Page 97 - Tài liệu Ebook cây cảnh Bonsai Basics

HEAL THY BONSAI 99

in a hot, humid atmosphere. Rust
growth is commonly encouraged by an
excess of potassium.

Black spot
Black spot is another formidable fungus,
which mostly attacks the leaves (elms
being particularly susceptible). The leaf
becomes progressively covered in black
patches, until it eventually shrivels and
falls off.
Preventive treatment with a general
fungicide is usually sufficient to deter
black spot. If you discover it has at-
Black spot is a fungal disease which attacks the
tacked your plant, use a sulphur or leaves of the plant.
maneozeb based product.

Chlorosis
This is one of the commonest deficiency
diseases, caused exclusively by a short-
age of iron in the plant nutrients. The
leaf blade or needles turn yellow, while
the veins usually remain green. Chlorosis
generally occurs in chalky or limy soil,
which locks up the iron. The best remedy
then is to change the potting compost
when repotting. Iron can also be given in
liquid form, by adding sulphate of iron
to the water when watering.

Some treatments should be given as a powder, rather
Other deficiency diseases than a liquid spray. Here, a powder dispenser.
Potassium deficiency can kill off the
leaves, which turn yellow at the edges,
shrivel and finally drop. Your choice of fertilizer should take
Shortage of nitrogen hinders photo- into account the precise needs of the
synthesis resulting in the leaves turning individual tree, with special reference to
pale, with brownish patches appearing the growth cycle of the tree.
on the stalks of plants.
Magnesium deficiency can depress
flowering, which becomes rather sparse. Mildew
This deficiency also causes a slight This fungal disease shows itself as a
yellowing of the leaves. greyish mould on the undersides of the
Phosphorus deficiency also restricts leaves and on the sepals of the flowers,
flowering and the leaves, though remain- marking them with a brown halo.
ing green, fall prematurely. Good ventilation and careful watering
All these deficiency diseases can be usually suffice to put an end to this
easily cured by a balanced feeding pro- trouble. If this is not enough, use a
gramme. This is why the choice of the suitable fungicide, preferably in powder
right fertilizers is so important. form, to destroy the fungus.