April 2020 - 34

technically speaking
BY ERIK RUNKLE AND JAN BYRNE
The Importance of Plant Diagnostics
What's wrong with my crop? That's one of the most common questions
for Extension educators. While some growers can be hesitant to talk about
production challenges, others realize that early and correct diagnosis of
a problem can save a lot of frustration, money and time compared to no
action - or even worse, taking a wrong corrective measure. Successful
diagnosis can make the difference between a marketable crop and one in
which some or all of the plants have to be thrown away.
Some growers are reluctant to submit a sample to a diagnostic lab
because of the time-sensitive nature of the crop production, where time
is money. In this situation, the grower could seek a diagnosis, and pursue
corrective action in the meantime. It also can be helpful to email images
of the symptoms to the diagnostic lab while also sending a physical
sample (however, photos alone are
usually inadequate). This gives the
diagnostician a " heads-up " that the
sample is coming, and allows time to
ask relevant questions.
Diagnosing a crop problem usually
necessitates gathering a lot of
information on the crop history and
culture, and some of that desired
information is below. Obviously
knowing which crop(s) is affected
is important, because some crops
are particularly sensitive to specific
environmental, cultural, insect and
disease problems.
Symptoms. What are the
symptoms, how prevalent are they,
and is the problem getting worse?
Gaining an understanding of the
specific symptoms, and how they
may have changed over time, is
very important to understand the problem. Is the problem only with a
particular crop, or are multiple cultivars and species impacted? Did the
problem suddenly appear (if so, when?), or has the problem gradually
gotten worse over time? Are most or all plants of a cultivar/species
affected, do the symptoms appear at random within the crop, or is there
a pattern to the problem? For example, are affected plants only along the
edge of a greenhouse wall, primarily under hanging baskets, or clustered
in a certain area?
pH and EC. What is the pH and electrical conductivity (EC) of the
growing substrate? A common production problem is related to the
watering and fertility of a crop. Crops have different tolerances to
substrate pH - some do best at a pH of 5.4 to 6.0 (those in the " petunia
group " including calibrachoa, bacopa, dianthus, etc.) while others
perform best at a pH of 6.0 to 6.6 (those in the " geranium group "
including marigold, lisianthus and New Guinea impatiens). Some crops
grow best at lower fertility rates (a low EC), and leaves may curl when
the fertility is too high. In contrast, some plants need higher amounts of
macro- and/or micro-nutrients for healthy growth.
34 APRIL 2020 GPNMAG.COM
Oftentimes, a
combination of two or
more factors leads to a
crop problem, which
makes diagnosis more
challenging.
While knowing the pH and EC helps with a diagnosis, more information
may be needed if the problem is suspected to be nutritional, especially
for micronutrient-related concerns. In this case, sending a sample of the
growing substrate (or sometimes plant tissue) to a lab for nutritional
analysis can be very informative.
Pesticides/PGRs. What pesticides and growth regulators have been
applied, when, and at what rate and volume? One of the possible causes
of a crop problem is phytotoxicity from application of, or an excessive
amount of, a chemical. If the grower suspected a plant disease was the
problem, it's useful to know the chemical history of a crop, especially in the
past few weeks. If a pathogen is suspected, a diagnostician may attempt
to culture it to identify the bacteria or fungus. It is best to avoid making
pesticide applications to plants
immediately prior to submitting
them for diagnostics. If a pesticide
application is necessary, collect a
small group of symptomatic plants
prior to making any applications.
Environment. What have been
the environmental conditions? In
particular, what are the day and
night temperature setpoints, and
has the greenhouse been cooler or
hotter than desired? Some crops
are more susceptible to chilling
damage than others, while other
plants don't tolerate heat as well,
especially if plants experience
drought stress. In addition to
temperature, it can be useful to
know about the light environment
as well as humidity conditions.
Oftentimes, a combination of
two or more factors leads to a crop problem, which makes diagnosis
more challenging. The more information about the crop history and
symptomology, the more accurately and quickly the problem(s) can be
identified. With an early and correct diagnosis, recommendations can
often (but not always) be made to overcome the problem. Unfortunately,
some problems cannot be overcome (such as herbicide damage), or may
be so severe that it's best to discard the plants.
While there is usually a small diagnostics fee, the amount is trivial
considering potential consequences of ignoring the problem or making
a wrong corrective action. In addition, an early and correct diagnosis
can limit the incidence and severity of the problem, and may eliminate
unnecessary pesticide applications. In the U.S., visit www.npdn.org to
find a diagnostic lab near you.
Erik Runkle is professor and floriculture Extension specialist in the
Department of Horticulture, and Jan Byrne is plant pathology diagnostician
in the Plant & Pest Diagnostics Lab at Michigan State University. Erik can
be reached at runkleer@msu.edu.
http://www.npdn.org http://www.GPNMAG.COM
April 2020

Table of Contents for the Digital Edition of April 2020
