused by mycoplasma-like organisms, similar to those that cause malaria. And like malaria, these diseases are vectored by insects, in this case leafhoppers. Because few people are aware of yellows diseases and diagnosis can be difficult, we know little about their distribution in Utah. Unlike fungi, which can be grown on agar media, mycoplasma-like organisms cannot. We confirm their presence by using molecular tests. All of these factors make field diagnosis or confirmation very difficult and time-consuming.
Ash yellows was first found in the eastern U.S., where it spread to north central and northeastern states, and southern Canada. It Utah, it was found on velvet ash by Dr. Wayne Sinclair while vacationing near Zion National Park in 1994. (He is co-author of the very useful book Diseases of Trees and Shrubs. If you don’t have a copy, I highly recommend it.) At the time, he found a high incidence of the disease, but few severe symptoms. It probably arrived to the State on infected nursery stock. White ash and green ash are most susceptible, and 10 additional ash species have been shown to exhibit symptoms.
Symptoms on ash include slow growth, chlorosis, reduced apical dominance, witches’ brooms, and a slow debilitating decline ending in death. Witches’ brooms, while characteristic, rarely appear until the tree is near death.
Elm yellows is commonly found in the eastern U.S., and sporadically in the northeast. It has not been confirmed in Utah via molecular testing, but I believe it does occur in the State. This disease, previously known as “elm phloem necrosis,” affects American, slippery, and winged elms, while Siberian elm is somewhat tolerant.
Infected trees have branch yellowing, wilting, and dieback. The interior wood often shows a brownish stain that may be mistaken for Dutch elm disease, though more “butterscotch” in color, and more spotted in appearance. Those with a keen sense of smell can detect a wintergreen odor coming from elm yellows-infected tissue.
I have seen this disease on a few occasions in Logan, where it has caused elms to appear generally unhealthy. One American elm located in Logan that I was monitoring appeared to have Dutch elm disease, which is caused by a fungus. I sampled the woody tissue showing the typical vascular discoloration on several different occasions in an attempt to isolate the pathogen for my forest pathology class. Both attempts were unsuccessful; I thought I was losing my touch. The city ultimately cut the tree down, and the following spring, a witches’ broom formed on the stump – an Aha! moment for me! The tree most certainly was infected with elm yellows. I have been observing another tree with similar symptoms. This tree is dwarfed compared to the other trees planted at the same time, and shows early fall color each year.
The vectors for elm yellows in Utah are unknown, and we have no idea how widely distributed it is in the State.
Yellows diseases are assuredly present in Utah, and are possibly overlooked for other diseases or abiotic causes. It is important for Utahns to be aware of these diseases to elm and ash species to avoid purchasing infected nursery stock, and to avoid wasting resources treating these diseases as a nutrient or water deficiency problem.