A Mushroom Mystery:
Mycologist Gary Samuels and
University of Maryland student
Lutorri Ashley discuss the
morphology of the Trichoderma
that causes green mold of mushrooms.
In early spring, after winter-dormant fields have been
tilled and seeded, farmers walk over their fields and look for signs
of life. They scan the newly furrowed earth for tiny shoots of green
and give a small sigh of relief when they find them.
Most farmers, that is.
If there's one thing a farmer of mushrooms fears, it's seeing the color green in his or her crop beds. Mushrooms are fungi and lack chlorophyll. Instead of relying on the sun and photosynthesis, they draw their nutrients from the ground and their growing medium. Healthy colors for the most popular commercially grown mushrooms in the United States are white, brown, and beige. Green spells disease. For mycologists with the Agricultural Research Service's Systematic Botany and Mycology Laboratory (SBML) in Beltsville, Maryland, green also spelled a challenge not long ago.
Per capita U.S. consumption
of mushrooms increased from
3.7 pounds in 1993 to 4.2
pounds in 2000. These button
mushrooms are a favorite in
or on soups, salads, pizza,
and many other dishes.
In the early to mid-1990s, mushroom farmers in Pennsylvania
were under siege. Commercial production of their crop was being seriously
affected by a green mold epidemic. According to the National Agricultural
Statistics Service, Pennsylvania farmers grow more mushrooms than farmers
in any other state, and in 1995 the farmers in Chester Countythe
state's mushroom meccaexperienced crop losses of 30 to 100 percent.
Initially, scientists identified the culprit as Trichoderma
harzianum, a common fungal species used commercially in the biological
control of other fungi that induce plant diseases, including Botrytis
gray mold. It also has the potential to enhance plant growth and has
been credited with degrading pesticides in soil and preventing mycotoxin
But if T. harzianum were the fungus causing the green mold epidemic, its commercial viability would be in jeopardy: It would be attacking a valuable and popular food commodity.
Americans love mushrooms, now more than ever. Per capita consumption
in the United States increased from 3.7 pounds in 1993 to 4.2 pounds
in 2000. Sales of the 2001-2002 U.S. mushroom crop totaled 851 million
pounds, and consumers spent $912 million on them. Mushrooms are a good
source of selenium, potassium, and copper, and some types have significant
amounts of three B-complex vitamins. In a fight between the commercial
production of mushrooms and the commercial production of T. harzianum,
mushrooms would win.
But the mycologists at SBML were not so sure that the beneficial biocontrol
fungus T. harzianum was to blame. They looked at the green mold
problem and saw that not just one, but four distinct T. harzianum
biotypes had been identified as the cause. And only two of those could
be associated with appreciable mushroom loss.
Says Gary Samuels, an SBML mycologist and world-renowned Trichoderma
expert, "We suspected that the four biotypes identified as causing
the green mold epidemic might not all be from the same species. A few
studies suggested genetic distinctions between them, but no one had
studied the differences closely."
That's when the systematic expertise of SBML researchers came into play. Systematics is the science of classification, and the researchers at SBML focus on describing and classifying fungi and plants. They use morphological (structural), biochemical, and molecular data to identify and characterize agriculturally important species and sort out their relationships.
The Aggressive Mold
Samuels and SBML mycologist Sarah Dodd examined 99 strains of the 4
Trichoderma biotypes found in cultivated mushroom beds. Only
two biotypes were associated with mushroom loss; the other two were
benign. SBML studies confirmed others' findings that the benign biotypes
were the real T. harzianum and T. atroviridea common,
"There were consistent genetic differences between the biotype
we knew to be T. harzianum and the two biotypes that were causing
the mushroom losses," says Dodd. She compared all four biotypes
through molecular analysis, using particular sequences from their nuclear
ribosomal DNA and a protein-coding gene called EF-1-alpha.
Samuels says, "The differences are detectable at more than just
the molecular level. We could also distinguish the benign fungi by their
rate of growth and odor. For example, only the real T. harzianum
grows well and forms spores at 35 °C [95 °F]. And T. atroviride
has a characteristic coconut odor."
Through their morphologic and molecular studies, the SBML researchers
were able to exonerate T. harzianum and to name a new Trichoderma
species as the mushroom killer.
"As we suspected, the two strains of Trichoderma causing
damage to cultivated mushrooms aren't from the species of good biological
control fungi," says Samuels. "They're from a different species
altogether." The scientists named the new species T. aggressivum
because of its aggressive nature.
An article containing a description of T. aggressivum and expanded
descriptions of T. harzianum and T. atroviride appeared
in the January 2002 issue of Mycologia.By Amy
Spillman, Agricultural Research Service Information Staff.
This research is part of Plant Disease, an ARS National Program
(#303) described on the World Wide Web at http://www.nps.ars.usda.gov.
Little-Known Mushroom Facts