Everybody knows that flowering plants exploit bees, using pretty colors and offering nectar to attract them so that they will pick up pollen and pollinate the next flower.
Now, two researchers have found, there is a fungus that does much the same thing. In fact, the fungus, which infects blueberry plants, achieves a similar goal -- getting its reproductive spores transferred to other blueberry plants -- by turning infected leaves into something that, to bees, looks like a flower. The scientists say it appears to be the first known example of a plant disease organism creating floral mimics to exploit insect behavior.
The phenomenon was discovered by Lekh R. Batra and Suzanne W.T. Batra, both of the U.S. Department of Agriculture's research center at Beltsville. Their report is published in the May 24 issue of Science.
The Batras became interested in the fungus because it can devastate commercial blueberry crops, cutting yields by 85 percent in some fields. The fungus, Monilinia, also afflicts wild blueberries and huckleberries. Infection causes blueberry flowers to produce inedible seedless fruits that shrivel and dry, forming what are sometimes called mummy berries.
The earliest symptom of infection is a wilting of young leaves. In a day or two the leaf turns brown, often with a tinge of violet. Soon the leaf develops an odor and a layer of fungal spores. About this time, the Batras found, bees and other pollinating insects zero in on the infected leaves with the same flight patterns they use to approach flowers. Then the insects lick the spores, smearing some on their bodies and ferrying them to the next blueberry plant they visit.
The Batras found that the wilted leaves reflect ultraviolet light, just as do flower petals. Healthy leaves absorb ultraviolet light. Bees and other pollinators are known to be able to see ultraviolet light as a color unknown to the human eye. The researchers also found that the coat of the fungal spores contains significant quantities of the three sugars found in nectar: sucrose, glucose and fructose.
The fungus, the Batras concluded, has evolved a survival mechanism that creates floral mimics to fool insects into carrying their progeny to the healthy plants needed to continue the fungal life cycle.