The best time to treat a stressed plant may be just before
you
can see it’s stressed.
The trick is to know when that is. And a University of Georgia
scientist may have the
answer.

It’s a matter of light, said Chi Thai, a biological and
agricultural engineering
scientist with the UGA College of Agricultural and Environmental
Sciences.

Plants, he said, are trying to tell us when they’re stressed.
We just can’t see it.

It’s not that we’re not trying. Seeing that plants are
stressed earlier than we
normally do could save farmers millions of dollars. We just
don’t
have the right
equipment.

Human Eyes
Limited

“Human eyes can see only at light wavelengths between 400
and 700
nanometers,” Thai said. “For the part of light that
gets through the atmosphere,
the region of interest to our plant-health research is from
ultraviolet wavelengths around
200nm to near-infrared wavelengths around 2500nm.”

The answer, as Thai saw it, was to improve the equipment.
“We’ve built a
field-portable spectral imaging system,” he said. “It
contains two spectrometers
sensitive from 300nm to 1700nm and is equipped with fiber optic
inputs and continuously
tunable spectral video imaging capability.”

Thai adds some Liquid Crystal Tunable Filters, then a
virtual-reality goggle (because a
computer monitor is unwieldy in the field) to see plants in
literally a whole new light.

Reading Plants’ Chemical
Signatures

“This equipment allows human users to visualize chemical
signatures of a plant
which usually are beyond unaided human vision,” Thai said.
The chemical signals let
people see the plant’s earliest signs of stress.

Thai took the process a step further, with an LCTF
alternating
between two fixed
wavelengths, 692nm and 755nm. By plotting the average of the
gray
values of the pixels
forming the plant canopy image, he can directly estimate the
chlorophyll and biomass
amounts in plants.

“The plant-health status was related to the degree of
brightness of the
image,” Thai said. “A brighter plant is a healthier
plant.”

Thai’s initial, encouraging research was on Bahia grass and
bush beans. Now he’s
studying peanut plants inoculated with Tomato Spotted Wilt
Virus.

The biggest trouble so far has been getting the virus to
infect the plants. “For
some reason, what happens naturally very easily in the field has
been hard for us to do in
the lab,” he said.

Research Could Help
Growers

If the studies work, though, it could greatly help growers of
the state’s $400 million
peanut crop, said Albert Culbreath, a CAES plant pathologist in
Tifton, Ga.

“It would help first in research,” Culbreath
said. “It would be a
nice tool for evaluating plants in greenhouse and small-plot
studies.”

Many plants don’t show symptoms right away but turn yellow and
wilt late in the season.
“Such a tool might enable us to know what’s really going on
in those plants,” he
said.

“In epidemiological studies,” he said,
“detecting infection before
symptoms occur might help pinpoint when management actions might
be taken to reduce the
… disease.”

Many Potential
Benefits

Spectral imaging could help evaluate varieties and management
practices, too. If it
could help farmers assess disease problems faster than they can
by seeing symptoms, it
could help even more.

“If we could come up with a chemical signal for assessing
problems that could be
detected from a few feet above or a mile overhead,”
Culbreath said, “it could be
of economic value.”

It may help farmers boost their yields, he said. And even if
it can’t, it may be able
to better predict losses, which would help growers in their
marketing.

For the time being, all the scientists really know is the
potential.

“We don’t know what reality can be, or how much
benefit,” Culbreath said.
“There’s much work to be done, but new tools such as this
can be of great importance
in the future.”