Agriculture Reference
In-Depth Information
field conditions, because the pathogens are transported in the droplets of water
which form on the equipment, tools, and skin.
e.
Chemical Control
Chemicals used to control plant disease pathogens include fungicides, bac-
tericides (disinfectants), and nematicides. The general term "fungicide" is often
used to describe chemicals which combat fungi and bacteria. Fungicides may
be classified as protectants, eradicants, and systemics.
Protectants
must be applied before or during infection by the pathogen.
In order to be effective, they must either persist or be applied repeatedly.
Most chemicals now available to combat plant diseases are protectants.
Eradicants
are less common and are applied after infection has occurred.
They act on contact by killing the organism or by preventing its further
growth and reproduction.
Syste mics
are used to kill disease organ isms on livin g plant s. Sys-
temic chemicals are transported in the sap stream from the application site to
other plant parts. This type of chemical may act as both a protectant and an
eradicant.
Successful chemical control of plant diseases requires proper timing.
Plant disease control on some crops must begin
before
infection occurs.
The protectant chemical must be applied when environmental conditions are
expected to be ideal for the development of plant pathogens. If the protectant
is not applied in time, major crop damage may result or the application of the
more expensive eradicant sprays may be needed. Label directions often call for
routine protectant applications every 7 to 10 days during periods of prime in-
fection risk. Almost all plant disease control chemicals are applied as cover
sprays. The purpose is to reach and protect all potential sites of infection.
IV. WEEDS
Any plant can be considered a weed when it is growing where it is not
wanted. Weeds are a problem because they reduce crop yields, they increase
costs of production, and they reduce the quality of crop and livestock products.
In addition, some cause skin irritation and hay fever, and some are poisonous
to man and livestock. Weeds also can spoil the beauty of turf and landscape
plants.
Weeds harm desirable plants by:
•
competing for water, nutrients, light, and space
