Many producers faced huge crop losses in 1994 and 1995 from green mold, a light green fungus that invades commercial mushroom beds and prevents the crop from emerging. "We faced well over $250,000 in losses in one year," Buster Needham says. "Other growers' losses were far higher, and some went out of business."

A new strain of Trichoderma harzianum, or "green mold," has cost Pennsylvania growers millions of dollars in lost mushroom crops.

Trichoderma harzianum, commonly known as green mold, lives in almost every environment on earth. Usually it is no more harmful than any other common fungus, but in 1985, a different, more aggressive form of Trichoderma suddenly appeared in cultivated mushrooms. Growers in Ireland and Britain reported a new strain of green mold in their operations. Scientists in Europe called the new strain Trichoderma harzianum 2 (Th2). By 1994, another virulent variety of green mold, called Th4, had appeared on Pennsylvania mushroom farms, causing crop losses of more than $1 million per year. "We needed to find out why green mold disease suddenly developed to epidemic proportions," explains mushroom scientist Peter Romaine.

Penn State mobilized its research teams to find methods growers could use to prevent outbreaks and discover where the disease came from. Plant pathologists Paul Wuest and David Beyer, using criteria pioneered by the Centers for Disease Control, designed a questionnaire for the state's mushroom growers to discover common factors that might have contributed to outbreaks. Data revealed that the initial battlefield between green mold and Agaricus occurs at the earliest levels of growth, when grain inoculated with the Agaricus fungus is planted into the substrate. The application of the fungicide Benlate, researchers found, greatly reduced outbreaks of Th4, but only if applied directly to the spawn before it is planted. At that point, the fungicide retards the growth of green mold, but does not affect the Agaricus fungus.

Green mold has been held in check for the past two years by applying Benlate and increasing on-farm sanitation practices. "Sanitation is the most important control method," Wuest says. "The fungus is sticky, and it travels on workers' clothing, their hands, on harvesting knives, and by other methods. Improved sanitation can eliminate it from the mushroom house."

Mushroom scientist Peter Romaine used DNA fingerprinting to seek the origin of the fungus responsible for recent green mold outbreaks.

Managing green mold has been easier than discovering where it came from. Romaine and Dan Royse used a process of elimination to try to pinpoint the origin of Th4. First, they showed that Pennsylvania's Th4 green mold disease was indeed a different organism than the European Th2 pathogen. Romaine and graduate student Xi Chen developed a polymerase chain reaction (PCR) test to genetically identify the Th4 pathogen. "The test was quickly adopted by scientific laboratories to help mushroom growers detect the disease early and make decisions about disease management," Romaine says.

Researchers analyzed green mold samples from major Pennsylvania outbreaks and found that 90 percent of them were caused by the new pathogen, Th4. "This told us it was a disease distinct from the European green mold," Romaine explains. "It did not tell us if Th4 had existed in cultivated mushrooms before the epidemic of the 1990s, if it was recently introduced into the industry, or if the epidemic was the consequence of the type of mushrooms the industry grows."

To see if Th4 existed previously, Romaine and Chen went to Penn State's Mushroom Culture Collection, where samples of green mold collected between 1950 and 1990 are stored. None of the samples matched the genetic fingerprint of Th4. Next, they examined the U1 hybrid, the Agaricus variety making up about 95 percent of the mushrooms grown in the United States and Europe, to see if the crop's lack of genetic diversity contributed to the epidemic. The researchers infected samples of Agaricus strains used before U1 was developed and found all the earlier strains were equally or more susceptible to Th4.

Finally, Royse examined other types of green mold to determine whether Th4 mutated from them. He focused on Trichoderma strains currently used as biocontrol agents for various fungal diseases of crop plants. He and graduate student Manuel Ospina-Giraldo used DNA fingerprinting to genetically identify the biocontrol agents and found that the Th4 strain was not related. "The theory was that one or more of the biocontrol agents had somehow mutated and devastated the mushroom industry, but that wasn't the case," Royse explains.

"The simplest explanation is that it's a new pathogen introduced into the industry at the onset of the epidemic," Romaine says of the outbreak. "We may never know where the new strain of Trichoderma originated or how it was introduced into the industry, because green mold exists nearly everywhere. Disease epidemics usually result when man steps into the path of a new pathogen. Clearly, this is a case where the mushroom industry unwittingly stepped into the path of Th4."