Ash Yellows This disease is relatively new so the disease cycle is mostly unknown, although it is thought that insect
vectors are used as the mode of transmission. The disease is difficult to diagnose because some symptoms match those of stressed environmental conditions such as drought, flooding, or shallow soil. When trees become infected they can survive for many years or immediately die depending on the environmental conditions and health of the tree. The major symptom of this disease is
witches broom which causes branches in tufts. Unfortunately, there are not many strategies for controlling the disease besides avoiding areas where it is more prevalent.
Host and symptoms Ash yellows refers to the disease that occurs on
ash trees, usually white ash (
Fraxinus americana) and green ash (
Fraxinus pennsylvanica). All ages and sizes of ashes are vulnerable to infection and symptoms will occur within three years of infection. Some of these symptoms include progressive loss of vitality, subnormal growth, and leaves that fail to reach normal size and are often light green to
chlorotic. Additional symptoms include branch dieback, cracks in the bark, early color change in the fall, and premature death of trees. This disease is often diagnosed by the presence of a distinct symptom known as
witches broom. Witches broom occurs when there is an overgrowth of the branches of the host, resulting in a clumps of branches that resemble the head of a
broom. This usually occurs near the soil line but can occur higher up in the tree as well.
Disease cycle Not much is known about the disease cycle of ash yellows. It is unknown how exactly the
Phytoplasma enters the tree and how it spreads but insect vectors are suspected. One likely candidate for spreading these
Phytoplasmas, also known as
Mycoplasma-like organisms (MLOs), is
leafhoppers because data shows they are the most common insect vector for MLOs. Additionally, the severity of the disease symptoms caused by the
Phytoplasma varies. In one study that was conducted, 12 different strains of
Phytoplasmas belonging to the ash yellows group were tested and it was found that these strains varied significantly in aggressiveness and the impact they had on growth of the host.
Environment The disease typically occurs in woodlots and forests, home landscapes, and urban settings. The geographic origin of ash yellows is unknown but currently the disease is only reported to be in
North America. One major environmental condition that has been shown to amplify the growth of the disease includes drought. One final control method is to use
insecticide, however it is unclear if the use of insecticide to control leafhoppers is an effective way to prevent the spread of the
Phytoplasma that causes ash yellows. Within the ash populations that were sampled, 50% of the trees had crown die-back of 10% or more caused by ash yellows. This study also found that disease incidence was lowest in the two populations where other plant species were present. In addition to this ash trees also provide food and habitat to a number of animals including
cardinals and
wood ducks. Symptoms of the LWB
Phytoplasma include
witch's brooms, shortened
internodes on new growth, twig dieback, overall loss of vitality and premature death. ==See also==