PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Mundulla Yellows
MUNDULLA YELLOWS
From Forest Health News 148, February 2005
Finding the cause of a new disease when there is no easily
recognisable primary pathogen can be a lengthy process. Possible
agents must be critically evaluated and many are eventually
eliminated as information steadily accumulates. One such disorder
is Mundulla yellows, a mysterious ailment of many species of
eucalypts and some other species of native flora in Australia.
Symptoms are expressed as a progressive yellowing of the foliage
followed by dieback of the crown, and eventually the death of
badly affected trees. Leaves exhibit a characteristic interveinal
yellowing or chlorosis, which usually starts in the new growth
and spreads to older leaves. These symptoms often begin on a
single branch before spreading to multiple limbs. New, initially
healthy, recovery shoots (referred to as epicormic ‘panic’ growth)
may be produced within the tree crown, and this is followed by
reduced flowering and seed set. Progression through the various
stages may take several years or even decades, ending ultimately
in extensive dieback and tree death.
Mundulla yellows is acknowledged as a real threat to the natural
biodiversity of many wildlife habitats in Australia. It was named
after the town of Mundulla in South Australia where it was first
described in 1975, but symptoms have now been found in all
States. Since investigations into the disease began fungi, bacteria,
and nematodes have all been eliminated as the cause of the
problem. Researchers then turned their attention to microscopic
agents that cannot live or replicate outside the host plant, such as
viruses, viroids (virus-like agents), and phytoplasmas (minute
bacteria-like micro-organisms lacking a normal cell wall), and it
was soon discovered that a viroid was frequently present within
the affected leaf tissues. However, such an association is not proof,
and to establish that an agent present within the host is the actual
cause of a disease requires a rigorous series of steps to test its
virulence or pathogenicity. In the case of Mundulla yellows
different groups of researchers have reported a variety of results
and the role of this viroid in the development of the disease remains
a matter of contention. Other possible causes have been
considered, including the likelihood that the symptoms may
develop after the roots on affected trees have absorbed certain
herbicides applied to control weed growth. One multidisciplinary
group of researchers in Victoria and South Australia have identified
some factors common to sites associated with the disorder. They
found a strong and consistent relationship with high pH alkaline
soils, low levels of iron and manganese in the soil and plant, and
high rates of carbonates and chloride in the foliage. They postulate
that habitat modification may be the primary cause of the
condition. Many parts of Australia are experiencing serious
vegetation problems due to increased soil salinity. This is a result
of rising water tables following the change from the natural
landscape of woodlands and shrublands to pasture and agriculture.
Thus 30 years after the recognition of Mundulla yellows as a new
disease the definitive cause or causes have not been unequivocally
established. Research is continuing but it may still be some time
before there is a final answer to the cause of Mundulla yellows.
For further information, visit sites such as:
http://www.deh.gov.au/biodiversity/invasive/publications/mundulla-eucalyptus/
http://www.burkesbackyard.com.au/1999/archives/25/in-the-garden/weeds-and-garden-pests?p=2394
(Margaret Dick, Forest Research, from information provided by
Ian Smith, Department of Sustainability and Environment,
Forest Pathology, Heidelberg, Victoria, Australia)
