Dodonaea Yellows
A‘ali‘i (Dodonaea viscosa: Sapindaceae) is a pantropical shrub or
small tree indigenous to Hawai‘i, where it is an important component of the native
flora. I first recognized what I have called the Dodonaea yellows disease
in Hawai‘i Volcanoes National Park (on the island of Hawai‘i) in 1977 (Gardner,
1980b; Gardner and Kageler, 1984). Although the disease probably had been observed
prior to that time, no record of its occurrence has been found. Since the time of
its discovery, symptomatic plants have been found elsewhere on Hawai‘i and wherever
the host has been found on other islands of Hawai‘i. Although no data are
available, my observations suggest that the disease has become more prevalent in
Hawai‘i Volcanoes N. P. during the years since its discovery, but as indicated, the
history of the disease in Hawai‘i is not known. Correspondence with plant
scientists and resource managers on other Pacific islands has not revealed any record or
notice of this disease in the Dodonaea populations of those islands. The
origin of the disease is not known, whether endemic or introduced, nor is the means of
transmission known. My coworkers and I have made attempts to compare it with the
sandalwood spike disease reported from India. This phytoplasma disease has been
reported to occur on a Dodonaea host there as well as on sandalwood.
However, the resemblance of Dodonaea yellows to sandalwood spike is still in
question.
Dodonaea yellows is characterized by stunted yellow leaves, distortion of leaf
margins, severe internodal elongation of branches and twigs to produce drooping
witches'-brooms, and eventual death of infected tissue, possibly including the entire
plant. Flowering and fruiting may be diminished or absent on symptomatic
branches. Entire plants may be affected, or the symptoms may occur on individual
branches of otherwise normal-appearing plants.
Among my coworkers, former graduate student Dr. Wayne Borth, of the University of
Hawai‘i, investigated the etiology (cause) of Dodonaea yellows as his
dissertation project and found evidence of both a virus and a phytoplasma (formerly
referred to as a mycoplasma-like organism) associated with symptomatic plants (Borth et
al., 1988, 1990, 1992, 1995; Borth and Gardner, 1993). The means of disease
transmission is the subject of current research, as is the possible roles of both the
virus and the phytoplasma in producing the symptoms.
Although phytoplasmas are suspected to cause certain crop diseases, the phytoplasma
associated with Dodonaea yellows disease is the first such organism demonstrated
to be associated with a plant disease in Hawai‘i.
Well-developed witches'-brooms of the Dodonaea yellows disease.
Note the cluster of yellow, red-veined leaves of reduced size (left), and the
"hairy" appearance caused by twig proliferation in the defoliated broom (right).
Various manifestations of Dodonaea yellows
disease in Hawai‘i Volcanoes National Park. The disease may affect isolated
branches or twigs on otherwise normal-appearing shrubs, or the entire plant may exhibit
symptoms.
Although the disease is slow-acting, it may eventually lead to the death of
the entire plant. These photos are of the same plant, showing the progression of the
disease. The photo on the right was taken approximately 18 months after the photo on
the left was taken.
Symptoms include abnormalities in leaf and twig
development. Leaves of the green, but infected branch (left) have wavy margins and
are less uniform appearing than those of the healthy twig on the right. Shoots of
the diseased twig also proliferate irregularly.
Plants may become symptomatic at a very young age, as shown by this
seedling, growing in a‘a lava, with diseased branches, the lowermost (arrow) being
the most prominent.
Because the means of transmission is not yet known,
laboratory study necessitates finding young diseased plants in the field and transplanting
them to the laboratory. Such relocation may eventually result in the loss of some
symptoms, such as yellowing.
Well-developed witches'-brooms of the Dodonaea yellows disease.
Note the cluster of yellow, red-veined leaves of reduced size (left), and the
"hairy" appearance caused by twig proliferation in the defoliated broom (right).
Although the disease is slow-acting, it may eventually lead to the death of
the entire plant. These photos are of the same plant, showing the progression of the
disease. The photo on the right was taken approximately 18 months after the photo on
the left was taken.
Symptoms include abnormalities in leaf and twig
development. Leaves of the green, but infected branch (left) have wavy margins and
are less uniform appearing than those of the healthy twig on the right. Shoots of
the diseased twig also proliferate irregularly.
Plants may become symptomatic at a very young age, as shown by this
seedling, growing in a‘a lava, with diseased branches, the lowermost (arrow) being
the most prominent.
Because the means of transmission is not yet known,
laboratory study necessitates finding young diseased plants in the field and transplanting
them to the laboratory. Such relocation may eventually result in the loss of some
symptoms, such as yellowing.