Leaf spots cause skewed abortions.

Who gets to be the dad of the next generation of mountain laurels--showy flowering shrubs that grow in eastern U.S. woodlands--may depend on a pesky leaf fungus.

Pathogens tweak their hosts in odd ways, note Maureen A. Levri and Leslie A. Real, who collaborated at Indiana University in Bloomington. In the July ECOLOGY, they report the "novel" effect of the fungus Cercospora kalmiae on plant parenthood.

Bumblebees perform most of the pollination of the mountain laurel. Their chunky bodies land heavily enough to release pollen-bearing stamens from a groove in the flower. The stamens bounce up and swat their pollen onto the bee. If no bees have arrived by the time the flower starts to crumple with age, the stamens snap up anyway and dump pollen onto the flower's own female organ. The flowers, however, may later abort some of the seeds that begin to develop.

For laurels suffering various levels of fungal assault, Levri and Real compared the amount of seed produced in bee-pollinated versus self-pollinated flowers. The researchers also mimicked a bad case of fungus by attacking leaves with a hole punch.

Flowers near damaged leaves were more likely to abort seeds developing from self-pollination than were flowers near healthy leaves. Yet, damaged leaves did not cause abortions in bee-pollinated flowers. The net effect, say the researchers, is that the fungus shifts the plant's mating system, inflating the role of cross-fertilization.