February 2023 - 23

PLANT HEALTH
quite right. The first test of
whether you are dealing with a
disease or pest is to determine
if it's nearly all the plants or
just a few. If it's nearly all the
plants, then the likelihood
is low that it's a disease or
pest. That's because things
like bugs and fungi take time
to grow their population and
spread, and they are generally
picky, choosing just one plant
type at a time.
Next, think about timing:
if it appeared over just a few
days, then the likelihood is
low that it's a disease or pest,
for the same reason. Things
like flooding and nutrition
issues may take time to show
up on a scale we can notice,
complicating the timing clue,
but the percentage affected
will still be high, guiding us
toward abiotic
stresses.
BUGS VERSUS
DISEASE
Assuming we
figured out by
distribution and
timing that the issue
is biotic, we now
have to determine if
it's likely a disease
problem or bug
infestation. Remember
when I noted that
pathogens are picky - attacking one type of plant at a
time usually? Well, bugs (insects and mites) may attack
a few types of plants at the same time, but the symptoms
will be similar across all the plants affected. With
bugs, we look for insect feeding damage, leaf or flower
distortion, spider mite webbing, and frass and sooty
mold, which tell us insects are feeding. Here is where the
zoom function on your phone or a magnifier of some sort
can be really helpful.
FUNGAL, BACTERIAL OR VIRAL
Since I am a plant pathologist, I have to assume you
have ruled out nutritional or other abiotic issues by timing
and distribution, and you
haven't found any evidence
of insect issues. That leaves
us with plant diseases. To get
to a management tool, we
have to decide if the disease
is likely viral, bacterial, or
fungal. Generally, I hope
it's fungal because we have
effective tools for many fungi
if we catch the disease early
enough, although for all
diseases, prevention is the
cheapest and most effective
management tool.
Fungi and bacteria tend
to start and spread in wet
areas - places with shade,
near sprinklers that keep
nearby plants wetter than
the rest of the crop or create
splash zones, low spots, or
where plants are packed
together, maintaining a humid
area within the canopy.
Viruses often get going either
sporadically throughout a
seedling or transplant crop,
which tends to indicate a seedborne
issue, or near air intakes
and doors, where insects might gain
entry to a greenhouse for instance,
and along traffic areas where people
might touch many plants in a row.
Fungi are bullies - they can
punch through major leaf veins,
so they are less restricted in their
growth and create round, targetlike
spots. They can also create tough structures that
survive in soil, in pots and on benches. Bacteria are a little
wimpier and wetter, so we tend to see that they cause
water-soaked spots with angular edges, defined by major
veins or starting at hydathodes and anywhere water
collects on the plant - edges, tips and petioles.
Virus symptoms are often just weirdness - mosaic,
spotty, twisted, and distorted, and often the newest
growth is affected the worst, as the virus builds up in
the plant.
FINDING THE RIGHT INFORMATION
The ag college in your state hires Extension faculty
like me to help you find the right solution for your plant
GREENHOUSE PRODUCT NEWS 23
Leaf damage
caused by
the Rugose
spiraling
whitefly.
February 2023

Table of Contents for the Digital Edition of February 2023
