Uncovering the Mysteries
of Gray Mold
A strawberry rachis completely
engulfed by a gray mold fungus,
|If you love strawberries, you've
probably seen it. You pick out a luscious, ripe berry from the basket, only to
turn it over and see a mound of yucky gray fuzz.
The fuzz, caused by gray mold (Botrytis sp.), ruins more than just
strawberries. Over 23 species of Botrytis reduce yield, soften fruit, or
affect color in a wide range of small fruits and nursery crops. In the Pacific
Northwest alone, the mold causes up to $125 million per year in crop losses.
Researchers at ARS' Horticultural Crops
Laboratory in Corvallis, Oregon, have discovered new characteristics of the
mold and powerful new control approaches that may help growers reduce
"Diseases caused by gray mold are among the most difficult to
control," says ARS plant pathologist Walter F. Mahaffee. That's because
the mold can remain dormant for long periods, waiting for environmental
conditions to turn favorable. Botrytis grows well on dead or dying plant
tissue, such as leaves, then spreads to live parts of the plant. It reproduces
prolifically and produces spores at all stages of its life.
On this monitor, the Botrytis cinerea
fungus on a plant specimen is easy
to spot for research associate Caroline
Press, of Oregon State University, and
ARS plant pathologist Walter Mahaffee.
The fungus glows green, thanks to
green fluorescent protein technology.
| Recently, Mahaffee and colleagues at
Oregon State University (OSU) in Corvallis discovered a new clue about
Botrytis' success: The mold can also live as an epiphyte. That means
mold spores germinate and grow unnoticed on the surface of leaves and other
plant parts. That allows it to be present constantly until the perfect
conditions arise for it to infect the plant and cause disease. This epiphytic
growth appears to be why the disease spreads so rapidly.
"I'd look at a leaf before going home in the evening and it would look
pretty healthy," says Mahaffee. "Then I'd come in the next day and
two-thirds of the leaf would show signs of infection. That's a lot of area to
be covered very fast."
Mahaffee (left) and Tom
Verhoeven, president, Peoria
Gardens, Inc., discuss uses
of biological control agents
to manage gray mold.
| He discovered that, in reality,
Botrytis had completely colonized the leaf surface epiphytically. Then,
when the time was right, the mold infected the leaf at multiple sites
simultaneously. Mahaffee found that the mold could move from one leaf hair to
the next without actually touching the leaf tissue itself.
"That type of spreading could reduce the efficacy of pesticides," he
says, "because it would reduce the mold's contact with the residues on the
This finding was made possible by green fluorescent protein (GFP) technology.
(See "Jellyfish Gene
Lights Up E. coli," Agricultural Research, March 2000,
"By using this technology, we could watch the development of a single mold
spore over time under the microscope," says Mahaffee. "We can also
use a different GFP to mark a biological control agent and watch how the two
organisms interact in real time. That's a first."
Botrytis cinerea sporulation on
a ripe strawberry.
| Taking Different Tacks
This work suggests new avenues for Botrytis control. "If we can
determine the conditions that allow the mold to live in this epiphytic state,
we may be able to make it harder for it to survive," says Mahaffee.
Growers use fungicides and biological control agents to keep the mold in check.
But Botrytis quickly develops resistance to pesticides. Available
biocontrols can help prevent infection, but they don't get rid of
Botrytis once it is established.
Mahaffee's team recently found a new bacterium that may lead them to better
biocontrols. A strain of Burkholderia, the bacterium eradicates even
established gray mold on geranium leaves in the laboratory. Unfortunately, the
bacterium is related to bacteria that can cause health concerns for cystic
fibrosis patients. Although that is likely to preclude its development into a
commercial biological control agent, it still gives the scientists new
strategies to pursue.
"We may be able to identify the genes responsible for the bacterium's
effectiveness and search for other bacteria that have similar genes. Or we may
be able to move the genes into a harmless biocontrol organism," says
Mahaffee. Another option: The researchers might be able to harvest the active
compounds produced by the bacteria and use them to develop a pesticide.
But the most exciting discovery about the bacterium is that it forms a filmlike
cluster of cells as it grows.
"This film seems to protect the bacterium from adverse conditions, like
rapid or extreme changes in moisture or temperature," says Mahaffee. He
and OSU plant pathologist Caroline Press found they could enhance this biofilm
production by spraying the organisms onto the plant in a mixture of natural
polymers already used as food additives.
"Adding polymers to the Burkholderia gives the same biological
control of Botrytis, but at a much lower bacterial concentration,"
Mahaffee says. And the polymer mixture helps with other biocontrols, too.
"Adding the polymer to some existing biological control agents gave us
Botrytis control in the greenhouse where there was none without the
polymers, or it improved control of other agents," he says. Mahaffee
suspects that the polymers help organisms colonize a leaf surface better,
giving them a higher, more constant population to stave off gray mold.
While this technology is just now being developed, Mahaffee hopes it could find
commercial application with growers in 5 to 7 years, giving consumers firmer
fruit and brighter flowers.By
Stelljes, Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program (#303)
described on the World Wide Web at http://www.nps.ars.usda.gov.
Walter F. Mahaffee is with
the USDA-ARS Horticultural
Crops Laboratory, 3420 N.W. Orchard Ave., Corvallis, OR 97330; phone
(541) 752-9455, fax (541) 750-8764.
"Uncovering the Mysteries of Gray Mold" was published in
2001 issue of Agricultural Research magazine.