Investigating How FHB Invades Kernels

The objective of this project (July 1, 2001- June 30, 2003) was to evaluate possible pathways of entry by Fusarium graminearum (the principal head blight pathogen) into heads of barley and wheat. Past work in our laboratory and elsewhere indicates that the outer surfaces of individual florets in wheat and barley spikes have thick-walled epidermal cells.  The head blight fungus cannot penetrate these armored epidermal cells. If, however, the fungus can gain entry into the interior of florets, which is lined with thin-walled epidermal cells, it can readily penetrate and invade interior tissues. We have investigated development of F. graminearum on and in inoculated heads of Robust barley to learn what pathways the fungus uses to enter florets.  Study conclusions:

•  The fungus gains access to the floret interior principally by growing over floret surfaces into the lemma-palea crevice or into the floret mouth.

•  Anthers are not required for fungal entry into florets.

•  Primary lesions occur on virtually all interior surfaces of the floret, including the palea, lemma, and caryopsis. Lesions sometimes spread, forming secondary lesions in contiguous tissues. The end result is a mixture of primary and secondary lesions in advanced stages of head infection.

•  Fungal penetration of epidermal cells in the floret interior usually occurs without formation of specialized infection structures. Penetration deserves additional investigation to learn what epidermal cells are most vulnerable, whether the epidermal cells have mechanisms for resisting penetration, and whether the structures seen at some epidermal surfaces/hyphal landing sites have a role in infection processes.

–  W.R. Bushnell, U of M Department of Plant Pathology, ARS-USDA Cereal Disease Laboratory, St. Paul, MN (now retired)