Agriculture Reference
In-Depth Information
Diseases
Diseases are caused by bacteria, fungi, viruses and other organisms, such as
mycoplasmas and viroids.
Bacteria
These organisms are the cause of moist foul-smelling wounds, necrosis, galls
and wilting, because they can infect the vascular system and clog it. Bacteria
can be controlled with antibiotics such as streptomycin, but these products
are fairly expensive. The measures to reduce their appearance and progress
are to select propagating material that is clean and avoid excess moisture.
Disinfecting any tool before using it on another plant and destroying any
diseased material by burning are other simple techniques.
Fungi
Fungi are the most common cause of plant diseases. For a fungal attack to be
successful it will depend on the status of the plant; the presence of mechanical
breaks in the cuticle, the plant's susceptibility to the fungal strain, the plant's
stage of development; the fungal development stage and the environmental
conditions. Relative humidity of 90-100% is optimal for fungal development,
with dew sometimes creating favourable conditions.
All fungi need a certain combination of temperature and relative
humidity to become a problem; hence many fungal diseases can be a serious
problem in some situations and seasons but not in other conditions. Fungal
diseases can be avoided through resistance in a few cases. Well-fertilized
plantings with a good drainage system and careful irrigation to avoid excess of
water will reduce the risk of root diseases. Disinfection of pruning and other
wounds, control of host weeds and spraying with specifi c fungicides all help
in controlling diseases. Preventive applications of fungicides are often used
with some crops, such as papaya against anthracnose, where all stages of fruit
development are present versus application only at certain stages, such as
during mango fl owering.
Fungicides act via contact or systemically, with many products being
available in the market. In most cases, fungicides have to be rotated to avoid
resistance development; this is especially acute for postharvest use. It is
important to check the toxicity of the product and the waiting time before
harvest. Fungicides in general are less toxic than insecticides, and in the last
years several new products have been developed that are more ef ective and
also friendlier to the environment. Several older products are still available
that are cheap and ef ective. Examples include copper sulfate, copper oxide,
Bordeaux mixture (a copper derivate), sulfur and some oils.
 
Search WWH ::




Custom Search