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

Many plant pathogenic fungi, such as the cereal pathogen Colletotrichum gra minicola, differentiate highly specialized infection structures called appr essoria, which send a penetration peg into the underlying plant cell. Appre ssoria have been shown to generate enormous turgor pressure, but direct evi dence 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 mic ronewtons.