The original article was written for Hong Kong Gardening Society.
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| Michelia spp. from nursery |
It was a moist week when one of my clients called me to inspect his
sick-looking White Jade Orchid Tree / White Champaca (Michelia x alba 白蘭) tree. The species is popularly planted in public
parks or private gardens locally. People
love the strong flower scent in early summer while the tree may still be very
small in size. Michelia x alba is
cultivated by grafting, so it would flower at the time when you get it from a
nursery. The real Champaca (M.
champaca 黃蘭) can be grown from seed in the contrary.
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| Aphids and sooty mold |
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| Shiny and sooting covers on leaves |
Michelia x alba grows quite fast,
however. Its wood breaks relatively easy
too. Sometimes owners may heavily prune the
trees (though not properly) because the crowns are thoughtto be too big and
dense as they mature. Under such
condition or when the site environment is too stuffy, occasionally a white waxy
or wooly layer may come to cover the underside of the leaves. That white wax first concentrates along leaf veins
or young shoots and affects mainly the sheltered part (usually the bottom) of
tree crown. Gradually the affected leaf
surface and the surrounding may become sticky and shiny, followed by black dusty
appearance. The affected trees will look
dirty. The leaf colour will become pale
or even turn yellow. There will be
pre-mature leaf drop as the situation gets worse and the whole tree will appear
declining.
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| Mealy bugs on bark |
The situation of my client’s tree showed similar symptom. A closer inspection of the branches and leaves
revealed the activities of rather stationary insects, including mealy bugs and
aphids. The former creates and carries
white wax at the back, while both of them secrete honey dew which results the
sooty mold. The two pests come from the
same insect suborder Homoptera (under order Hemiptera - the true bug). Members of this taxon are all sucking in their
diet habit and many are important pests to crop production. Mealy bugs and aphids do harm trees as well,
although trees – if healthy enough - usually recovers better from the
infestation. Nonetheless, the pests can reproduce
quite rapidly on the hosts under favorable condition and if the population is
massive, health of the hosts or the nearby trees can be seriously impacted. The affected parts may further extend to the
bark of major branches or tree trunks. Other
than the two Michelia species, Elephant Ear Tree (Macaranga tanarius
血桐) is also a popular host for such Homoptera ‘pandemic’.
Eradication of these pests is not easy, but it helps if we can improve
tree health and the site ventilation. Spraying
oil-base or soap-base pesticide to the infected parts direct can suffocate the
insects and get down the population.
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| Readily closing flowers in the morning |
It is not recommended to apply systemic pesticide while a tree is flowering
as this may also kill pollinators. Michelia
belongs to the plant family Magnoliaceae, a primitive plant group that is
believed existing on the earth much earlier than some other common pollinators
(e.g. honey bees and butterflies). Michelia
spp. (and many Magnoliaceae species) blossom at night and attract beetles,
which feed on the flower parts, by scent instead of colour. This also explains why most flowers of Michelia
x alba close at daytime, but we surely can still enjoy the sweet odor which
lasts quite long!
