Normal red color of newly developing shoots of ‘ohelo.The endemic shrub ‘ohelo (Vaccinium reticulatum) is a frequent understory component of upper elevation native forests and is particularly prominent in Hawai‘i Volcanoes and Haleakala national parks where it fills an ecologically important niche. Its fruit is also valued for local human consumption. Plants have long been known to show conspicuous reddening of leaves of particular branches. Since normal new growth of this species is also red, the condition usually has been overlooked as an abnormality. Upon close examination, however, the red leaves are seen to be produced on a witches'-broom rather than normal twigs, and red leaves and twigs die, rather than continue vigorous tissue development as would be the case with normal growth (Gardner, 1980b).
Earlier investigators who recognized the symptoms as abnormal but who were unfamiliar with the disease considered a mycoplasma-like organism (phytoplasma) as a possible cause, and treated symptomatic plants with oxytetracycline, to which phytoplasmas are frequently sensitive. However, the disease was later recognized as being caused by the basidiomycete Exobasidium, and referred to E. vaccinii, the cause of a similar disease of North American Vaccinium spp. (Gardner, 1985a, b). The disease also has been found on tree ‘ohelo (V. calycinum), which, as its name implies, is a large shrub or small tree. Infection of the later species is less common or conspicuous, however, than is infection of V. reticulatum.
Since the causal fungus was only tentatively referred to E. vaccinii, the possibility remains that the fungus represents a similar, but endemic species that has evolved with its host. The origin of the fungus, whether introduced or endemic, has implications for the management of the disease in a natural area. That is, if a disease of an endemic species is caused by an introduced pathogen, the management action probably would be toward the control or elimination (if possible) of the disease. On the other hand, if the disease were caused by an endemic pathogen, it should be preserved along with the host as part of the native system. Thus, a more thorough investigation should be directed toward establishment of the correct taxonomic identity of the fungus.
Normal red color of newly developing shoots of ‘ohelo.
Abnormal red leaf condition of ‘ohelo shrubs is sometimes
mistaken for normal new growth. However, the 'red leaf' condition is associated with
eventual death of leaves and twigs, in contrast to normal new growth which is vigorous and
healthy (above).
Witches' brooms, caused by abnormal twig proliferation, is frequently
associated with red leaf disease.
Comparison of a diseased branch, showing an
abnormally thickened stem and proliferation of twigs to form a witches' broom (left), with
a normal branch of 'ohelo (right).
A basidium (large arrow), as
viewed with a microscope, on the lower leaf surface of a leaf infected with Exobasidium
vaccinii (?), with four basidiospores (two of which are clearly visible, small arrow)
borne on sterigmata.