Optical measurements of invasive forces exerted by appressoria of a plant pathogenic fungus

Many plant pathogenic fungi, such as the cereal pathogen Colletotrichum graminicola, differentiate highly specialized infection structures called appressoria, which send a penetration peg into the underlying plant cell. Appressoria have been shown to generate enormous turgor pressure, but direct evidence for mechanical infection of plants by fungi is lacking. A microscopic method was developed that uses elastic optical waveguides to visualize and measure forces locally exerted by single appressoria. By this method, the force exerted by appressoria of C. graminicola was found to be about 17 micronewtons.