Zymoseptoria pseudotritici, a pathogenic fungus that damages wheat crops, is a good example. This recent hybrid resulted directly from the coupling of two genetically distinct, nonpathogenic fungi that had been brought into contact through human trade and agricultural practices. The offspring, unlike its parents, is a killer.
"If there is some new environmental condition in which they can't thrive, some fungi change their reproductive strategy and they reproduce sexually. Fungal sex is far more common than we ever thought. In terms of sheer numbers, they're among the most successful organisms on the planet," Fisher says.
One of the most sinister weapons in fungi's survival arsenal is its ability to hide in any life-form that is being shipped from one country to another—and then to wait out poor conditions. For instance, Phytophthora ramorum, the funguslike oomycete that has caused the die-off of native oak trees in California and Oregon in the past decade, probably hitched a ride on a non-susceptible host in Asia, mostly likely a rhododendron, through the ornamental plant trade. "There were no warning bells for this disease," says Matteo Garbelotto, extension specialist at the Department of Environmental Science, Policy and Management at the University of California, Berkeley. Once the fungus arrived on the California coast, the warm weather and intermittent rainfall enabled it to move easily among hosts. "What we think happened was the pathogen moved from the rhododendron to the California bay laurel, where it can lay dormant for years. There is no direct oak-to-oak transmission," Garbelotto says.
When environmental conditions aren't perfect, phytopthora bides its time. "In dry years, they don't propagate," Garbelotto says. "Phytophthora is a zoospore, which means it goes through a swimming phase during its life cycle. If it's dry or the temperature is low, there is no outbreak. But when the conditions are right, an epidemic can go from nonexistent to affecting innumerable trees within a few weeks."
Not all fungi are bad—far from it. Without fungi we wouldn't have Penicillium which ages blue cheese, rots oranges and from which the antibiotic drug penicillin was extracted. "Without fungi, life on Earth would look very different," Fisher adds. "Forests themselves depend on fungi for their survival." Mycorrhizal, or symbiont fungi, have evolved mutualistically with plant-root vascular systems. They transfer nitrogen and phosphorus from the soil to the plant roots through these associations.
Fungi are among evolution's most successful organisms. Yet it took only a few generations for some of nature's most awe-inspiring assets to become some of its most fearsome liabilities for other species on the planet.