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NectriaNectria - peritheciaperithecia FusariumFusarium - conidiaconidia** Images and lecture material were not entirely created by J. Bond. Some of this material was created by others.**

How pathogens attack plants

Remember: most microbes are not pathogens on most plants!

Apple scabSouthern leaf rust of corn

Infection Process

Pathogens must gain access into plants to cause disease

• Viruses – Wounds• Bacteria – Wounds, natural openings• Fungi and Fungal Like Organisms –

Wounds, natural openings, and direct penetration

• Nematodes – direct penetration

Pathogens use plants for food, but most of the “food” is inside the cell.

To get to this food, pathogens must first penetrate the cell wall.

To penetrate the cell wall, pathogens must first get past the plant cuticle (usually).

Regular cell wall•2 layers: cellulose, pectin, hemicelluloses, lignin

•Tough to get through

Epidermal cell wall•2 layers plus cutin and wax covering.

•VERY tough to get through

Mechanical force

• Only fungi, nematodes, and parasitic plants use mechanical force to get into the cell

• Most use force to get through wax layer (although they must adhere to the wax during penetration)

• Adherence is usually by appressoria

Appressoria can exert tremendous pressure!•Melanin strengthens fungal cell wall in

appressoria and appears to be an important pathogenicity factor

•Appressoria also secrete cutinases, cellulases, pectinases

- Once past the cuticle, fungus must continue to secrete cellulases, pectinases, proteinases to enter each and every cell.

Utilization of the protoplast

• The protoplast contains the “food” that most pathogens are after

• But this food must first be degraded, then absorbed

• Degradation by enzymes - amylases, lipases, phospholipases, proteinases