basics of plant diseases
“Some foliar diseases
are obvious when you see symptoms
of infected leaves compared to non-infected leaves.”
Keeping leaves dry during the day will
stop those new infections. Next year,
or next time you start a tomato plant,
avoid overhead irrigation. Secondly,
clean up all of the fallen, infected
leaves on the ground as they will be
the source of next year’s infection.
Some foliar diseases are obvious
when you see symptoms of infected
leaves compared to non-infected
leaves. For example, some maple trees
grown in landscapes are susceptible to
powdery mildew. This fungus disease
causes superficial infections on leaves,
but the tell-tale symptom is the production of white, powdery masses
of spores on the infections. Some
infected landscape plants, however,
lack the abundant sporulation that
would distinguish the problem as
powdery mildew. The powdery mildew
pathogen, and some other diseases
caused by fungi, such as rusts and
downey mildew, cannot be cultured in
the lab, so diagnosis has to be done by
symptoms alone and observing spore
production under the microscope.
So, what if your plants do not exhibit
any of the tell-tale symptoms discussed
above? Perhaps most of your plants
show the same stunting or yellowing
discoloration, or maybe the plants are
still green but are not growing as well
as you would like. Assuming there have
been no environmental extremes, one
should consider irrigation and watering
practices. Over-watering can damage
the roots enough to impair their function; under-watering can simply cause
the plant to experience drought stress.
During drought stress periods, plants
are not able to function well, and
growth can be slowed as a result.
What about fertilization? Too little
fertility in the soils could deprive
the plant of needed nutrients. For
example, the needed phosphorus in
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many soils is unavailable because it is
bound to clay particles or has formed
an insoluble precipitate. That means
phosphorus is immobile in the soil.
Once again, the solution is to add
more fertilizer to meet your plants’
needs, or inoculate your plants with
mycorrhizal fungi at planting. These
symbiotic fungi colonize roots and
help the plant acquire phosphorus
from more distant places and pipe it
back to the plant.
Left: Powdery mildew on
maple leaves.
Right: Septoria leaf spot on a
tomato plant.
Next time
The next installment
of this series on
plant diseases
will discuss the
pathogens
themselves.
Pathogens are
grouped as
fungi, bacteria, viruses
and nematodes, and I
will describe
the life cycles
of these agents
with enough
detail for you to get
a sense of the enemy
and its potential to cause
plant diseases. The most
likely pathogens to cause diseases in
gardens are fungi or bacteria.
“Over-watering can damage
the
roots enough to impair their function;
under-watering can simply cause the
plant to experience drought stress.”