Agriculture Reference
In-Depth Information
Mycoplasmas are the smallest known independently living organisms.
They can reproduce and exist apart from other living organisms. They obtain
their food from plants. Yellows diseases and some stunts are caused by my-
coplasmas.
4.
Nematodes
Nematodes are small, usually microscopic, eel-like roundworms. Many
nematodes are harmless. Others attack and feed on plants grown for food, feed,
ornamentals, turf, or forests. Some species attack above-ground plant parts,
such as leaves, stems, and seeds. But most pest nematode species feed on or in
the roots. They may feed in one location, or they may constantly move
throughout the roots. The root-feeding nematodes directly interfere with water
and nutrient uptake. Typical host plant symptoms include stunting, yellow-
ing, loss of vigor, and general decline. Nematode damage may go unrecog-
nized or be blamed on something else, such as nutrient deficiencies.
All nematodes that are parasites on plants have a hollow style which they
use to puncture plant cells and feed on the cell contents. Nematodes may de-
velop and feed either inside or outside of a plant. They move with an eel-like
motion in water, even water as thin as the film of moisture around plant cells
or soil particles. Their life cycle includes an egg, several larval stages, and adult.
Most larvae look like adults, but are smaller.
In adverse conditions, the females of some species, such as root knot and
cyst nematodes, form an inactive, resistant form called a cyst. The cyst is the
hard, leathery, egg-filled body of the dead female. It is difficult to penetrate
with pesticides. Cysts may provide protection for several hundred eggs for as
long as 10 years.
Figure 3.10
shows the life cycle of a typical nematode.
B.
DIAGNOSIS OF PLANT DISEASE
Attempting to control plant diseases without sufficient information usu-
ally results in failure. For maximum effectiveness, the first step is to diag-
nose the disease correctly.
Diseased plants may be recognized by comparing them with healthy
plants. Knowledge of normal growth habits is necessary for recognition of a
diseased condition. To identify the cause of plant disease, you must observe:
•
symptoms—reaction of the host plant to the disease agent
•
signs—actual evidence of the presence of the disease agent
Many plant diseases cause similar
symptoms
in the host plants. Such things
as leaf spots, wilts, galls on roots, or stunted growth may be caused by many
