basics of plant diseases
Such wilt diseases are unlikely to
occur in soilless media where the
pathogen would not be present. In a
few plants that we know of, the pathogen is seed-borne, including spinach
and basil.
Diagnosing Problems from
Symptoms and More
Diagnosing a plant problem may be
done based on symptoms alone in
some cases. Take the fusarium wilt
disease, for example: a single tomato
plant in your garden suddenly wilts in
spite of adequate irrigation. Sometimes
the wilting occurs on only one side of
the plant and if you slice downward
into the stem of that branch, you may
see brown streaks, which could indicate
a vascular wilt disease. A cross section
of the stem might also show the browning in spots. Most tomato plants from
nurseries have built-in resistance to
fusarium wilt, but you may be growing
an older variety that does not have that
resistance. Sending your wilted tomato
to a diagnostic lab where the pathogen
can be isolated and identified would be
the clincher in diagnosis.
What if your plant wilts, but there are
no tell-tale symptoms in the stems that
would point to fusarium wilt? Check
the roots and see if they look healthy
or diseased. If enough of the roots look
rotten, that may be enough impaired
root function to cause the plant to wilt
when water becomes limited. If the roots
look good, then the cause may be due to
some cultural problem, such as too litt le
water. On a side note, it is well known
that plants that have mycorrhizal fungi
in their roots can withstand soil drought
much better than plants without.
Root examination might also reveal that
the roots are not so brown, but they have
swollen areas or knots on them. That
would be a tell-tale sign the plant has root
knot nematode infection. Nematodes
are tiny worms that infect the roots to
complete their life cycle, and in so doing,
disrupt the root function enough to
cause stunting and even wilt.
Take another example like tomato
leaf spot. Your tomato plants looked
good in the early part of the growing
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season, but
then spots started
to appear on the leaves.
You are sure your overhead sprinkler
system is working to keep the plants
well watered, but the disease continues to spread to more leaves as the
plants grow—and it is spreading fast.
Your plants could have fungal leaf
spot caused by the fungus septoria.
When leaves become infected, the
fungus produces many new spores
that splash onto other leaves when the
sprinklers come on. The spores germinate, penetrate the leaf tissue and
begin to kill the cells. Eventually the
tomato fruit will also have those spots.
This disease is on a rampage, and
only two things will slow it down. First
of all, change your irrigation system
from overhead sprinklers, as standing
water on the leaves allows the spores
to germinate and begin infection.
Wilt of a tomato plant caused
by the fungal pathogen
fusarium. Left: foliage wilt
symptoms. Center: vascular
browning that blocks water
transport. Right: Cross
section of infected stem
showing brown vascular
tissue where water transport
is impaired.
“If enough of the roots
look rotten,
that may be enough impaired root
function to cause the plant to wilt
when water becomes limited.”