Page 181 - Tài liệu cây cảnh Bonsai4me Bonsai Basics

are kept indoors. The flies themselves are no more than an irritation, however

their grubs feed on the root system of the bonsai. Fungus Gnats are drawn to
overly wet soils, particularly if they contain moss. Though the Gnats are simple
to kill with the use of insecticides or household fly sprays; it is also important to
improve the condition and drainage of the soil as well as ensuring that the soil is
not kept permanently wet.

Viruses and Fungi

Viruses are most commonly detected by the presence of leaves or flowers
that are distorted or discoloured, growth can be stunted abnormally and the plant
can dieback. Treatment should include removal of all infected growth. Plants

suffering with a virus should not be grouped together with other plants of the
same Genus as to avoid cross-contamination and tools should be sterilised after
use. Mildew, rust and black spot are all common to weak and stressed trees. If a
tree is affected by these diseases it is important to try and discover the cause of
the underlying weakness in the bonsai itself.
Mildew is a fungi that thrives in damp, poorly ventilated conditions causing
the presence of a white mould to form on foliage. The fungi extract sap from the
host plant causing loss of vigour, distorted growth and dieback. The fungus
overwinters in buds so that young foliage emerges in Spring already infected.
Spores are produced that can be spread to healthy foliage via water; hence
mildews can spread quickly during warm, rainy periods. Confusingly though,
whilst water droplets can aid the dispersal of the mildew spores, water stress
brought on by lack of water to the root system in hot weather, reduces the natural

resistance of the tree to infection.Once affected, it is not possible to rid a leaf of
mildew. Infected shoots and leaves should be removed as soon as possible and
healthy foliage should be sprayed with fungicide to prevent further infection.
Rusts are fungal diseases that cause raised, brown or orange areas to develop
on the underside of leaves (which can sometimes be seen from above the leaf)
particularly on Beech and Birch species. Rust is not only unsightly but causes
loss of vigour to the plant. As with Mildew, Rust is dealt with by removing
affected leaves and applying fungicide, again, good air circulation will help trees
avoid infection.
Black Spot on Chinese Elms can be prone to developing clusters of black
spots less than 1mm in diameter on the surface of their leaves. The foliage then
goes on to yellow and drop. This is caused by a virus known as Black Spot. As
with mildews and rusts, once a leaf is found to be infected, it must be removed to
halt the spread of the disease. Care should be taken not to spray the foliage as