Fungus may be good for your chocolate

It’s a hard sell to someone who has been afflicted with athlete’s foot or jock itch. It’s counter-intuitive. Yet fungus infections may be most desirable for the folks who bring you Milky Way bars and M&Ms.

Allen Herre, a Smithsonian Tropical Research Institute biologist whose current work is in part funded by the Mars corporation that makes the above-mentioned candies, explained why at a recent Tupper Center lecture.

The company wants cacao trees --- from whose beans cocoa and ultimately chocolate are derived --- that lose fewer fruit to the various common blights. One might presume that this would mean trees with fewer fungi living on and within them. But after five years of research into “what the fungi that people find inside the plants are actually doing there,” Herre and his associates have discovered that cacao leaves that are loaded with fungi suffer substantially less damage than fungus-free leaves when they are attacked by the same pathogens.

The work has involved a great deal of field sampling, culturing of micro-organisms in labs and DNA analysis, the first point of which is to know the fungi that inhabit cacao leaves. Part of the process is monitoring leaves by age, to plot the usual course of fungus infections. Once this work has been done, various experiments with some of the fungi that have been isolated and identified, and with the pathogens that cause the black pod, frosty pod and witch’s broom cacao blights, hold the promise of agricultural techniques that reduce losses that in turn affect the chocolate market.

“Any time you see a plant anywhere,” Herre explained, “what you are seeing is not just a plant but a mosaic of plant and fungi.... In two square feet of leaves, you have more distinct species of fungi than species of trees in the canal area.” The applied science of plant fungi is most developed with respect to grasses. Herre noted, for example, an effort at Rutgers University to manipulate fungus infections in order to develop better golf course turf.

The diversity and life cycles of fungi in tropical woody plants, however, are a lot more complex than those found the temperate grasses that have been most closely studied.

“Perfectly healthy plants are shot full of fungi,” Herre said, describing a process by which new leaves come into the world with few or no fungi, are quickly invaded by a wide variety of fungus species that reach a saturation point when the leaves are about a month old, and then as the leaves age the rarer fungi tend to die out while a few more common types tend to thrive.

In the grasses, fungi are transmitted from plants to their offspring through the seeds. With cacao trees, they are transmitted “horizontally,” from plants to their neighbors through processes that are not completely understood. It seems, for example, that the fungi that infect cacao leaves do not sporulate when the leaves are on the tree but rather after they have fallen.

In any case, and conveniently for researchers, it has been found that the way to keep a cacao leaf from picking up a fungus infection is simply to keep it dry. That makes it easier to do controlled studies, comparing on different leaves of the same plant the difference between how pathogens affect leaves with fungi and those without.

It appears that cacao tree fungi inhibit the damage caused by blights. The protective mechanism seems to be local on the leaves rather than systemic through the plant. (Think of the difference between local and systemic actions as something like the distinct ways of fighting off an infection implied in putting iodine on a wound to kill the germs on the site versus ingesting antibiotic pills to provoke an immune system reaction throughout the body.) The fungus protection seems to be more effective for old leaves than young ones.

To add extra complications, it seems that the mix of endophytic fungi at the five sites around Panama that Herre and his associates studied vary with geography. Plants of different species standing right next to one another will have different mixes of fungi living on them, but reasonably close cacao trees will have more or less similar fungus cultures. But cacao trees in Bocas del Toro are hosts to a substantially different mix than cacao trees on an old plantation near Gamboa.

The practical applications of Herre’s research are now being conceived and tested. Researchers are looking for techniques to foster a “fight fungus with fungus mutualism” in which cacao trees are inoculated with beneficial fungi to fight off blights.

The first attempts involved culturing the fungi and spraying them on trees, but it was found that the rain would wash the inoculation right off. Technicians from the War on Drugs, part of which now involves spreading plant diseases that harm coca, marijuana and poppy plants, were consulted about developing stickier sprays.

In early field testing the results seem to be positive, although the spraying was of fungi that seem effective against one blight when the season’s main problem was another disease.

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When a fungus infection is a good thing